Updated the relevant bit of code to reflect the actual distance, and changed it from int to float:
const float distance = 23.5; // distance covered by the vehicle for every revolution of the drive line
I left the MPH variable as int, so that should take care of rounding off the decimals for me. Dad decided that the one second interval was close enough for his purposes. I uploaded the new code and started mashing the button like mad, and it looks like it’s matching up with the values from the spreadsheet quite nicely, at least up to the 7 presses per second maximum that I can manage to hit. Once I get the Hall sensor, I’ll rig up a magnet on a little wheel and see what I get.