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Tire Size - What's The Diff?

- provided by "Transmission Digest"

No one will question the fact that transmission repair is on the cutting edge of rapidly changing technology.  Automatic and standard transmissions have evolved into very sophisticated units using advanced friction materials and electronic controls.

There are now a great many vehicles on the road with four-wheel-drive, all-wheel-drive and viscous-coupling differentials.  Transfer cases are equally complex, using viscous couplings, differentials and electronic controls.  We must understand complex theories of operation and be able to troubleshoot involved electrical systems.  With so much to know and understand, it becomes very easy to overlook simple, obvious problems that cause unnecessary removal and teardown of transmissions and transfer cases.

We handle a large volume of tech calls that involve transfer cases, all-wheel-drive units and front-wheel-drive transaxles with various shift problems and internal failures that have a root cause outside the unit.  Let's look at some common problems.

  1. A late-model Jeep is brought into the shop with an NP 242 transfer case that will not shift out of 4WD.  The shop spends a lot of time on diagnosis, inspects the case internally only to find nothing wrong.  After a frustrating waste of time, it is found that one tire is 5 lbs. low on air pressure.  When the tire pressures are corrected, the unit performs properly.

  2. A Hyundai with a KM 175 transaxle is towed into the shop.  The fluid is badly burned and oxidized, the clutches are worn and the differential carrier is wasted.  The shop rebuilds the transmission and gives it back to the customer in good working order.  Two months later, the unit is back in the shop with the differential wasted again and the valve body is full of differential dust.  The unit is repaired again and returned to the customer at no charge under warranty.  Three months later the unit is back again with differential failure.  While it is being repaired for the third time, one of the installers notices that the passenger-side front tire is an oddball and a full size smaller than the one on the driver's side.  The oddball tire is replaced with the spare, and the unit stays out on the road.

  3. An all-wheel-drive Ford Aerostar with a Dana 28 transfer case is brought into the shop with a complaint of bind-up and trailer hitching on turns and in straight-line driving.  The shop diagnoses the computer and electronic system and finds no trouble codes.  Speed sensors are replaced just to be sure, with no fix for the problem.  The unit is removed and inspected internally.  No damage is found, but the shop then finds that the rear tires are bald and the front ones are new.  They are the same size and brand, but the tire stagger (difference in diameter at the center of the tread) is at least one inch.  The rear tires are replaced, and the complaint is gone.

  4. An Eagle Talon with an all-wheel-drive 5-speed Mitsubishi transaxle comes into the shop with a failed center differential and viscous coupling.  The driver-side front tire is brand new.  Upon being questioned by the shop service writer, the customer explains that while he was returning from vacation the tire went flat on the freeway and was replaced with the "space-saver" spare.  The car was then driven at 70mph for 750 miles to the owner's home and used for normal commuting for three days, until the owner bought a new tire on his lunch hour.  Although the space-saver spare had a warning that it should not be used at more than 50mph for 50 miles, the owner through his ignorance paid for an expensive transmission repair.

As you can see from these sad tales, any transaxle, transfer case, all-wheel-drive unit that is capable of differentiating torque can suffer from mismatched, underinflated, or overinflated tires.  The difference in the rolling radius of the tire puts great stress on differentials and viscous couplings.  The greater the difference, the greater the potential for damage.  The length of time the vehicle is operated under these conditions increases the chance of severe damage.