ok I have an 87 mustang with the 89 140mph cluster in it .. i just put an 83 T-5 transmission but i still used the 87 speedo gear and i know my speedometer has to be at least 10 mph slower than what i'm actually going. Does anybody know what speedometer gear I might need and also down the road i'm putting 3.73 gears what geat would i need when i do that as well? thanks guys