i just bought a 99 35th ann mustang GT, the previous owner had a full cold air intake in it along with new headers and exsaught, and i beleave some type of chip and my friend thinks diffrent gears, and i followed a friend at 40 mph and my gauge was reading 10 mph more then what i was going, any suggestions on how to fix or what the problem is, please help

 

Its your gears. The guy probubly didnt put a speedo gear in it. Ive never done it but alot of guys in here have they can help ya out on that one.

 

Sounds like the previous owner changed rear end gear ratios (as you expected.) Luckily, there's an easy fox for this. If you can get your hands on a tuner (or know someone that has one) you can reprogram the car with the new gear ratio.

You should be able to find the new ratio using one of the speed calculators (inputting your tranny, tire size, and RPM.) Chances are, you either have 3.73 or 4.10 as they seem to be the most common.

 

Yeah, it can be a pain in the ***. With older Mustangs (95 and older) you had to physically put a new speedo gear in. But the newer ones (96 and up) can simply be reprogram med. Lucky bastards

 

You'll have to buy your own tuner. Hand held tuners will become married to the car once used and can't be divorced unless the car is returned to the stock settings. You can also get Speed Cal if you don't need the other functions of the hand held tuner. It'll be about half the price of a tuner.