The gear ratio in any gear assembly refers to the ratio of teeth between two connected gears. A gear ratio of 5:1 can be understood as five teeth on the idler gear, or the gear to which power is transferred, for every one tooth on the pinion gear, which is directly connected to and powered by the motor. In drag racing, it may seem that a higher gear ratio will automatically make your vehicle faster, but other variables play a large role in determining which gear ratio is best for your drag racing vehicle.
Measure the overall diameter of the tires on your drag racing vehicle, using a tape measure. You'll need to lift the car with a hydraulic jack to relieve the pressure on your tire. With the pressure relieved, take your tape measure and measure the diameter of your tire from the top of one tread to the bottom of the other. Make sure that your tape measure travels through the exact center of the hubcap; measurements must be accurate to 1/8 of an inch for the calculations to be made correctly.
Calculate the loaded radius of your tires on your drag racing vehicle. You can find the loaded radius of your tires by dividing the overall diameter of your tires in half.
Calculate the tire revolutions per mile for your drag racing vehicle. You can find the tire revolutions per mile by dividing 1,0084 by the loaded radius of the tire.
Find the gear ratio that's best for your drag racing vehicle by calculating the maximum speed for the vehicle using peak RPM and tire revolutions per minute. The formula for calculating the maximum speed is Speed = (RPM x 60) / (Gear Ratio x Tire Revolutions per Mile). Your peak RPM is the revolutions per minute on your engine at peak horsepower; this information should be included with the owner's manual for your vehicle, or you may need to contact your vehicle's manufacturer for more information. Substituting the known variables, calculate the maximum speed with this formula using multiple gear ratios for the calculations. When you have found the maximum speed possible, you've discovered the best gear ratio for your drag racing vehicle.
Cross-reference the maximum speed calculated for your drag racing vehicle with National Hot Rod Association (NHRA) speed records to find if your calculations are reasonable. You will want to compare your calculated top speeds to the top speed record for your drag racing vehicle's NHRA class. If your calculated speed is much higher or lower than the NHRA speed for your vehicle's class, you will want to redo all your work to make sure there were no errors in calculation.
