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Hummer Knowledge Base

The Hummer Knowledge Base

Effect of Different Tire Sizes:

The engine computer reads the Transmission Input Speed Sensor (TISS) and Transmission Output Speed Sensor (TOSS) and compares them to make sure the transmission is functioning properly. If the ratio of the reported speeds does not match one of the gear ratios of the tranny, it reports a problem.

Similarly, it compares the TOSS with the Vehicle Speed Sensor (VSS) which is located in the t-case and senses the rear driveshaft speed. (It also uses the high / low range switch input so it knows if you are in low range.) In high range operation, the TOSS should show the same speed as the VSS. With different size tires on one axle, the output speeds are different.

The assumption is that the front output and rear output of the t-case is the same. Whether they are the same or not, they will average the same speed as the TOSS, but the computer only knows about the speed of the rear driveshaft from the VSS.

With the front and rear output shafts turning at different rates, the t-case differential is working overtime. If you drive far or fast in this situation, you can cook the fluid, and possibly overheat the gears.

When driving on a runflat (just about the worst case "different size tire" situation you can imagine) AMG says to restrict your speed to about 20 mph (lower if more than one runflat is in use). This is, among other concerns, to prevent the overuse of the various differentials (front, rear, t-case). Any time you run different-size tires, you should consider this possibility. Small differences in tire size are usually not a problem, but the bigger the difference the bigger the potential problem.

The AMG distance restriction for driving on a runflat (about 20 miles) is based on the runflat rubbing on the inside of the tire and the usual amount of runflat lubricant/fire retardant inside the tire. If you cut the tire off and run the runflat directly on the ground, the distance limit is removed. The speed limit is not, however.

In the situation you were in, one axle with larger tires than the other, I would recommend disconnecting one or the other drive shaft and using HL to drive in 2WD mode. (This was mentioned in a couple of the posts, IIRC.) If you remove the rear driveshaft, you will need to plug the rear of the t-case to prevent fluid loss. If you remove / disconnect the front, you avoid this problem.

Dave B.

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