How to Calculate Quarter Mile ET and MPH
In drag racing, the traditional length of a strip is one quarter of a mile (1320 feet), or about 402.3 meters. To some extent, you can predict the elapsed time (ET) and terminal miles per hour (MPH) of a quarter mile race from the total weight of the vehicle and the horsepower. Although there are many other factors that influence speed and time--drivetrain gearing, tire friction, driver skill, etc.--the horsepower and weight are the most important factors and the easiest to obtain numbers for.
One set of formulas for predicting MPH and ET from the weight and horsepower is
MPH = 230∛ H/W
ET = 6.269∛ W/H ,
where H is the horsepower and W is the weight in pounds, including the weight of the driver. The units for ET are seconds. The formulas above were developed using ordinary street car data.
If you have a car that is adapted for racing, these formulas may return a MPH value that is too low and an ET value that is too high. Instead, you should use these formulas, developed with data from racing vehicles:
MPH = 234∛ H/W
ET = 5.825∛ W/H .
The first set of formulas is due to Geoffrey Fox and the second is due to Patrick Hale. You can use the calculator above to predict MPH and ET, or apply the above formulas using a hand calculator. Note that taking the cube root of a number is the same as raising it to the power of 1/3.
A regular street car weighs 2830 pounds with the driver included, and has a horsepower of 220. If it is drag raced on a quarter mile track, the predicted terminal speed is
230∛ 220/2830 = 98.161 miles per hour.
The total elapsed time is
6.269∛ 2830/220 = 14.689 seconds.
Keep in mind that these are only estimates and actual values may be higher or lower depending on other factors.
© Had2Know 2010