Archive-name: rec-autos/part5  [this article is one of a pair of articles containing commonly asked automotive questions; the other article contains questions more geared to the automotive enthusiast, and so is not crossposted to misc.consumers.  -- rpw]     [changes as of 14 April 1993: revised brake fluid section, as     non-silicone DOT-5 fluids are now apparently available -- rpw]                   Commonly Asked Automotive Questions   Tire Questions:  Q:  What do the funny numbers on the sides of my tires mean?  A:  Typically, you will see something like 195/60HR14.  the 195 is the     overall width of the tire in millimeters, the tread is usually     narrower.  The 60 is the `aspect ratio'; it indicates the height of the     sidewall of the tire relative to the overall width.  Our example tire     has a sidewall height of 0.60 * 195 ~= 117 mm.  The 14 is the wheel     diameter in inches; there are also some special tires called `TRX'     tires which have three digit metric wheel diameter designations, like     390, which is in millimeters.  The R means Radial, and the H is a speed     rating indicating the highest speed at which the tire, when properly     inflated and carrying an appropriate load, may safely operate.  Common     speed ratings are S (112MPH), T (118MPH), H (130MPH), and V (up to     150MPH.)      Recent changes to the method for specifying tire sizes move the speed     rating to a different part of the designation; you may therefore find     designations like 195/60R14 85H; the 85 indicates the per-tire load     associated with the speed rating -- exceeding this load in continuous     operation at the rated speed is dangerous practice.  What follows is     a table showing a number of `load indices' and corresponding maximum     per-tire loads:      Load Index       50   51   88   89   112   113   145   149   157     Max Load (Kg)   190  195  560  580  1120  1150  2900  3250  4125      Note that the usual mass vs. weight confusion occurs in this table.      In some cases, the letters P or LT may be found in front of a tire     size; the LT designation indicates Light Truck, and the P designation     indicates Passenger car.  If no letter is given, then the application     of the tire is Passenger car usage.  As far as I know, these letters     only appear in the US market.  The LT designation is prinicipally of     interest to owners of light trucks and other utility vehicles.  For     the owner of a passenger vehicle, there is no meaningful difference     between a tire with a P designation and one with no designation at     all.      If the aspect ratio is omitted, it is probably 80, but may be 78.      Tires with an MS (mud/snow) designation may have their speed rating     reduced by 20 km/h (about 12mph.)      There is an additional set of ratings on tires for temperature,     traction, and treadwear.  Temperature and Traction are graded     A, B, and C (with A the best and C the worst); treadwear is     a numeric rating.  These values are of limited value, as they     are assigned somewhat arbitrarily by tire manufacturers and are     thus not useful in comparing different brands of tires.  Q:  My car has tires with a funny size designation:  185/65HR390; can i put     normal tires on the car?  A:  Your tires are called TRX tires; they were devised by Michelin.     Because of a somewhat different bead design, they are incompatible     with normal tires; Michelin used a different diameter wheel for them     so that they could not be mounted on the wrong type wheel (and so that     more conventional tires could not be mounted on TRX type wheels.)     You will need to aquire different wheels to put a normal tire on your     car; it is barely possible to fit normal tires on TRX wheels, and horribly     dangerous to do so (the tires may simply peel off the rims in a corner,     or possibly blow out at high speed.)  TRX type tires are becoming hard     to find; in addition to Michelin, Avon makes suitable tires.  Goodyear     has apparently discontinued their line of TRX tires.  Q:  Can I rotate radials from side to side, or rotate them only on one side     of my car?  A:  Car and tire manufacturers have differing views on this subject; many     say that swapping radials between the left and right hand sides of a     car is now ok (this group includes Michelin and Goodyear); others     (for example, Pirelli and Nokia) will void warranties if such swapping     is done.  The best advice is to read your tire manual carefully before     rotating your tires, and stick to the manufacturer's recommendations.  Q:  How many snow tires should I buy, and if I buy 2, which end of the     car should I put them on?  A:  In short, 4, and both ends.  To explain, many drivers in areas that don't     get excessive snow or who don't drive agressively (or at all) in snow     conditions get away with only two snows on the drive wheels, but there     are circumstances where this can be dangerous practice.  With a rear     wheel drive car, you can choose between being able to start the car     going (a function of the rear axle) or stopping and turning the car     (a function of the front axle.)  In a front wheel drive car, you start,     stop, and turn with the front end.  The primary risk of putting the     snow tires on the front only is that if you have to put on the brakes     while going downhill, you run a serious risk of going downhill backwards.  