<PAGE>   1
                                                                   EXHIBIT 12-1

MCN ENERGY GROUP INC. AND SUBSIDIARIES                                      
COMPUTATION OF RATIO OF EARNINGS TO FIXED CHARGES
(Dollars In Thousands)


<TABLE>
<CAPTION>
                                                        Twelve Months           Twelve Months          Twelve Months
                                                            Ended                   Ended                  Ended  
                                                        March 31, 1998        December 31, 1997      December 31, 1996     
                                                     ---------------------   ---------------------   ----------------------
     <S>                                             <C>                       <C>                   <C>                    
     EARNINGS AS DEFINED (1) (5)
     Pre-tax income (2)                                  $ 190,082                 $ 181,299             $ 146,607
     Fixed charges (3)                                     132,454                   125,338                99,944
                                                         ---------                 ---------             ---------

          Earnings as defined                            $ 322,536                 $ 306,637             $ 246,551
                                                         =========                 =========             =========

     FIXED CHARGES AS DEFINED (1)  (4) (5)
     Interest, expensed                                  $  88,111                 $  86,453              $ 77,781
     Interest, capitalized                                  19,601                    18,190                13,235
     Amortization of debt discounts, premium                                                     
          and expense                                        2,267                     2,426                 2,217
     Interest implicit in rentals                            2,375                     2,181                 2,339
     Preferred securities dividend requirements
          of subsidiaries                                   36,615                    31,090                12,390
                                                         ---------                 ---------             ---------

          Fixed charges as defined                       $ 148,969                 $ 140,340             $ 107,962
                                                         =========                 =========             =========

     Ratio of Earnings to Fixed Charges                       2.17                      2.18                  2.28
                                                         =========                 =========             =========
</TABLE>



(1)  Earnings and fixed charges are defined and computed in accordance with 
     Item 503 of Regulation S-K.

(2)  This amount represents the aggregate of (a) the pre-tax income from
     continuing operations of MCN and its majority-owned subsidiaries, (b)
     MCN's share of pre-tax income of its 50% owned companies, and (c) any
     income actually received from less than 50% owned companies.

(3)  Fixed charges added to earnings are adjusted to exclude interest
     capitalized during the period for nonutility companies and the preferred
     securities dividend requirements of MichCon included in fixed charges but
     not deducted in the determination of pre-tax income.

(4)  Fixed charges represent (a) interest, whether expensed or capitalized, (b)
     amortization of debt discount, premium and expense, (c) an estimate of
     interest implicit in rentals, and (d) preferred securities dividend
     requirements of subsidiaries, increased to reflect the pre-tax earnings
     requirement for MichCon.

(5)  In June 1996, MCN completed the sale of The Genix Group, its computer
     operations subsidiary. For purposes of calculating the Ratio of Earnings
     to Fixed Charges, it has been classified as a discontinued operation and
     therefore excluded from the ratio for all periods presented.