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

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.

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.

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!

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.

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?

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?

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.

OK thank you, that helps tremendously!

When the vehicle is in motion, the vehicle speed sensors, (usually 3 of them on the vehicle) generate pulses that are sent to the PCM, (Powertrain Control Module). The PCM calculates speed by how many pulses it receives in a given amount of time. Each different tire size generates a different amount of PPM, (Pulses Per Mile). The PCM uses this data for calculating speed, distance, shift points, ABS, (brakes) and other engine calculations. The TCCM, (Transfer Case Control Module) both 2 speed selectable and Auto trac, also uses the data. P225/75R15 tires have a smaller circumference than the 235/75R15 tires so they generate more PPM. If/when tire size is changed, the PCM needs to be flashed, (reprogrammed) according to the PPM of the tires being used. It requires a bi-directional scan tool capable of flashing. GM dealers have them for sure, (Tech2 scan tool). Other than the dealer, you'd have to check around with different shops and see who has one, and knows how to do the flash. Depending on labor rates, the charge will vary. Typical labor charge is around 1 hour. And yes, if you want your vehicle to perform correctly, it is necessary.

i dont see it effecting the ABS because the ABS simply notices differences in wheel speed regardless of how fast you are traveling. with the smaller tires the PCM will think that he is going faster than he actually is BUT, the ABS will not know the difference. once the ABS sees the difference in wheel speed, thats when it kicks in. i would say that the ABS would be the system that is the LEAST effected by a change in tire size.

i would however, have the PCM reflashed because of the difference in speed and odometer readings. you dont want to put extra miles on your car because if you end up in an accident then the insurance company is going to make out better than you are because now you may have a "high mileage" vehicle.

just my .02