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Old 10-31-2012, 07:52 AM
GMR GMR is offline
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positraction and tire sizes

I have P215/70R15 tires all around on a '64 Chev and positraction. Can I use a P175/70R14 as a spare?

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Old 10-31-2012, 11:33 AM
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You must use tires of the same diameter on posi units width doesn't matter! or you will cause excessive wear to clutches or cones! If its a locker you even create more problems!

Jester
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Old 10-31-2012, 01:09 PM
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Originally Posted by painted jester View Post
You must use tires of the same diameter on posi units width doesn't matter! or you will cause excessive wear to clutches or cones! If its a locker you even create more problems!

Jester
If that were true, 1) you wouldn't be able to use a donut spare, and 2) you wouldn't be able to go around corners. I certainly wouldn't run different diameters for any length of time, but as a spare to get you a few miles down the road, it will work. Yes, it will cause the clutches to wear, but so does going around corners with the same size tires. Keep it under 50 miles (preferably under 10).
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Old 10-31-2012, 01:36 PM
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Quote:
Originally Posted by joe_padavano View Post
If that were true, 1) you wouldn't be able to use a donut spare, and 2) you wouldn't be able to go around corners. I certainly wouldn't run different diameters for any length of time, but as a spare to get you a few miles down the road, it will work. Yes, it will cause the clutches to wear, but so does going around corners with the same size tires. Keep it under 50 miles (preferably under 10).
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?
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..
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Old 10-31-2012, 02:45 PM
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Originally Posted by E.Furgal View Post
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.

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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.
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Old 10-31-2012, 03:22 PM
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I got the 30 miles straight from my 85 t/a owners manual..
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Old 10-31-2012, 04:30 PM
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Look why take a chance! Get a spare the size of the rears, you can run a bigger spare on the front with no problems! To each his own any one with any sense with a performance car wouldn't run a smaller tire on a posi!

Its easier to change a tire then clutches or cones!


Jester
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Old 10-31-2012, 05:01 PM
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215/70/15 is 26.9"
175/70/14 is 23.7"

if you want to keep the 14" spare maybe find a 215/75/14 to mount on it..
26.7" tall
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Old 10-31-2012, 05:34 PM
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put the spare on the front,,,,,,,,,
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Old 10-31-2012, 06:52 PM
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I've been handing out candy LOL Happy Halloween!

Any way something that wasn't mentioned I couldn't respond to before: When turning right or left with the same diameter tires every once and a while and then going straight you dissipate heat and friction from side to side, it isn't the same as running a smaller tire!! When running a smaller tire for 30 or 50 miles its like turning a circle continually for 30 or 50 miles in one direction building up heat and with heat your increasing friction and wear all on one axle and one side of the posi! I don't care what kind of math you use its bad!


Jester
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Old 10-31-2012, 07:00 PM
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Good stuff fellas, learned something, thanks to all.

Happy Halloween to you too jester and everyone.
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Old 10-31-2012, 07:42 PM
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Quote:
Originally Posted by joe_padavano View Post
........... How about a little math? .....
Quote:
Originally Posted by vinniekq2 View Post
put the spare on the front,,,,,,,,,
Great thread... why do math when you have vinniekq2 and his Keep It Simple approach!

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Old 10-31-2012, 10:44 PM
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Why not just call AAA?? Towed my dually 95 miles no expense to me and I didn't even get dirty .....untill I started working on it.
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Old 11-01-2012, 07:34 AM
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You're right about vinniekq2. I admit, I didn't think of that. I did consider the towing though since I too have 100 miles free from AAA. Thanks again all.
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