# Michigan: Don’t Believe the Speeds

All the talk at Michigan International Speedway about high speeds and the hoopla over passing the 200-mph barrier prompts me to offer this caveat:  Take the speeds you hear with a grain of salt because the average lap speeds are estimates, not measurements.

The quantity being measured is time.  The speeds you see in NASCAR timing and scoring are obtained from the classic equation:

speed=\frac{distance}{time}

Nobody is making exactly 2.0 mile laps around Michigan.

## How Long is Michigan International Speedway?

The whole business of how tracks are measured is sort of murky.  Some tracks measure the distance around the apron.  Some tracks measure the distance along a line 15 feet from the apron and others measure the distance along a line 15 feet from the outside wall.

For simplicity, I’ll approximate a track as a circle with radius r.  The distance around the circle at the radius r is 2 πr.  Let’s say that is 2.0 miles.

If I change r by 15 feet, I change the distance I have to travel by 94.2 feet (0.01785 mile).  Given that the lap time is the same (we’ve measured it), changing the distance changes the speed.

Mark Martin ran a lap time of 35.805 seconds.  At 2.0 miles distance, that translates to 201.0892 mph.  If he actually ran a shorter path (1.98215 mile, which corresponds to a 15-foot difference), then his speed was actually only 199.2945 mph.  That’s about 2 mph from a very small shift.  Of course, if he ran more than 2.0 miles, he would have been going even faster.

The point is that we don’t know exactly how far the car travels on each lap.

If you assume that the distance traveled in the straights doesn’t vary much with your position, this is probably pretty accurate.

## TL;DR

The conclusion:  it’s nice to deal with round numbers like 200 mph, but all of the hoopla about passing some magic mark is just silly.  The number gets attention  and sounds impressive, but it’s not real.  Trust in the lap times.

NOTE: This post was cleaned up a little on 8/17/2021 to correct some grammatical errors.

### Other Michigan Stories:

The Myth of the 200-mph Liftoff Speed

#### 1 Comment

1. Doug Scholl says:

Not only that, but the speeds mentioned are an average speed. So if they are hitting turn one and turn three at 215-218 mph entry speeds. That means the car must slow to approximately 175-178 mph through the turn which the car must then accelerate through. I for one are not worried about the speeds recorded yesterday. As temperatures increase the track loses speed and also as the track rubber ups it’s will also add more drag to the cars.