There has been some discussion about back axle ratios and top speed. If the final differential ratio is too high the car will have good acceleration but will have to pull ridiculous revs to give a high top speed. To calculate what the top speed will be for a car you need to find how many MPH per 1000 engine revs the car will do in top gear then multiply by the max revs the engine will pull in top. To follow the formula you will need to determine: Tgr : Top gear ratio for the gearbox to be used. Look in a manual. It is often 1 or 1.1 for Ford gear boxes. Rr : Rolling radius of tyres and wheel. Measure from center of wheel to the ground in mm or calculate as follows: Wheel diameter in inches * 12.7 + (Tyre size * aspect ratio/100) For my car with 13 inch rims fitted with 175/70s 13 * 12.7 + (175 *70/100) = 287.6 Dr : Differential ratio. For Vauxhall Viva/Magnum axle the ratio is indicated by a letter stamped on the pinion housing - on the top for 1800&2300cc OHC engines, bottom for 1256cc: A - 3.88, B - 4.125, D - 3.45, K - 3.727, none on later 1256 cars - 4.125 First calculate revs of back wheels per hour : (60000 * Tgr ) / Dr = Rph For my car : (60000 * 1)/ 3.45 = 17391 Secondly calculate distance per rev of back wheels in miles: (Rr * 6.263) /1000 * 0.000621 = Dpt For my car : (287.6 *6.263)/1000 * 0.000621 = 0.00011185 Finally : (Rph * Dpt ) = miles per 1000 rpm 17391 * 0.00011185 = 19.45 mph per 1000 rpm This means at 4000 RPM (comfortable cruising) the car will do (19.45 * 4) = 77.8 mph. At max revs (19.45 * 5.5) = 107 mph. You can use the same formula for the other gears by using that gear ratio in Fdr to get the top speed in any gear. You can also see the affect of changing a wheel and tyre size by altering the rolling radius and reworking the figures. Note: Max engine revs may be higher than best cruising revs.