I've got the spare tire on the Firebird right now and it is maybe an 1/8 of an inch or less shorter than the tire on the other side. I can't remember my high school geometry right off hand but I'm sure that translates into at least a couple inches in circumference. What is considered 'within spec' with a posi rear?

Mark

The tires need to be the same size and have the same air pressure. There is no acceptable variation. Any difference in circumfrence will cause excessive clutch wear and heat build up in the oil. If you are running the spare then you need to get the other tire fixed as soon as possible and get it back on the car. Rear tires should be replaced in pairs. You should not install a new tire on one side and leave a used tire on the other side. If you are replacing a standard differential with a posi and you have been doing one wheel burnouts then you need to replace both rear tires when the differential is replaced. You need to check the air pressure often to be sure that it is kept equal.

Circumference = wheel diameter x 3.14 (Pi)

alan

Circumference = wheel diameter x 3.14 (Pi)

alan= 0.37 inches, 3/8 inch.

Thanks for the response Freddie, and thanks to Alan and Everett for the geometry refresher. :D

The tires are about 28 inches tall so it comes out to a hair over 3/8 of an inch per tire revolution. That doesn't seem like much, and is a lot less than I was thinking.

Freddie, I'm with you on the equal tire size and pressure and will get matching tires back on it. You're right exact height between the two tires is optimal, BUT in a pinch how much wiggle room do you have to play with. I just wondered if there was something that GM considered 'within spec'.

Mark

I don't know of any tolerances in the tire size. I know that many people have tires that are off a little. I also know that every Ford 9 inch that I have disassembled out of a Ford truck with the Traction Lock had the clutches completely worn out. I figure that this is caused by unequal air pressure due to lack of maintanance from the owners. I've also seen a lot of blue clutches in the Eaton Posi and I know of a lot of people who have popping issues due to hot clutches caused by unequal tire size. Ive fixed a few rear ends just by putting air in the tires.

Ive fixed a few rear ends just by putting air in the tires.

I like that. :D

Thanks for the replies. :thumbsup:

Mark

The tires are about 28 inches tall so it comes out to a hair over 3/8 of an inch per tire revolution. That doesn't seem like much, and is a lot less than I was thinking.
Mark

When you consider how much slippage there is going around a corner, the 3/8" difference in circumference is inconsequential.

One way to look at it is 88" circumference/.375" = 231 revolutions of one tire while the other tire has 232 revs.

Get the correct tire on there when you can, but I wouldn't be concerned with it causing accelerated wear.

When you consider how much slippage there is going around a corner, the 3/8" difference in circumference is inconsequential.

One way to look at it is 88" circumference/.375" = 231 revolutions of one tire while the other tire has 232 revs.

Get the correct tire on there when you can, but I wouldn't be concerned with it causing accelerated wear.

Not sure what your math is representing but it comes out to about 23-24 feet difference per mile of straight line driving.

I agree, if you're doing a lot of in town driving around corners it's probably just as much wear and tear.

Mark