Speedometer off, how to fix?

[pokertramp]

OK so I knew I was off so I tested my car last night with my friends GPS which also tells how fast you are going. My speedometer is 10 MPH off. It it says I am going 35-MPH I am really doing 45-MPH. If I get up to 65 or 70-MPH then it goes up even higher I am really doing 13-MPH faster.

Car has a 350, TH-350 transmission, stock rear and 15 inch rims with almost the exact same height as the stock rims and tires.

What would I need to do to correct this? Would I put a higher or lower gear in the transmission for the speedometer?

 

[pontiacgp]

the almost exact height of your tires is probably causing the speedometer to be off.

try this and see if this helps

http://www.sccoa.com/faq/speedgr.html