sorry for late reply...I'm not gonna say your math is wonky, because I'm not confident that mine isn't, but I came up with a different discrepancy- about 1.2 miles on a 45 mile ride*. This was after a series of calculations that basically confused the crap out me- figuring out how many revolutions a wheel of circumference 'x' would have to make to roll 45 miles, then multiplying that by circumference 'y' to see the difference...along with the conversions between metric and English units, I could have very well screwed up.

I should have "shown my work" so I could go back and verify my results


* I shouldn't be so quick to discredit myself, but looking at this now, it seems highly improbable. Perhaps I used centimeters in one breath and millimeters in the next or something like that