Hi There, I just found out that when I had the original tires replaced on my GMC Envoy XLT that the place that put on two replacement tires put on P235/65R17's instead of P245/65R17's. So I was driving around with two different types of tires on the front and back. I didn't know the difference. I eventually (10 months later) replaced the other two tires and took my vehicle back to the same tire place. They replaced the two original older tires and put on a pair of P235/65R17's to match the two that were already on there. So now I have all 4 tires that are P235/65R17's. 10 months after that I had a problem with a valve stem and took my vehicle into the tire shop and had them replace it, which is when they told me that I had the wrong tires for my vehicle on my car! Once I found this out, I asked the owner to replace the P235's with P245's which was what was supposed to be on there. They acknowledged that they made the mistake in the first place and said they would give me a credit on the tires that were on my car (pro-rate is the term I believe that they used). I took my vehicle in and they replaced all 4 tires with P245's and then proceed to charge me full retail price offering only a $50.00 credit. The bill came to $468.40 after the $50.00 credit. I couldn't believe that this was all that they were willing to do considering that they were the ones who sold me something other than what I thought I purchased which were replacement tires for my vehicle. Now that I have explained all that...could this situation have damaged my vehicle in any way? The front end doesn't seem right and I am going to take it in to see if it needs an alignment but am now wondering if having the tires replaced like this has anything to do with it and if so, what would you do if you were me? Thanks for any replies, Pammy  

I've seen a lot of this same issue with the Fords that I worked on! I don't know what year your Envoy is but it should have set off some thing on the dash. GMC also has tire size compensation programs built in to the PCMs and that needs to be corrected when buying bigger. It gets reset with their scanner to the new tire size so that your speedo does not get affected. If it is a 4x4, then your transfer case internals could have suffered some damage du to the different  tire sizes! To what extent is unclear til you actually use it. If not a 4x4 then there is no reason for worry. It is some thing that happens all the time when people go to get tires, unfortunately. The other problem that i've corrected in the past is the wrong gross wieght or load range letter and the speed rating... they always get downgraded and it is reflected in the price of the tire. If it is a low price tire than what you know it may be a lower performance tire than what is required by the manufacturer. The other problem is tread pattern... If you buy one tire make sure that it matches the tread pattern that is on your exsisting tires as well as the other important performance info. Some places are good and some are not. It eems that they need to have some standard to make sure that they know what goes on your car or truck but they don't. The only weopon you have against this not happening to you in the future is to document what you have on your vehicle and keep it on hand when you purchase new units. PSI is another issue that I have with these places... they never set them to the cars spec. they always seem to set them to the tires max inlation spec. that's a totally different spec for every day use. A little self education on your cars replacement parts goes a long way. I talk from experience!  

Blackmonkey, I can assure you that no General Motors vehicle  computer can tell you that the tire on the car is the wrong size. Feel free to call them and ask.

They only tire monitoring systems are for air pressure. I know because I have a new GM vehicle in my driveway every 3 months.
-------------------------------------------

Beyond that, black is right that you should pay closer attention to that tires, and pressure they set it at. If it is 2wd, you have nothing to worry about. If it is 4x4, and you used 4x4, there could be some damage. The rolling diameter is supposed to be within a certain percentage on AWD/4x4 vehicles. The last time I took the 50th AE in, they screwed up and it was at 48psi when I pulled out of the bay. Sorry your experience was so poor. Who did you take it to?  

How often have you used a scanner to change the wheel "parameters" on your GM or on a Caddy suv, BOOsted? When I worked at my friends Caddy dealership, to cover one of his injured employees, I used the GM scanner to change those parameters due to Town Fair tires, and the such, installing bigger than average tires or sport wheels. That changed wheel info processing for the PCM and you had to change it through the "parameters" function. Ford calls it that, I forget what GM calls it! Ford and Lincoln also have this "parameters" function where you can change tire and wheel configurations. Did enough of them to know. It actually messes with shifting ranges and speedo function. On Ford it will come up as a instrument cluster module problem. If you're not in the know, you wouldn't think of that. I don't think that GM has the same error codes or scan method to hint at that issue but I know that those "parameters" are there to change if needed to correct issues with shifting and speedo readings due to wrong tire sizes!  

As far as the two different tire sizes on the same vehicle, there would have to be some kind of light that comes on. The PCM funtion, or module, that monitors the psi would see the bigger tire as a drag and assume it was low. A bigger tire spins at lower rpm range than a smaller one and, thus, would be seen as an issue at that tire or be confused and do nothing but give false readings or codes. I'm just assuming this for GM 'cause it's been a while since I diaged one. Things have more than likely advanced since then and diag procedures could be different, but the concept is still the same. It's not too difficult to diag those systems, said I while trying not to sound too techy.  

Having one in your drive way every now and then is not the same as working on them to know these things.