<PAGE>

                                                                   EXHIBIT 10.19

                                                                    July 7, 2004

Smithway Motor Xpress, Inc.
2031 Quail Avenue
Fort Dodge, Iowa 50501

And

East West Motor Express, Inc.
1170 JB Drive
Black Hawk, South Dakota 57718

RE:   TWELFTH AMENDMENT TO AMENDED AND RESTATED LOAN AND SECURITY AGREEMENT

Gentlemen:

      SMITHWAY MOTOR XPRESS, INC., an Iowa corporation ("SMITHWAY INC.") and
EAST WEST MOTOR EXPRESS, INC., a South Dakota corporation ("EAST WEST")
(Smithway Inc. and East West each a "BORROWER" and collectively the "BORROWERS")
and LASALLE BANK NATIONAL ASSOCIATION, a national banking association ("LENDER")
have entered into that certain Amended and Restated Loan and Security Agreement
dated December 28, 2001 (the "SECURITY AGREEMENT"). From time to time
thereafter, Borrowers and Lender may have executed various amendments (each an
"AMENDMENT" and collectively the "AMENDMENTS") to the Security Agreement (the
Security Agreement and the Amendments hereinafter are referred to, collectively,
as the "AGREEMENT"). Borrowers and Lender now desire to further amend the
Agreement as provided herein, subject to the terms and conditions hereinafter
set forth.

      NOW, THEREFORE, in consideration of the foregoing recitals, the mutual
covenants and agreements set forth herein and other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged, the
parties hereto hereby agree as follows:

      1. The Agreement hereby is amended as follows:

<PAGE>

Smithway Motor Xpress, Inc.
East West Motor Express, Inc.
July 7, 2004
Page 2

      (a) The definitions of "BUSINESS DAY", "MAXIMUM LOAN LIMIT" and "PRIME
RATE LOANS" set forth in Paragraph 1 of the Agreement are deleted in their
entirety and the following are substituted in their place:

            "BUSINESS DAY" shall mean any day other than a Saturday, a Sunday or
            (i) with respect to all matters, determinations, fundings and
            payments in connection with LIBOR Rate Loans, any day on which banks
            in London, England or Chicago, Illinois are required or permitted to
            close, and (ii) with respect to all other matters, any day that
            banks in Chicago, Illinois are permitted or required to close.

            "MAXIMUM LOAN LIMIT" shall mean Twenty Million and No/100 Dollars
            ($20,000,000.00).

            "PRIME RATE LOANS" shall mean the Loans bearing interest at the
            Prime Rate plus the Prime Margin, as set forth in Paragraph 4 of the
            Agreement.

      (b) Paragraph 1 of the Agreement is amended to add the following
definitions:

            "INTEREST PERIOD" shall have the meaning specified in Paragraph 4 of
            the Agreement hereto.

            "LIBOR RATE" shall mean with respect to any LIBOR Rate Loan for any
            Interest Period, a rate per annum equal to the offered rate for
            deposits in United States dollars for a period equal to such
            Interest Period as it appears on Telerate Service page 3750 as of
            11:00 a.m. (London time) two Business Days prior to the first day of
            such Interest Period. " TELERATE SERVICE PAGE 3750" means the
            display designated as "Page 3750" on the Telerate Service (or such
            other page as may replace page 3750 of that service or such other
            service as may be nominated by the British Bankers' Association as
            the vendor for the purpose of displaying British Bankers'
            Association interest settlement rates for United States dollar
            deposits).

            "LIBOR RATE LOANS" shall mean the Loans bearing interest at the
            LIBOR Rate plus the LIBOR Margin, as set forth in Paragraph 4 of the
            Agreement.

<PAGE>

Smithway Motor Xpress, Inc.
East West Motor Express, Inc.
July 7, 2004
Page 3

            "TAX" shall mean in relation to any LIBOR Rate Loans and the
            applicable LIBOR Rate, any tax, levy, impost, duty, deduction,
            withholding or charges of whatever nature required (i) to be paid by
            Bank and/or (ii) to be withheld or deducted from any payment
            otherwise required hereby to be made by Borrower to Bank; provided,
            that the term "Tax" shall not include any taxes imposed upon the net
            income of Bank.

