A record is not measured to 0.001 mph. Although record speeds are reported to three decimal places. The measured parameter is time, which may be recorded to many decimal places but is then truncated to 3dp for (FIA) record purposes.
Hmm . . .
So if you're timed at 7.2009 for the mile, which figures to 499.9375 mph, you'd be credited as going 500.
Yep. It needs someone smarter than me to explain why this is considered to be the "correct" way to do the calculation.
Not sure if the rule book helps, but the FIA rules follow. At no time could 499.9375 be anything greater than 499.937.
FIA D12.3.2 Time accuracy. Record time with an accuracy of 1/1,000th of a second and calculate the mean time with an accuracy of 1/1,000th of a second with no rounding off.
FIA D12.3.3 Speed Accuracy. Calculate and record speed with an accuracy of 1/1,000th of mph or kph
FIA D12.3.4 Conversion. Convert speed thus calculated to kph or mph with no rounding off using the defined conversion factor.
FIA D12.3.5 Precision. If the timekeeping equipment has accuracy greater than 1/1,000th of a second, its precision shall be set to record times to 1/1,000th of a second with no rounding off to allow direct use of all readings.