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4x4 Spare different size

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Old 02-19-2013, 05:33 PM
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Default 4x4 Spare different size

Hey Y'all,
I am putting P225/75R15 on my blazer, my previous tire size was a 235/75R15.

My Question is, will a 235/75R15 work as a spare for the P225/75R15s or do I need to get a spare that's the same size as the tires I'm running?

I have a 1996 Chevy S-10 4-door 4wd Blazer LS.

Thanks for any help you can give me,
Ken
 
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Old 02-19-2013, 05:51 PM
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i would go ahead and get a new spare. your new tires are going to be shorter than your spare. i just dont know by how much. but on 4wd you dont want to mis-match tire sizes. its hard on the differentials because then they are constantly rotating at different speeds. another thing is when was the last time that you looked at the spare? it might be all weather checked and need to be replaced anyway. but you can talk to your tire guy and see how much difference it will be, or you could get a nice used tire to save a few bucks.
 
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Old 02-19-2013, 05:56 PM
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Temporary only. The 235/75R15 would work as a short time spare to get you home or to the garage to get the other tire repaired. The G80 limited slip probably wouldn't engage with the difference in tire size either.
 
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Old 02-19-2013, 05:59 PM
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actually I check on the spare quite often as the original fell off and I keep a spare in the back of the truck now.
I needed new tires and got a really good deal on four used tires already mounted on rims from a guy I know.
I will start hunting for a used spare now thanks for the help!
 
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Old 02-19-2013, 06:42 PM
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Originally Posted by Diaita View Post
Temporary only. The 235/75R15 would work as a short time spare to get you home or to the garage to get the other tire repaired. The G80 limited slip probably wouldn't engage with the difference in tire size either.
Correct me if I am wrong but I believe the g80 will unlock if if go fast enough. So I don't think it would be a problem.
 
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Old 02-19-2013, 07:01 PM
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Not only will it affect the differentials and transfer case if you use the odd size, but another thing to keep in mind when changing tire size is speedometer, odometer, ABS and transmission shift points will all be adversely affected unless you have the PCM flashed. Add that to the price of the tires and see if it's still a good deal.
 
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Old 02-19-2013, 07:53 PM
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Originally Posted by Captain Hook View Post
Not only will it affect the differentials and transfer case if you use the odd size, but another thing to keep in mind when changing tire size is speedometer, odometer, ABS and transmission shift points will all be adversely affected unless you have the PCM flashed. Add that to the price of the tires and see if it's still a good deal.
OK sorry if this is a NewBe question but what is a "PCM Flash", who does it, how much does it usually cost, and how necessary is it?

From what I've been reading there is not a whole lot of difference between a P225/75R15 and a 235/75R15 I believe its only the width of the tire that's affected not the height, am I wrong hear?
 

Last edited by Ken87; 02-19-2013 at 08:01 PM.
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Old 02-19-2013, 08:06 PM
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Originally Posted by Ken87 View Post
OK sorry if this is a NewBe question but what is a "PCM Flash", who does it, how much does it usually cost, and how necessary is it?

From what I've been reading there is not a whole lot of difference between a P225/75R15 and a 235/75R15 I believe its only the width of the tire that's affected not the height, am I wrong hear?
It's about a 2% difference in tire diameter, which will mean that your speedometer will read around 2% faster than you're actually travelling. At 60mph actual speed your speedometer will read approximately 61-62mph. All this depends on how accurate your speedometer was with factory size tires, tire wear, and other things.
 
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Old 02-19-2013, 08:15 PM
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Originally Posted by El_Beautor View Post
It's about a 2% difference in tire diameter, which will mean that your speedometer will read around 2% faster than you're actually travelling. At 60mph actual speed your speedometer will read approximately 61-62mph. All this depends on how accurate your speedometer was with factory size tires, tire wear, and other things.
OK, I have a GPS that tells me the speed that I am going so is there any real need to do a PCM plash, are there potential problems that may arise if I do not have a PCM flash done?
 

Last edited by Ken87; 02-19-2013 at 08:17 PM.
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Old 02-19-2013, 08:23 PM
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People run different tire sizes all the time without correcting their PCM. Ideally you should have your PCM flashed so that your truck's computer has the correct information to calculate shift points, how the ABS should act, and stuff like that. That being said, a 2% difference is quite small and I really doubt that it would have any major effect on your truck's performance. You will likely see a small reduction in your highway fuel economy.
 

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