The tires on a tandem axle truck are an integral part of the powertrain system. Maintaining equal tire sizes on all four drive axles is extremely important. Tire size mismatch is one of the most common reasons for premature differential failures.
Tire size mismatch is the difference in the number of revolutions the drive tires make per mile. Coastline Transmission recommends maintaining a tire mismatch no greater than one percent. The easiest method to calculate tire mismatch is to measure the circumference of the tires with a tape measure running around the outside of the tire. The average 11×24.5 truck tire is about 136 inches in circumference; one percent of 136 is 1.36 inches. So, if 136 inches is the measurement of the largest tire, all 4 drive tire measurements would need to be between 136 inches and 134.6 inches.
The effects of tire mismatch on a differential depend on which axle the mismatch occurs. Side to side mismatch on the same axle will increase stress and wear on the differential side gears and pinions. While front to back axle mismatch will decrease the life of the inter-axle differential commonly known as the power divider. If tire mismatch is great enough it can cause the same spinout failure as running different ratio differentials. Typical failures are excessive wear to the gear set, pinions welded to the spider arms, cracked or broken thrust washers, over heated lubricant and metal debris on the magnetic drain plugs all causing the differential to wear and fail prematurely.
We have seen highway trucks in our shop with mismatched tires in excess of 4 inches. A tire with the rolling circumference of 136 inches revolves 466 times per mile while a 132 inch tire revolves 480 times per mile, this equates to a mismatch of 14 revolutions per mile. If a truck with a 4 inch mismatch operates on dry pavement with the power divider and the axles locked the tires will actually have to skid to compensate for 14 revolutions per mile. Needless to say, this practice will drastically shorten the life expectancy of both the tires and the differentials.
Therefore; by following the one percent tire mismatch rule you could greatly increase the life of your truck’s differentials.
If you have any truck Powertrain related questions or problems, call Coastline Transmission at 604-533-4651 or toll free at 1-888-686-4327