I'm hoping to get some help from you guys, even though my question is about a GTO. My car must have had the rear gears changed from the original factory one. I'm also running larger tires than factory correct. Here's my problem... The speedometer reads 14% higher than the actual speed. At 35 mph in town the speedometer reads 40. At 65 on the highway it reads 74 mph. It's 14% higher in both cases. I'm thinking that I need to add more teeth to the gear that's driven by the transmission. For example, if the driven gear has 34 teeth, I would need one with 14% more teeth or 39 teeth. Is my thinking correct?