Radar Detectors and Speed Limits:  Q:  Why aren't there any comments on Radar Detectors and Speed Limits     in this Q&A posting?  A:  Because questions about detectors and speed limits crossposted between     misc.consumers and rec.autos.* always start long, tedious, and pointless     flame wars.  If you want to talk about either of these topics, please     subscribe to rec.autos or alt.flame and keep it there.  Safety Equipment:  Q:  Do airbags really work?  A:  Preliminary statistics suggest the following:  Airbags work much     better than no belts; good 3 point belts alone work much better than     Airbags alone, and AirBags + 3 point belts work slightly better than     3 point belts alone.  The con to airbags is that some designs tend     to burn the driver's hands when venting the byproducts of the     explosion that occurs inside the bag, and that some designs (but     not all) may knock the driver's hands from the wheel, making retention     of control of the vehicle after the bag deflates more difficult.  Brake Questions:  Q:  Do I always need to get the rotors on my disk brakes turned?  Midas     always wants to do this.  A:  No.  There are specific conditions that mandate turning or replacing     rotors; some shops try and make a little extra money by replacing rotors     more often than is strictly necessary.  if the rotors are not warped     warped, and only lightly grooved, then there is no need to replace or     to turn them.  Note also that some rotors (the rotors on many Hondas     are a good example) are so narrow to begin with that it is not practical     to turn them; they must be replaced when they become too thin, warped,     or badly grooved.  Q:  They tell me I should downshift when braking to slow my car down.  Is     this really a good idea?  A:  It used to be a very good idea, back in the days of mediocre, fade     prone drum brakes.  In modern disc brake equipped cars, use of     downshifting to slow the car is not really necessary, except in cases     of long, steep downhill runs.  Otherwise, modern disc brakes are more     than adequate to stop a passenger car in all circumstances, and they     are much cheaper to repair than clutch linings.      On the other hand, many standard driver's license tests in the USA     still specify that the driver being tested downshift under braking; I     suggest that before taking a US driver's test, you either 1) learn to     do this smoothly (which takes some time and practice) or 2) borrow a     car with an automatic to take the test.  Q:  How often should I replace my brake fluid?  A:  Probably more often than you do.  Traditional brake fluids tend to     absorb water; this water can corrode internal parts of the brake     system and also lower the boiling point of the fluid.  DOT-3 type     are older fluids; DOT-4 and DOT-5 are newer specifications.  The     principal differences are in wet and dry boiling points; the dry     boiling point is important in fresh brake fluid, but the wet boiling     point is important in older brake fluid.  DOT-3 fluids have the lowest     wet and dry boiling point _requirements_; DOT-4 fluids have better     boiling point requirements; and DOT-5 fluids have the best boiling     point requirements (but DOT-5 fluids are not clearly superior; see     the next Q&A for more details.)  While the requirements imply that     DOT-4 fluids are better than DOT-3 fluids, there may be specific     cases where a DOT-3 fluid is preferable, but these are mostly     competition applications.  Otherwise, DOT-4 type fluids offer _much_     improved brake pedal feel.  Replacement once a year is recommended for     DOT-4 fluids, although agressive drivers may profit by changing out     fluid more frequently, or at least bleeding a modest amount of fluid     out of the brake calipers fairly regularly.  Q:  What about DOT-5 brake fluids?  A:  This breaks down in to two parts.  The DOT-5 specification looks     excellent for performance, but the first DOT-5 fluids were Silicone     based.  Silicone fluids are a tricky proposition.  Unlike DOT-3 and     DOT-4 fluids, they do not absorb water at all.  While this may sound     like a feature, the problem is that any water present pools up in     such systems, interfering with braking performance and corroding any     metals at the spot where the pooling is occuring.  The water will tend     to migrate downwards in the braking system to the brake calipers, where     most of the corrosion occurs.      Because of this phenomenon, it is essential when converting to     Silicone to empty the entire brake system and flush it throughly     beforehand; some even recommend replacing all rubber parts in the     brake system when converting to Silicone fluids.      Two other issues that come up with silicone fluids:  1) they are     difficult to pour cleanly (that is, without air bubbles), which     interferes with getting a good brake pedal feel, and 2) while they     generally have much higher boiling points than DOT-4 fluids, they     do have high temperature failure modes which are indistinguishable     in effect from boiling DOT-4 fluids.  SIlicone fluids may make sense     in some street car applications, but they are certainly not     recommended for high performance driving applications, and the     economics are questionable for street use.      