1999 GT speedo problem

Hey everyone.
I have a 1999 GT with the 3:73 gears in the back end, and 2003 Cobra R SVT rims on the car (275/40R17)
The combination of these things throws the speedometer off about 10km/h from 60km and then gets worse as the speed increases.
I phoned the local mustang shop and they quoted me $397.00 for a scanner that would fix these problems.
I am a student who doesn't want to drop $400 on a scanner that I'd use once.
Is there a cheaper scanner (or way to fix this) that I could buy online?
I know American Muscle has a chip that you install that plugs into the stock speed sensor, but that wouldn't fix all of my problems, would it?

Any help is appreciated

Thanks
 
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