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If a racetrack is measured around the middle at a given distance, say 3/8, but the cars run around the bottom (a shorter distance, like you said), then the the distance vs. time elapsed formula is going to spit out a mph number that is too high. Conversely, if the cars were to run around the top, the mph figure would be too low.
You can look at Eldora as a perfect example of the misleading mph numbers. They list it as a half mile, yet we know it's somewhere around 7/16 (.4375). So in reality the cars are traveling a shorter distance than listed. Jerry Coons Jr. clocked the non-wing track record lap of 14.712 in 2006. Put up against the listed distance of 1/2 mile, his speed is 122.348 mph. But, if we use the real (shorter) distance he traveled (.438, let's call it), his speed was actually 107.177 mph.
Obviously we don't know where every race track was measured, be it on the inside, in the middle or 10 feet off the wall. And we also don't know if the measurements are accurate (Eldora). I'd say the best way to get a true feel for speed is to ask your promoter or owner what and where the measurement is, and go from there.
PS - The formula for finding speed is easy. First take the seconds elapsed and divide it by 3600 (this converts the number to hours). Then, divide that number into the distance in miles. It's easy.
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