I have recently become aware of new fluids that meet the DOT-5 standard     that do not contain Silicones; these fluids appear to be reasonably     compatible with the older DOT-3 and DOT-4 fluids, but I have little     information at this time.  Q:  ABS is available on some of the cars I'm looking at, but it costs     more.  Is it worth it?  A:  This does not have a cut and dried answer; therefore, this answer will     be quite long in order to cover the pros and cons.  The short answer     is that ABS costs more, both initially, and to maintain, will generally     work better for the `average driver' (that is to say, a driver who does     not have extensive experience in high performance driving techniques),     and may require the `unlearning' of some obsolete braking techniques     like pump braking which should be unlearned in any case.      Now for the long answer.  ABS works by monitering the wheels of the     car, looking for signs of locked brakes.  It may or may not be able     be able to distinguish between the different wheels (there are several     systems on the market.)  It cannot detect impending lockup (which is     what you would really want in an ideal world), but only the existence     of lockup.  The sensors used vary; some of the less well designed     sensors are sensitive to tire size, and to brake pad material, and     may cease to function properly if the owner deviates from original     equipment or OE-equivalent components.      When the sensors detect lockup, the ABS system responds by unlocking     the brakes (either individually, or all at once, depending on the     system.)  If the driver keeps their foot firmly planted, the ABS     will end up cycling between the locked and unlocked states (if a     sensor existed that could detect _impending lockup_, then we could     sit right at that point, which is where maximum braking effect is     achieved.)  This pulsing can often be felt in the brake pedal, as     the system cycles.  The percentage of the time that the brakes are     truly engaged is called the `duty cycle'; typically in an ABS system     this is about 40%  On dry pavement, a trained driver can beat this     duty cycle quite reliably using a technique called threshold braking;     on wet pavement, braking is so chancy that ABS will outperform     threshold braking nearly every time.  Unfortunately, on mud and on     snow, often maximum braking effect can be acheived with the brakes     locked; only Audi, of the manufacturers producing ABS-equipped cars,     has seen fit to provide a disable switch for the ABS system for this     eventuality.      A particularly important feature of ABS is that it preserves     steering control.  This is the case simply because, if you are     braking near the limit and turn the wheel, the ABS will release     the brakes if it sees steering-triggered lockup, and back off     on the percentage of the time that the brakes are applied.     Braking distances will lengthen accordingly.      An important caution:  ABS cannot exceed the maximum theoretical     braking force in any given situation; if you start sliding on     glare ice, don't expect an ABS system to help you out very much.     The coefficient of friction is not changed by the presence of an     ABS system in your car.      As far as maintenence goes, in addition to the potential     restrictions I've listed above, you have to worry about the     following:  1) parts costs are much higher; the OE master cylinder     for my obscure european sedan lists for $185, but the OE master     cylinder for the ABS-equipped version of the same car lists for     over $1000.  Most manufacturers explicitly forbid use of DOT-5     (silicone) brake fluids in ABS-equipped vehicles.  Because of     the potential cost of replacement of corroded brake system     components, regular (I suggest annual) replacement of brake fluid     becomes very important.  Q:  What about this threshold braking business?  A:  [normally, I'd not put this in the consumers Q&A, but recent     publicity about a number of accidents involving police drivers     in ABS-equipped Chevy Caprices suggests that this section is     needed here -- rpw]      Threshold braking is a technique practiced by all serious high     performance drivers; if made a habit, it replaces the `stab     the pedal and lock 'em up' panic habit entirely, and is much     to be prefered.  Basically, the premise is that tires generate     maximum braking force when they have just started to slide, but     just before the wheels lock up entirely.  Drivers who threshold     brake learn to feel what this `threshold' feels like, and learn     to search for it and hit it on the application of the brake     pedal.  In many cars, you can feel that you are near the     threshold when the pedal starts to firm up as you depress it.     In any case, if you can't hear the tires whine just a bit, you're     not very near the threshold.      In a car with ABS, often there is a twinge in the pedal just     before the system starts cycling; if the driver backs off on     the pedal just a tad when the twinge is felt, then they are very     close to the threshold and they'll probably achieve better     stopping distances than if they just punched it and let the ABS     take over.      Recently, there has been a rash of publicity over a number of     accidents, and one death, involving police cars equipped with     ABS systems.  