      (c) Subparagraph 8(a) of the Agreement is deleted in its entirety and the
following is substituted in its place:

            (a)   Borrower shall direct all of its Account Debtors to make all
                  payments on the Accounts directly to a post office box (the
                  "LOCK BOX") designated by, and under the exclusive control of,
                  Bank or another financial institution acceptable to Bank.
                  Borrower shall establish an account (the "LOCK BOX ACCOUNT")
                  in Borrower's name with Bank or such other financial
                  institution acceptable to Bank, into which all payments
                  received in the Lock Box shall be deposited, and into which
                  Borrower will immediately deposit all payments received by
                  Borrower for Accounts in the identical form in which such
                  payments were received, whether by cash or check. If Borrower,
                  any Affiliate or Subsidiary, or any shareholder, officer,
                  director, employee or agent of Borrower or any Affiliate or
                  Subsidiary, or any other Person acting for or in concert with
                  Borrower shall receive any monies, checks, notes, drafts or
                  other payments relating to or as proceeds of Accounts or other
                  Collateral, Borrower and each such Person shall receive all
                  such items in trust for, and as the sole and exclusive
                  property of, Bank and, immediately upon receipt thereof, shall
                  remit the same (or cause the same to be remitted) in kind to
                  the Lock Box Account. If the Lock Box Account is not
                  established with Bank, the financial institution with which
                  the Lock Box Account is established shall acknowledge and
                  agree, in a manner satisfactory to Bank, that the amounts on
                  deposit in such Lock Box Account are the sole and exclusive
                  property of Bank, that such financial institution has no right
                  to setoff against the Lock Box Account or against any other
                  account maintained by such financial institution into which
                  the contents of the Lock Box Account are transferred, and that
                  such financial institution shall wire, or otherwise transfer
                  in immediately available funds in a manner satisfactory to
                  Bank, funds deposited in the Lock Box Account on a daily basis
                  as such

<PAGE>

Smithway Motor Xpress, Inc.
East West Motor Express, Inc.
July 7, 2004
Page 4

                  funds are collected. Borrower agrees that all payments made to
                  such Lock Box Account or otherwise received by Bank, whether
                  in respect of the Accounts or as proceeds of other Collateral
                  or otherwise, will be applied on account of the Liabilities in
                  accordance with the terms of this Agreement; provided, that so
                  long as no Event of Default has occurred, payments received by
                  Bank shall not be applied to the unmatured portion of the
                  LIBOR Rate Loans, but shall be held in an interest bearing
                  cash collateral account maintained by Bank until the earlier
                  of (i) the last day of the Interest Period applicable to such
                  LIBOR Rate Loan and (ii) the occurrence of an Event of
                  Default; provided further, that so long as no Event of Default
                  has occurred, the immediately available funds held in such
                  interest bearing cash collateral account may be disbursed, at
                  Borrower's discretion, to Borrower so long as after giving
                  effect to such disbursement, Borrower's availability under
                  Paragraph 1 of Exhibit A of the Agreement at such time equals
                  or exceeds the outstanding Liabilities at such time. If the
                  Lock Box Account is established with Bank, Borrower agrees to
                  pay all fees, costs and expenses which Bank incurs in
                  connection with opening and maintaining the Lock Box Account
                  and depositing for collection by Bank any check or other item
                  of payment received by Bank on account of the Liabilities. All
                  of such fees, costs and expenses shall constitute Loans
                  hereunder, shall be payable to Bank by Borrower upon demand,
                  and, until paid, shall bear interest at the highest rate then
                  applicable to Loans hereunder. All checks, drafts, instruments
                  and other items of payment or proceeds of Collateral shall be
                  endorsed by Borrower to Bank, and, if that endorsement of any
                  such item shall not be made for any reason, Bank is hereby
                  irrevocably authorized to endorse the same on Borrower's
                  behalf. For the purpose of this paragraph, Borrower
                  irrevocably hereby makes, constitutes and appoints Bank (and
                  all Persons designated by Bank for that purpose) as Borrower's
                  true and lawful attorney and agent-in-fact (i) to endorse
                  Borrower's name upon said items of payment and/or proceeds of
                  Collateral and upon any Chattel Paper, document, instrument,
                  invoice or similar document or agreement relating to any
                  Account of Borrower or goods pertaining thereto; (ii) to take
                  control in any manner of any item of payment or proceeds
                  thereof; and (iii) to have access to any lock box or postal
                  box into which any of

<PAGE>

Smithway Motor Xpress, Inc.
East West Motor Express, Inc.
July 7, 2004
Page 5

                  Borrower's mail is deposited, and open and process all mail
                  addressed to Borrower and deposited therein.

