Tire spin and MPH myth [ Home ] [ Up ]

A popular rumor or misconception is a car that has poor traction has a higher 1/4 mile MPH. The false idea is the spinning tires and slower ET allow for more time to apply power. This rumor is as bad as the "engine overheats because water flows too fast without a thermostat or restrictor" myth.

MPH or velocity is a result of the time and the "push" or torque from the the tires to the ground, provided time is a preset value. The formula is:

a = ∆V /  ∆ t

a is the acceleration

∆V is the change in velocity

∆ t is the change in time

For distance, the formula is:

Distance

 distance ∆d = V *  t time t =∆d / V velocity at given distance ∆V = d / a average velocity V = ∆d /  t

The third formula above  ∆V = d / a  is another way of saying difference in start and finish speed is simply the distance times the acceleration.

People claiming spinning tires accelerate a car to a faster trap speed have ignored the basics of acceleration. Final speed difference from starting speed is always average force applied to the ground over a fixed distance. If tires are slipping and spinning, and if distance is the same and average engine horsepower is the same, we always will have less speed. There is no way around this.

Fortunately, you do not need to know the math. You can use online calculators like this Velocity Calculator or you can do a test.

One test would be to accelerate your car to the fastest speed possible when the track is dry, and then repeat the test in winter with a snowy track. Another would be to replace slicks or drag radials with small street tires, and watch the MPH change.

Summary

Some of the confusion behind this might be elapsed time. When the clock is running, we want to capitalize on maximum possible acceleration from zero MPH. Since speed is zero at the start, we travel no virtually no distance at all at the start in a one second period. Traveling sixty feet, the standard distance for the first time clock reading, takes about 2 seconds in a relatively fast street car.

1 foot per second   =  0.68182 MPH

Our car's average speed in MPH over any distance in feet is  MPHavg  =  .68182 *   (d/t)

 60' time formula Avg MPH 2.5 .68182  * 60 / 2.5 16.35 2 .68182  * 60 / 2 20.45 1.5 .68182  * 60 / 1.5 27.27 1 .68182  * 60 / 1 40.91

The ultimate MPH at 60 ft depends on how the car accelerates at every point along the 60 feet, and cars are never perfect. The exact formula is  Vf 2d / t = at + 2Vi

 60' time typical MPH @ 60 ft 2.5 29 2 35 1.5 45 1 62.2

The more tires spin, the faster MPH has to be for the same ET. This is not the same as saying more tire spins results in higher MPH. Actually, the exact opposite is true. The more tires spin, with power and distance constant, the slower the final speed will be. ET slows right along with MPH when traction is reduced over a fixed distance, although traction generally affects ET much more than MPH.