Originally Posted by E.Furgal
isn't that a weak arguement as seen as the donut is to get you to a service station.. not to drive on..for days like people do?
My 84 Custom Cruiser came with 28" tall tires from the factory and a much smaller donut spare. It also has an 8.5 axle with posi. GM says I can travel up to 50 miles that way without "burning up" the posi or the tire. Would I go that far? Only if forced to because there was no alternative.
to the o/p.. if you use your spare to get off the road and to a repair shop to fix or replace the reg tire, the smaller tire will be fine, but because most times thats not a real world option, I'd stick with the same height tire..
FYI 30 miles with different sized(height) tires is all it takes to render a limited slip diff, junk..
30 miles will render the limited slip junk? How about a little math? The OP's two tire sizes are 26.85 and 24.65 inches in diameter, respectively. The rolling radius of each is probably even closer, but let's use these numbers. Over 30 miles, that results in a little over 2000 revs more of one axle than the other. I'm assuming that wear on the posi clutches is caused by this differential rotation.
Now, how many miles of normal driving does it take to see the same number of differential revs from one axle to the other? We have to make some assumptions about turn radius and frequency, but 15 ft is the minimum inside turn radius of a passenger car used by many state DOTs for road design. Let's double that to 30 ft since not all turns are that sharp. That's the inside radius, and assuming a 5.5 ft track width, the outside radius is 35.5 ft. The difference in arc length for a 90 deg turn is a little over 8.5 ft, which is about 1.3 revs difference for the turn. Now how many turns to you make when driving? Pure guess here but let's say one turn per 10 miles, or 0.13 revs difference per mile. Do the math and you get about 16,400 miles to equal the differential axle revs from driving 30 miles with the different sized tires. Of course, that assumes you're only driving straight ahead with the different sized tires, so increase it to 20,000 miles to cover turns.
Anyone here burn up their posi after 20,000 miles of street driving?
Of COURSE it's not a great idea to run these two different sizes (about 2" diameter difference) for any length of time, but even driving 30 miles on the spare in an emergency isn't going to kill the posi. Frankly, I'd be more worried about the lower load capacity of the 175 tire than the life of the posi.