      (d) Subparagraph 8(c) of the Agreement is deleted in its entirety and the
following is substituted in its place:

            (c)   For purposes of calculating interest, Bank shall, within two
                  (2) Business Days after receipt by Bank at its office in
                  Chicago, Illinois of (i) checks and (ii) cash or other
                  immediately available funds from collections of items of
                  payment and Proceeds of any Collateral, apply the whole or any
                  part of such collections or Proceeds against the Liabilities
                  in such order as Bank shall determine in its sole discretion.
                  For purposes of determining the amount of Loans available for
                  borrowing purposes, (i) checks and (ii) cash or other
                  immediately available funds from collections of items of
                  payment and Proceeds of any Collateral shall be applied in
                  whole or in part against the Liabilities, in such order as
                  Bank shall determine in its sole discretion, on the day of
                  receipt, subject to actual collection.

      (e) Paragraph 4(a) of the Agreement is deleted in its entirety and the
following is substituted in its place:

            (a)   INTEREST RATE: Subject to the terms and conditions set forth
                  below, the Loans shall bear interest at the per annum rate of
                  interest of the Prime Rate plus the Prime Margin set forth
                  below or at the LIBOR Rate plus the LIBOR Margin set forth
                  below, at Borrower's option, based on the following matrix.
                  Initially, the Loans shall bear interest as reflected in Level
                  II of the matrix through December 31, 2004, with the first
                  test upon the submission by Borrower of fiscal year end 2004
                  audited financial statements (and shall be tested quarterly on
                  a rolling twelve (12) month basis by Bank thereafter) and, if
                  applicable, reset by Bank.

<PAGE>

Smithway Motor Xpress, Inc.
East West Motor Express, Inc.
July 7, 2004
Page 6

<TABLE>
<CAPTION>
                                        REVOLVING LOANS            TERM LOANS
                                   --------------------------      ----------
                FUNDED DEBT                            LIBOR          PRIME          LIBOR
LEVEL            TO EBITDA         PRIME MARGIN       MARGIN         MARGIN          MARGIN
-----        ------------------    ------------       -------      ----------       -------
<S>          <C>                   <C>                <C>          <C>              <C>
I               >or= 3.5              50 BPS          300 BPS        50 BPS         350 BPS
II           >or= 2.5 AND < 3.5       25 BPS          275 BPS        25 BPS         325 BPS
III          >or= 1.5 AND < 2.5       25 BPS          250 BPS        25 BPS         300 BPS
IV                < 1.5                0 BPS          225 BPS        25 BPS         275 BPS
</TABLE>

      "EBITDA" shall mean with respect to any period, Borrower's net income
      after taxes for such period (excluding any after-tax gains or losses on
      the sale of assets, other than the sale of Inventory in the ordinary
      course of business) and excluding other after-tax extraordinary gains or
      losses) plus interest expense, income tax expense, depreciation and
      amortization for such period, plus or minus any other non-cash charges or
      gains which have been subtracted or added in calculating net income after
      taxes for such period.

      "Funded Debt" shall mean, with respect to any period, all Debt of Borrower
      for borrowed money, whether or not evidenced by bonds, debentures, notes
      or similar instruments.

      "Prime Rate" means the publicly announced prime rate of LaSalle Bank
      National Association (which is not intended to be LaSalle Bank National
      Association's lowest or most favorable rate in effect at any time) (the
      "Prime Rate") in effect from time to time, said rate of interest to
      increase or decrease by an amount equal to each increase or decrease in
      the Prime Rate effective on the effective date of each such change in the
      Prime Rate; and

      "Interest Period" shall mean any continuous period of thirty (30), sixty
      (60) or ninety (90) days, as selected from time to time by Borrower by
      irrevocable notice (in writing, by telex, telegram or cable) given to Bank
      not less than two (2) Business Days prior to the first day of each
      respective Interest Period; provided that: (i) each such period occurring
      after such initial period shall commence on the day on which the
      immediately preceding period expires; (ii) the final Interest Period shall
      be

<PAGE>

Smithway Motor Xpress, Inc.
East West Motor Express, Inc.
July 7, 2004
Page 7

      such that its expiration occurs on or before the end of the Original Term
      or any Renewal Term; and (iii) if for any reason Borrower shall fail to
      timely select a period, then such Loans shall continue as, or revert to,
      Prime Rate Loans. Interest shall be payable on the last Business Day of
      each month and on the date of any payment hereon by Borrower.

      Upon the occurrence of an Event of Default, the Loans shall bear interest
      at the rate of two percent (2.0%) per annum in excess of the interest rate
      otherwise payable thereon, which interest shall be payable on demand. All
      interest shall be calculated on the basis of a 365-day year.

