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i have a 1995 Jeep Grand Cherokee Laredo 4x4 that used to be my fathers. For years he took his car to discount tire co to replace his tires. Now that I have it and i know a little bit more about cars than he does, I've found a slight problem.

I was on the highway and according to my speedometer i was going 75mph. But my GPS said that i was only going 70mph. And to be honest i thought it was strange that everyone seemed to speed all the time even around town...I thought that was very odd, so i did a little research and looked at a few things. The spare tire that I know for a fact is the same one that came with the car, is a 215/75R15. But the tires on the car are 225/70R15s. The tire plackard on the door says 215/75R15. So i looked for my old receipts for my tires from discount tire. As far back as 2006 they were putting on the wrong tires. Now i don't care what their computers say, but the 215s are cheaper, add almost .5 inch to the diameter of the wheel, and the car is tuned to run on 215s. So i was pretty pissed.

I've done a lot of looking online, and i've even contacted discount tire and I'm going over there to see if i can make them put the right tires on my car. So I want an unbiased, professional opinion from someone who either works with or knows about this...i know i'm new to the forum but i'm just wondering if i'm raising hell for no reason or if i have a legitimate complaint here. thanks!  

Dude, you need to chill!

First the difference in diameters between the 2 sizes in question is about 1/4" - about 1%.  So your speedometer error is NOT due to the difference in tire size.

You also need to be aware that there can be considerable difference in diameter between tires - meaning make and model.  There aren't any regulations that govern the diameter of a tires versus its size.  That means the particular tires you have may be purposely built small - which is a reflection of the manufacturer and not the "size".

Besides, these 2 sizes are so similar that it is a common exchange - and one i would hardily endorse!  

The 8MM or so in height differential is less than the difference between a new tire and one that is worn out so I agree with the other post(r), look elsewhere to solve your problem.  Ever stop to consider the P225/70R15 is what dad wanted on his jeep once he was presented with all the options?  

deff need to relax,the size is a minimal difference,its prolly what they had in stock or had a deal on at the time,they had to have asked if you wanted to change sizes.you are raising hell for no reason people put huge tires on jeeps