Ok, so I have this 92 AWD with welded center differential. I bought my car with tires of different brands and no welded center diff. They are all the same size (205/55/16) but the actual measured diameters are different. I installed the welded center diff and drove the car. After about 20 highway miles crusing, I smelled gear oil and saw smoke in the rearview. I stopped and noticed the rearend was smoking hot, literally. I parked it and made it front wheel drive the next day and took it home.

Then I rebuilt the rearend. I installed it and drove the car a short distance, it was still getting hot. Finally I broke down and measured the tires' circumference with a cloth measuring tape. The front two were the same at 47", but the rear two were at about 46' and 45.5"

I swapped one rear one for one front one. The car drove better, I mean that it would coast properly. Before it acted like one rear brake was sticking slightly and wouldn't roll for shit. The rearend also whined a bit before and the whining is gone.

Anyway I wanted to share the lesson I learned so that everyone else knows too. I was surprised that 1" diameter difference between front and rear axles would cause such a problem, but after I worked out the math it made sense. This is especially important for those with welded center diffs./spools, but everyone should know this, I think.

For those of you who wish to offer advice about now having left/right tire diameter differences causing front and rear carriers to be always working, save your breath, I know that is the case. I am not going to keep the mismatched tires on the car, it is only a temporary fix until I buy a "set" of 4 tires. The point here is to make others aware so they don't get confused if anything like this ever comes across their path.

The bottom line is: If you don't have 4 matching tires with equal tread wear, you could be wearing out other drivetrain parts more quickly than normal (for a DSM).

Enjoy,
-Jesse

hmmm

I am going to measure the tires on my project GVR4.

It has many of the problems you are describing, overheating rear end, whine, etc.

I will check first thing tomorrow.

Thanks!

What did you find out? Were your tires off?? How far??

Just a sec. You had 47" front tires? thats almost four feet tall.... tell me that is centimeters!

Laser4G63 said:

I think he meant the circumference.

Click to expand...

We have a winner. 4' tires would be bigger than almost anything made for the consumer vehicle market.

If you had two 47" tires in the front and one 46" and one 45.5" in the rear, then you had a .5" difference in the rear and a 1.25" difference front-to-rear. Maybe the .5" difference in the rear was a main source of the problem in the rear end, but you also have to consider what was going on with the front-to-rear difference coupled with a welded diff. Assuming the front tires maintained grip (due to the extra weight), the rears must have been slipping 3% (or maybe I should say "anti-slipping" since the ground was moving faster than the tires' treads). This rear slip could have been spread across the two rears or it might have been concentrated in the smaller of the two. This would cause the rear diff to be operating at a bit more that 5% (as well as dragging the smaller rear at this rate). And it would not surprise me if the rear's VC couldn't handle that.

Not that I suggest the experiment, but I'd be curious to know what would happen if the you put both 47" wheels in the rear. The front diff is open (if stock), so this might cause less grief.

In any event, it's an interesting post.

- Jtoby

I am pretty confidant that it was the rear-to-front difference is what was causing my problem. I swapped the 45.5(rear) for a 47(front), and everything is happy enough to drive on occasion. It's really happy enough to drive anywhere. I don't drive it often though.

Anyway, to everyone else, there is a long post involving tires, gear ratios, spools, and VC's that everyone may also find interesting. It has a lot of math, and I learned something very cool and new. Link to thread Read it for yourself and be highly informed. Jtmcinder and I already know of the thread because we spent time learning and communicating in it.

P.S. I changed the original post to say "circumference" instead of "diameter." Sorry about that.

-Jesse

When I bought my car is had 215-40-r17's all around, 2 were worn on the inner edges so i replaced them with the only 2 same size tires I could find in my area. Now my front 2 tires look to be a little taller than the rears is this a problem? 1990 awd with front and rear lsd stock all 4 tires are 215-40-r17 2 new tires up front the rears are getting o where they need replaced. I've been driving like this for about 1500 miles.