 Main Gear Ratio Speed Calculator

Gear Ratio Speed Calculator
By AssassinRacing.com

Calculate the theoretical speed for a car in each of its gear ratios. If a car has less than 6 speeds, only enter gear ratio numbers for the gear ratios that apply (i.e. no need to enter anything in '6th' gear for a car with only 5 speeds). Use the redline RPM to figure top speed for each gear. Note that you will get ridiculous speeds for overdrive gears. This is because in reality top speed is physically limited in one of two ways, either by being geared relatively low or by aerodynamic drag. Use the horsepower calculator to find out how much horse power a car would require to run at a given speed. If the engine doesn't make that much power, it can't go that speed no matter how it's geared.

Tech Tip:
The most accurate way to measure tire circumference is to use a string and measure the actual distance (circumference) around the tread of the tire. A quick way to approximate the tire circumference is to add the wheel diameter to twice the height of the tire sidewall and then multiply by Pi (3.14159). For example, a car with 225/40R tires has a sidewall height of 3.54" (225 mm x 40/100 is 90 mm which is 3.54"). If it has 18" wheels then the tire diameter is roughly 25" (18 + 3.54 + 3.54). Thus, 25" multiplied by Pi (3.14159) yields an approximate tire circumference of 78.54" for this example.

Use the Drag Performance Calculator to calculate the theoretical 1/8 Mile ET, 1/4 Mile ET, 1/4 Mile Top End Speed, and Ideal 1/4 Mile Gear Ratio and the 0-60 MPH Time Calculator to calculate the theoretical 0-60 Time for a given application.

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