Quarter Mile Calculator — 1/4-Mile ET and Trap Speed from Power and Weight

Quarter Mile Calculator: Estimate Your Car's 1/4-Mile ET and Trap Speed

If you're thinking of entering your car into a drag race and are wondering how well it would perform, then this quarter-mile calculator is for you. It combines a quarter-mile time calculator and a quarter-mile speed calculator. Using only your car's power and weight, it will estimate its 1/4-mile elapsed time (ET) and final speed (trap speed). With this quarter-mile ET calculator, you can:

• Set a target time for your drag race.
• See how a power upgrade to your engine affects your ET.
• Learn how reducing the car's weight affects its performance.

In this article, you'll also learn about the history of attempting to calculate the elapsed time and speed over a quarter-mile using only the vehicle's power and weight.

Huntington's Quarter-Mile ET and Speed Formulas

Back in the 1950s, engineer and automotive enthusiast Roger Huntington wanted to determine a mathematical relationship between a vehicle's power, weight, and performance over a 1/4-mile distance (a typical track length used in drag racing). His solution was to record the elapsed time and final speeds of different vehicles, plot the data on a graph, and find a line of best fit.

Common sense dictates that the heavier a vehicle is, and the less horsepower it has, the longer the ET over a quarter-mile distance will be. So he plotted Weight/Power against the 1/4-mile elapsed time and derived the following empirical equation:

ET = 6.290 × (Weight / Power)^1/3

For the final speed, it makes more sense to plot the power-to-weight ratio (more power and less weight means more speed). The Huntington empirical formula for the final speed is therefore:

Final speed = 224 × (Power / Weight)^1/3

• ET — elapsed time over a quarter-mile distance;
• 6.290 / 224 — empirically derived constants;
• Weight — total weight of the vehicle (including the driver) in pounds; and
• Power — peak engine power at the clutch (net power) in horsepower.

Don't worry if you only know your vehicle's weight and power in metric units — our 1/4-mile calculator will handle all the unit conversions for you!

Fox's Quarter-Mile Time and Speed Equations

Later on, in the 1960s, a physics professor named Geoffrey Fox at the University of Santa Clara proposed a theoretical basis for Huntington's formula. He wrote up his theory in a 1973 article in The American Journal of Physics entitled "On the Physics of Drag Racing," identifying the following significant variables that affect ET and trap speed:

• Vehicle weight;
• Engine power;
• The friction of the tire on the track;
• Aerodynamic drag;
• Mass moment of inertia;
• Frictional loss due to moving parts;
• Gear ratios and gearbox type (e.g., manual or automatic); and
• The vertical and horizontal center of gravity.

Fox concluded that the power and the weight of the vehicle are the major factors in determining the final speed, making it more useful than the ET if you want to estimate the horsepower of a vehicle. He refined the constants in his equations, resulting in the following formulas:

ET = 6.269 × (Weight / Power)^1/3
Final speed = 230 × (Power / Weight)^1/3

Hale's Quarter-Mile Speed and ET Formulas

In the 1980s, drag racer, engineer, and computer programmer Patrick Hale produced software that took all the variables listed above into account to estimate the ET and trap speed. He also produced similar formulas to estimate the performance values from just the power and weight:

ET = 5.825 × (Weight / Power)^1/3
Final speed = 234 × (Power / Weight)^1/3

How to Use the 1/4-Mile Calculator

1. First, decide which formula to use. Fox's or Hale's is likely to give more accurate results than Huntington's, as they are more recent and more likely to apply to modern vehicles.
2. Enter the total weight of the car and driver. For a standard car, you can look up its weight in the manual or search for it online. If you have a custom vehicle, visit a weighing station to measure its mass.
3. Enter the car's peak power output. For an unmodified engine, you'll find it in the manual, quoted at a particular engine speed (rpm). If you have modified the engine, take it to a garage to test its peak power output.
4. When performing the quarter-mile run, you need to maintain the peak power rpm over the entire distance. The start will be the hardest part, as you are accelerating from a standing start. As you drop the clutch, the engine revs could significantly decrease if you don't smoothly engage the first gear.
5. The results section shows you the estimated elapsed time over the 1/4-mile and the final speed at the quarter-mile point.

Just like our horsepower calculator, this quarter-mile calculator can also be used (in reverse) to estimate your car's power given a measured quarter-mile elapsed time or final speed.

Comparison of the Three Equations — A Worked Example

Consider a car with a total weight of 3,000 lb and a peak power of 300 hp. The weight-to-power ratio is 10 lb/hp, and the power-to-weight ratio is 0.1 hp/lb. The cube root of 10 is about 2.154, and the cube root of 0.1 is about 0.464:

As you can see, the three models agree closely on the elapsed time, while Hale's more recent constants predict a slightly quicker run and a higher trap speed. Try the calculator above with your own numbers and compare the three equations side by side.