Two formulas for speedometer correction are presented "High-Performance Jeep Cherokee Builder's Guide." Formula 1: Old Tire Diameter / New Tire Diameter x New Gear Ration / Old Gear Ratio x Old Tooth Count = New Tooth Count Formula 2: (63360 / (MSOD x 3.14159) x Axle Ratio) / 74.5 = # of Teeth in Speedo Gear My algebra days are long behind me, so can I get someone to double-check my numbers here and explain to me how these formulas work? Since I was too lazy to pull my existing gear to start with, I used Formula 2 first after getting actual measurements on my tires. I have GPS verified the accuracy of my speedo with my old tires with a diameter of 28.25". (63360 / (28.25 x 3.14159) x 3.55) / 74.5 = # of Teeth in Speedo Gear 28.25 x 3.14159 = 88.7499175 63360 / 88.7499175 = 713.91615659811740106687986498692 713.91615659811740106687986498692 x 3.55 = 2534.4023559233167737874235207036 2534.4023559233167737874235207036 / 74.5 = 34.018823569440493607884879472531 34 teeth and lots of decimal places! Now, back to Formula 1... I didn't change my gear ratio, meaning old / new would be 1, so I'll leave that part out. 28.25 / 30.625 x 34 = New Tooth Count 28.25 / 30.625 x 34 = 31.363265306122448979591836734694 So I need to go all hillbilly and lose 3 teeth? But seriously, what is represented or determined by the numbers 63360 and 74.5?