For years ATSG has produced a wide range of issues related to improper tire sizes on vehicles. Even under-inflated tires have been known to cause issues. Problems such as premature failure with an active 4WD transfer case will occur with incorrect tire sizes.
Additional issues include the following:
- AWD vehicles can be negatively affected as well causing a binding wheel hop type of a feel while in turns.
- Incorrect tire sizes affect 2WD cars that have curve recognition programs.
- Curve recognition programs monitor the lateral movement of the vehicle to prevent shift busy-ness while in turns.
- Incorrect tire sizes provide a false signal that the vehicle is in a turn when it is not. This will impede shift scheduling.
- Under-inflated tires have been known to cause quirky coast downshift bumps and clunks.
As a result of some of these issues, we often mention the use of a stagger gauge as a quick and easy method in checking the tires’ circumference (see Figure 1, above).
Aside from a stagger gauge being inexpensive to purchase, and how easy it is to use, what makes it such an effective tool is that it checks these tires with the full weight of the vehicle on them. This allows for a very accurate measurement. The idea is that all four tires should be very close in rolling radius. Generally, they should be within 1/8- to 1/4-inch of each other in circumference, not diameter. However, there are some vehicles that require the front and rear tires to be different in circumference.
As a sneak preview of ATSG’s 2024 Technical Training Seminars, one of the pieces of tech that will be presented is an incident that occurred with John and Dominic from Affordable Transmission Specialist. They had a local dealer send to them a 2010 Mercedes E350 W212 chassis equipped with a 722.9 transmission and 4MATIC transfer case with a complaint of a grinding noise. A road test confirmed the severity of the noise which was coming from the bearings in the transfer case unit. After the transfer case was rebuilt with new bearings and a viscous clutch, during a road test, the vehicle would exhibit a binding sensation on turns, just as if there were the wrong size tires on the vehicle.
When John and Dominic looked, they saw that all four tires were brand new and they were all the same size. They decided to look at the door jam sticker to see if the correct size tires were placed onto this vehicle. It was then discovered that the front and rear tires are supposed to be different in size. Figure 2 shows that the front tires are supposed to be 245/40 R18 while the rear tires are to be 265/35 R18. Instead, there were 245/45 R17’s all around (Figure 3).
These are completely different in both aspect ratio and circumference, causing the binding sensation on turns, damaging components in the transfer case. Once the correct size tires were installed, the 4MATIC function of the transfer case was restored which will also prevent repeat failure of the unit. So, with not all vehicles being created with equal tires, it is best to always compare the door tag information to the tires on the vehicle. A great lesson learned.