      (f) Subparagraph 4(b)(iv) of the Agreement is deleted in its entirety and
the following is substituted in its place:

            (iv)  AMENDMENT FEE: Borrowers shall pay to Lender an amendment fee
                  of One Thousand and No/100 Dollars ($1,000.00), which fee
                  shall be fully earned and payable upon execution of this
                  Amendment.

      (g) The following subparagraph is hereby added to the end of Paragraph 4
of the Agreement:

            4.1   OTHER LIBOR PROVISIONS:

                  (a)   Subject to the provisions of this Agreement, Borrower
                        shall have the option (i) as of any date, to convert all
                        or any part of the Prime Rate Loans to, or request that
                        new Loans be made as, LIBOR Rate Loans of various
                        Interest Periods, (ii) as of the last day of any
                        Interest Period, to continue all or any portion of the
                        relevant LIBOR Rate Loans as LIBOR Rate Loans; (iii) as
                        of the last day of any Interest Period, to convert all
                        or any portion of the LIBOR Rate Loans to Prime Rate
                        Loans; and (iv) at any time, to request new Loans as
                        Prime Rate Loans; provided, that Loans may not be
                        continued as or converted to LIBOR Rate Loans, if the
                        continuation or conversion thereof would violate the
                        provisions of subparagraphs 4.1(b) and 4.1(c) of this
                        Agreement or if an Event of Default has occurred.

                  (b)   Bank's determination of LIBOR as provided above shall be
                        conclusive, absent manifest error. Furthermore, if Bank
                        determines, in good faith (which determination shall be
                        conclusive, absent manifest error), prior to the
                        commencement of any Interest

<PAGE>

Smithway Motor Xpress, Inc.
East West Motor Express, Inc.
July 7, 2004
Page 8

                        Period that (i) U.S. Dollar deposits of sufficient
                        amount and maturity for funding the Loans are not
                        available to Bank in the London Interbank Eurodollar
                        market in the ordinary course of business, or (ii) by
                        reason of circumstances affecting the London Interbank
                        Eurodollar market, adequate and fair means do not exist
                        for ascertaining the rate of interest to be applicable
                        to the Loans requested by Borrower to be LIBOR Rate
                        Loans or the Loans bearing interest at the rates set
                        forth in Paragraph 4 of this Agreement shall not
                        represent the effective pricing to Bank for U.S. Dollar
                        deposits of a comparable amount for the relevant period
                        (such as for example, but not limited to, official
                        reserve requirements required by Regulation D to the
                        extent not given effect in determining the rate), Bank
                        shall promptly notify Borrower and (x) all existing
                        LIBOR Rate Loans shall convert to Prime Rate Loans upon
                        the end of the applicable Interest Period, and (y) no
                        additional LIBOR Rate Loans shall be made until such
                        circumstances are cured.

                  (c)   If, after the date hereof, the introduction of, or any
                        change in any applicable law, treaty, rule, regulation
                        or guideline or in the interpretation or administration
                        thereof by any governmental authority or any central
                        bank or other fiscal, monetary or other authority having
                        jurisdiction over Bank or its lending offices (a
                        "Regulatory Change"), shall, in the opinion of counsel
                        to Bank, make it unlawful for Bank to make or maintain
                        LIBOR Rate Loans, then Bank shall promptly notify
                        Borrower and (i) the LIBOR Rate Loans shall immediately
                        convert to Prime Rate Loans on the last Business Day of
                        the then existing Interest Period or on such earlier
                        date as required by law and (ii) no additional LIBOR
                        Rate Loans shall be made until such circumstance is
                        cured.

                  (d)   If, for any reason, a LIBOR Rate Loan is paid prior to
                        the last Business Day of any Interest Period or if a
                        LIBOR Rate Loan does not occur on a date specified by
                        Borrower in its request (other than as a result of a
                        default by Bank), Borrower agrees to indemnify Bank
                        against any loss (including any loss on redeployment

<PAGE>

Smithway Motor Xpress, Inc.
East West Motor Express, Inc.
July 7, 2004
Page 9

                        of the deposits or other funds acquired by Bank to fund
                        or maintain such LIBOR Rate Loan) cost or expense
                        incurred by Bank as a result of such prepayment.