The police departments in question quickly blamed     the new ABS systems, but according to Autoweek magazine, it now     seems clear that the problem was a lack of training; none of the     involved officers had any recent performance driving training.     There is reason to believe that the drivers reacted to the pulsing     brake pedal by `pump braking', an old and discredited technique     of stabbing and releasing the brake pedal, the goal being to     try and get brakes back with a failing hydraulic system.  If you     think about it for a minute, you'll realize that pump braking must     cut the effective operation of a working brake system by at least     1/2, so if you cut the 40% duty cycle of an ABS system by that     much, you are giving up most of your brakes for the wrong reason.     Threshold braking has the advantage in that it is an effective     and useful technique regardless of whether your car has ABS; if     you do fear a failed hydraulic system, then one or two stabs at     the pedal will be sufficient.      Gas Questions:  Q:  Does High Octane gasoline help?  A:  Maybe, maybe not.  Some cars have knock sensors, and can adjust the     engine timing or turbocharger boost to suit the gasoline being used.     On most cars, however, you should use the cheapest gas that makes your     car run well.  Check your owner's manual for details on what your car     needs.  Q:  My car was made for leaded gasoline.  Will unleaded gas hurt it?  A:  It is possible that unleaded gas may *slightly* increase valve wear,     although the Amoco Oil company argues otherwise.  The actual increase     in valve wear will be almost unnoticeable, however, as modern leaded     gasolines actually contain very little lead.  You should, however,     check your owner's manual; many cars from the early 1970s do not     actually require leaded gasoline.  Q:  Do fuel treatments help?  What kind should I use?  A:  Some do and some don't.  During the winter, it is a good idea to use     dry gas; however, some may be harmful to fuel injection systems.     Never use an additive containing Methanol (sometimes called Methyl     Alcohol); such additives may damage fuel systems in cars with carbs     and almost certainly will damage cars with fuel injection.      Manufacturer's opinions vary on additives containing Ethanol (sometimes     called Ethyl Alcohol); if your car has fuel injection, check the owner's     manual on your car before  using these.  Most manufacturers consider     10% Ethanol acceptable in gasoline.  Additives with Isopropyl Alcohol     (Isopropanol), and Petroleum  Distillates are fine in fuel injected     cars.      An occasional bottle of fuel injector cleaner is helpful in cars with     fuel injectors, although many premium gasolines contain detergents that     do the same job.  Some off brands of fuel injector cleaners contain     Ethanol or Methanol; always check the ingredients before putting anything     in your gas tank.      There are a small number of particularly good additives; these are     noticeably more expensive that the run-of-the-mill ones, but work     much, much better.  Among these are Chevron Techron, Redline SL-1,     Wurth `Lubrimoly Ventil Sauber', and BG 44K.  A bottle of one of these,     once every six months, is highly recommended.  Q:  What about detergent gasolines?  A:  The quality of detergent packages in gasolines varies somewhat; BMW     has instituted a testing program, and gasoline brands which pass this     test may advertise that fact.  Stickers indicating passage of the test     are now beginning to appear on fuel pumps at gas stations.  If such     gasolines are used, then fuel injector cleaners are probably optional.     Beforewarned that while use of BMW approved gasolines will keep a clean     engine clean, they may not clean a motor with bad valve deposits.  Lubrication Questions:  Q:  What do the numbers and letters in a motor oil designation mean?  A:  There are several different items encoded.  There is a two-letter     code indicating the type of detergent package that the manufacturer	     uses in the oil; this looks like SE,SF,CD or such.  The S codes are     for gasoline engine applications; the C codes are for diesel engine     applications.  The second letter is assigned in sequence as new levels     of protection are developed; thus SF is considered better than SE,     SE is considered better than SD, and so forth.      The more noticeable designation is the oil weight.  This is either     a single number (e.g., 30 weight) or a pair of numbers separated by     the letter W (e.g., 10W30.)  The latter type is much more commonly     used these days, and are the only type that most automobile     manufacturers specify in operators manuals.  The first number in the     designation (10W) is the apparent viscosity of the oil when it is cold;     the W stands for `winter'.  The second number (30) is the viscosity     of the oil when hot.  There is a trick here; the oil doesn't actually     get thicker (turn from 10 weight to 30 weight) as it gets hotter.  What     is actually happening is that when the oil is cold, it has the viscosity     of a cold 10 weight oil.  as it gets hotter, it doesn't get thin as fast     as a 10W oil would; by the time it is up to temperature, it has the     viscosity of a hot 30 weight oil.      Note that these numbers actually specify ranges of viscosities; not     all 10W oils have exactly the same viscosity when cold, and not all     30 weight oils have the same viscosity when hot.  