Find the diameter of your tires: (tire size X aspect ratio / 25.4 = sidewall height in inches) For a 225/60R16 225 X .60 = 135/25.4 = 5.31 (Diameter = 2 X sidewall + rim) 5.31 X 2 = 10.62 + 16 = 26.62 inches diameter (Circumference = Diameter X Pi) 26.62 X 3.14147 = 83.62 inches 5280 feet X 12 inches = 63360 inches in a mile To compute the number of tire revolutions per mile, divide 63360 by the Circumference. 63360 ---------------------------- = Tire Revs/Mile 83.62 63360 / 83.62 = 757.8 Speed: - dividing the original tire revolutions by the new tire revolutions and multiplying by the indicated speed on the speedometer, we arrive at the new actual speed. Orig. Tire Revs/Mile) ---------------------- X Indicated Speed = Actual Speed (New Tire Revs/Mile) Using the above tire sizes: (775) ----- X 60 mph = 62 mph (747) Rear Axle Ratio: - Changing tire diameters will also change the effective rear axle gear ratio. Divide the new tire revolutions by the original tire revolutions and multiply by the original axle ratio. For example, a car that had 215/65s and a rear axle ratio of 3.42 (what else?) and moved to a 255/60 will net an effective ratio of 3.30. (New Tire Revs/Mile) --------------------- X Orig. Axle Ratio = Effect. Axle Ratio (Orig.Tire Revs/Mile) (747) ----- X 3.42 ratio = 3.30 ratio (775) As you can see, this will theoretically raise the top speed of the vehicle but hurt its acceleration. To regain the original effective ratio with the new tires, a 3.55 ratio is required. (Orig.Tire Revs/Mile) --------------------- X Orig. Axle Ratio = Equiv. Axle Ratio (New Tire Revs/Mile) (775) ----- X 3.42 ratio = 3.55 ratio (747) Maximum Speed To calculate the maximum speed of a vehicle, take: Overall Tire Diameter X Achieved Engine RPM (redline) ------------------------------------------------------ = Max Speed Differential Ratio X Top Gear Ratio X 336 26" X 5250 ----------------- = 177 mph 3.42 X 0.67 X 336 This differs from actual speed due to the cars aerodynamics. Lateral Acceleration To measure lateral acceleration (cornering ability), use: 1.227 X R g = ----------- (T X T) R is the radius of the turning circle and T is the time (in seconds) required for one lap.