                  (e)   If any Regulatory Change (whether or not having the
                        force of law) shall (i) impose, modify or deem
                        applicable any assessment, reserve, special deposit or
                        similar requirement against assets held by, or deposits
                        in or for the account of or loans by, or any other
                        acquisition of funds or disbursements by, Bank; (ii)
                        subject Bank or the LIBOR Rate Loans to any Tax or
                        change the basis of taxation of payments to Bank of
                        principal or interest due from Borrower to Bank
                        hereunder (other than a change in the taxation of the
                        overall net income of Bank); or (c) impose on Bank any
                        other condition regarding the LIBOR Rate Loans or Bank's
                        funding thereof, and Bank shall determine (which
                        determination shall be conclusive, absent any manifest
                        error) that the result of the foregoing is to increase
                        the cost to Bank of making or maintaining the LIBOR Rate
                        Loans or to reduce the amount of principal or interest
                        received by Bank hereunder, then Borrower shall pay to
                        Bank, on demand, such additional amounts as Bank shall,
                        from time to time, determine are sufficient to
                        compensate and indemnify Bank from such increased cost
                        or reduced amount.

                  (f)   Bank shall receive payments of amounts of principal of
                        and interest with respect to the LIBOR Rate Loans free
                        and clear of, and without deduction for, any Taxes. If
                        (1) Bank shall be subject to any Tax in respect of any
                        LIBOR Rate Loans or any part thereof or, (2) Borrower
                        shall be required to withhold or deduct any Tax from any
                        such amount, the LIBOR Rate applicable to such LIBOR
                        Rate Loans shall be adjusted by Bank to reflect all
                        additional costs incurred by Bank in connection with the
                        payment by Bank or the withholding by Borrower of such
                        Tax and Borrower shall provide Bank with a statement
                        detailing the amount of any such Tax actually paid by
                        Borrower. Determination by Bank of the amount of such
                        costs shall be conclusive, absent manifest error. If
                        after any such adjustment any part of any Tax paid

<PAGE>

Smithway Motor Xpress, Inc.
East West Motor Express, Inc.
July 7, 2004
Page 10

                        by Bank is subsequently recovered by Bank, Bank shall
                        reimburse Borrower to the extent of the amount so
                        recovered. A certificate of an officer of Bank setting
                        forth the amount of such recovery and the basis therefor
                        shall be conclusive, absent manifest error.

                  (g)   Each request for LIBOR Rate Loans shall be in an amount
                        not less than Two Million and No/100 Dollars
                        ($2,000,000.00), and in integral multiples of, Two
                        Million and No/100 Dollars ($2,000,000.00).

                  (h)   Unless otherwise specified by Borrower, all Loans shall
                        be Prime Rate Loans.

                  (i)   No more than four (4) Interest Periods may be in effect
                        with respect to outstanding LIBOR Rate Loans at any one
                        time.

      2. This Amendment shall not become effective until fully executed by all
parties hereto.

      3. Except as expressly amended hereby and by any other supplemental
documents or instruments executed by either party hereto in order to effectuate
the transactions contemplated hereby, the Agreement and Exhibit A thereto hereby
are ratified and confirmed by the parties hereto and remain in full force and
effect in accordance with the terms thereof.

                                  LASALLE BANK NATIONAL ASSOCIATION

                                  By  /s/ PETER WALTHER
                                    ------------------------------------

                                  Title  Vice President
                                       ---------------------------------

ACKNOWLEDGED AND AGREED TO
this 7th day of July, 2004:

SMITHWAY MOTOR XPRESS, INC.

By  /s/ G. LARRY OWENS
  ---------------------------------
    G. LARRY OWENS

TITLE: PRESIDENT, CEO AND CHAIRMAN

<PAGE>

Smithway Motor Xpress, Inc.
East West Motor Express, Inc.
November 11, 2004
Page 11

EAST WEST MOTOR EXPRESS, INC.

By  /s/ G. LARRY OWENS
  --------------------------------
    G. LARRY OWENS

TITLE: PRESIDENT, CEO AND CHAIRMAN

Consented and agreed to by the following guarantor(s) of
the obligations of SMITHWAY MOTOR XPRESS, INC. and EAST
WEST MOTOR EXPRESS, INC. to LASALLE BANK NATIONAL
ASSOCIATION.

SMSD ACQUISITION CORP.

By  /s/ G. LARRY OWENS
  --------------------------------
    G. LARRY OWENS

TITLE: PRESIDENT, CEO AND CHAIRMAN

DATE: July 7 2004

SMITHWAY MOTOR XPRESS CORP.

By  /s/ G. LARRY OWENS
  --------------------------------
    G. LARRY OWENS

TITLE: PRESIDENT, CEO AND CHAIRMAN

DATE: July 7, 2004