Note also that the     novel behaviour of multi-grade oils is caused by additives, and it has     been reported that with the sole exception of Castrol GTX, 10W40 oils     do not retain their multi-grade characteristics well over time.  10W30,     15W40, and 20W50 oils work very well, though.  Q:  Are `quick lube' places any good?  A:  Some do adequate work, but there are quite a few incompetent ones out     there.  Let the buyer beware, watch them while they work, make sure     that they don't overtorque the oil drain plug, and keep your hand on     your wallet at all times.  Q:  Are oil additives like Slick-50 or Tufoil any good?  A:  Slick-50 and Tufoil are PTFE-based additives.  Many of these have     come onto the market recently; they are different from the moly-based     additives that have been around since the early 50's.  PTFE is the     chemical name for Teflon(TM), a trademark owned by DuPont. In general,     auto manufacturers do not recommend use of these products.  Most USENET     responses to questions on these additives are favorable (slight     increase of MPG after application, smoother revving) but long term     results (whether PTFE additives are effective after 5K miles) are     debatable.  Some manufacturers (such as Saab) claim that either the     product or the engine flush that preceeds application causes     deterioration of the oil seals and eventual leakage.  Some BMW owners     have reported death of valve seals shortly after the addition of     Slick 50 to their cars.  This writer has been cautioned by a Slick-50     Dealer (!) that Slick-50 should _not_ be used in Japanese motors, as     it may clog the oil return passages in the engine.  Otherwise, there are     no known reports of damage caused by PTFE additives.      On the other hand, there are satified Slick 50 customers in the world.  Q:  Do synthetic oils really work?  A:  Yes, but.  More specifically, most auto manufactuers accept synthetics,     but disagree with the extremely long oil change intervals claimed by     oil manufacturers.  Auto manufacturers recommend that you continue to     change oil at the intervals recommended in the owners manual for your     car.  Even if you decide to try the longer intervals, at least change     the oil filter at regular intervals, as synthetics get just as dirty     as conventional oils.      Synthetic gear lubricants for manual transmissions are another matter     entirely; Amsoil, Redline, and AGIP are very highly regarded and very     effective.  Mobil 1 synthetic gear lube gets mixed reviews, however.  Q:  Manufacturers are specifying longer and longer oil change intervals.     How often should I change my oil?  A:  It depends on how you drive.  If your car always (or nearly always)     gets warmed up, and you don't drive it very hard and keep the revs     down, the manufacturer's recommendation is probably fine.  If, however,     you drive it hard, drive it at high revs, or alternatively, if you     only drive it to and from the supermarket so that it doesn't get up     to temperature, then you may wish to change oil much more often,     perhaps at 3000 mile intervals (given that most manufacturers are     now specifying 7500 mile intervals.)  If you don't drive your car     much at all (say 7500 miles a year), then you probably want to change     oil every six months anyway.  If you are storing a car during the winter,     then change oil before storing it and change oil when you bring it out     of storage.  Misc. Questions:  Q:  My car has a timing belt.  I hear that bad things happen when they     break.  What's the story?  A:  It depends on the internal design of the motor.  Early Ford Escorts,     for example, will suffer severe valve damage if the belt breaks, but     the newer cars will just require a tow and installation of a new belt.     Some Honda motors will not be damaged, but others will be.      If no replacement interval is specified for your car, then change the     belt at least every 60,000 miles; some cars may require more frequent     replacement.  Ask your dealer or independent mechanic.  Also, ask if     there are any related repairs that should take place at the same time     (for example, the same Ford Escorts that suffer valve damage also have     a timing belt driven water pump, which has been known to seize,     destroying the timing belt, and which then causes major valve damage     as a side effect.  Replacing the timing belt while ignoring the water     pump can be a costly mistake.)  Q:  Why would anyone be stupid enough to design a motor so that it     self destructs when the timing belt breaks?  A:  For performance reasons.  Compromising piston design so that the     valves and pistons will not collide requires that the compression     ratio of the engine be reduced significantly; this is why you are     more likely to avoid valve damage in economy cars than in performance     oriented vehicles.  Q:  What causes Unintended Acceleration?  A:  The final report of the National Highway, Transportation, and Safety     Administration concluded that unintended acceleration could not be     caused by any mechanical failure of the vehicle in question and at the     same time be consistent with the physical evidence.  The NHTSA report     goes on to conclude that `pedal misapplication' by the vehicle operator     is probably the cause.