I know how many miles I commute because I don't use the bike for anything else, and I have an odometer.

I do make black marks on my calendar when I drive though. Or, I did until the last 2 months, when my son wound up on crutches and needed to be driven to school (for 6 weeks so far, UGH!). Salvation is at hand however, the cast came off on Tuesday and hopefully his ankle will loosen up and the muscles get strong enough to lose the crutches this weekend!!!