my spedometer is incorrect. depending on the speed it can overstate from 5-15mph. it is a problem because it is putting on more miles than I actually am. what is the problem? i also think the tires in the back are slightly bigger

 

you need to change your speedo gear in the tranny. It's a $12 part from GM.
Find out your rear gears, your tire size, and find out what the drive gear is in the tranny, then you can find out what driven gear you need.

Or post that info on here and we can tell you.

 

thanks a lot! very helpful. I will talk to my mechanic tomorrow and see what he can do

 

One more question....If I took my car to my mechanic and told him to do this. Will it be hard job that is time consuming and expensive. Or is it like changing a fuse.

 

If he's a competent mechanic sure. I have yet to find one.... I've been to many shops lately and have seen a string of incompetence.
Phone first and ask if they can replace a speedometer gear for you. Go get the part from GM first, unless you trust your mechanic to figure that out for you too...