Please note: these formulas are for estimating only. Contrary to popular belief, using this site will not increase your horsepower.
This calculator will help you choose the rear gear ratio that suits your desire based on a few variables: transmission gear ratio when you finish the quarter mile, height of your tires in inches and maximum RPM that you spin your motor to.
| Transmission Ratio: | |
| Tire Height (inches): | |
| Trap\RPMs | 5800 | 6000 | 6200 | 6400 | 6600 | 6800 | 7000 | 7200 |
| 110 | ||||||||
| 115 | ||||||||
| 120 | ||||||||
| 125 | ||||||||
| 130 | ||||||||
| 135 | ||||||||
| 140 | ||||||||
| 145 |
Here is a list of formulas: [back to top] •Rear Gear Ratio = (Tire Diameter * RPM) / (Trans Ratio * MPH * 336) •Tire Diameter = (Trans Ratio * MPH * 336 * Rear Gear Ratio) / (RPM) •RPM = (Trans Ratio * MPH * 336 * Rear Gear Ratio) / (Tire Diameter) •Trans Ratio = (Tire Diameter * RPM) / (Rear Gear Ratio * MPH * 336) •MPH = (RPM * Tire Diameter) / (336 * Rear Gear Ratio * Trans Ratio) •Tire Diameter (inches) = (((section width * profile) / 2540) + (wheel diameter / 2)) * 2 •HorsePower = TORQUE * RPM / 5252 •TORQUE = (5252 * HP) / RPM •1 Liter = 61.02374 Cubic Inches •Displacement (in^2) = Number Of Cylinders * 3.141592653589 * (Bore/2)^2 * Stroke •HP from MPH (1/4 mile) = WEIGHT * (MPH/234)^3 •HP from ET (1/4 mile) = WEIGHT * (5.825/ET)^3 •ET from HP (1/4 mile) = (WEIGHT / HP)^.333 * 5.825 •HP from cylinder head intake flow = CFM x 0.2575 x number of cylinders •1/4 mile ET = 1/8 mile ET x 1.5832 (prestage.com) •1/8 mile ET = 1/4 mile ET / 1.5832 (prestage.com) •piston speed in fpm = stroke in inches x rpm / 6 •rpm = piston speed in fpm x 6 / stroke in inches •bhp loss = elevation in feet / 1000 x 0.03 x bhp at sea level •theoretical cfm = rpm x displacement / 3456 •volumetric efficiency = acutal cfm / theoretical cfm x 100 •street carb cfm = rpm x displacement / 3456 x 0.85 •racing carb cfm = rpm x displacement / 3456 x 1.1 ->Formulas for g force & weight transfer •drive wheel torque = flywheel torque x trans gear x final drive x 0.85 •wheel thrust = drive wheel torque / rolling radius •g (force) = wheel thrust / weight •weight transfer = weight x cg height / wheelbase x g •lateral acceleration = 1.227 x radius / time^2 •lateral weight transfer = weight x cg height / wheel track x g •centrufugal force = weight x g ->Formulas for shift points •rpm after shift = ratio shift into / ratio shift from x rpm before shift •driveshaft torque = flywheel torque x transmission ratio ->Formulas for weight distribution •percent of weight on wheels = weight on wheels / overweight x 100 •increased weight on wheels = [ distance of cg from wheels / wheelbase x weight ] + weight ->Formulas for center of gravity •cg location behind front wheels = rear wheel weights / overall weight x wheelbase •cg location off-center to heavy side = track / 2 - [ weight on light side / overall weight ] x track •cg height = [ level wheelbase x raised wheelbase x added weight on scale / distance raised ] x overall weight Check out Prestage.com for more useful formulas. email me at aaronmurray@msn.com if you have any formulas you would like to see added (as a calculator) or if you have any suggestions.