Just miles logged. But there are two aspects to miles logged in my case:
1. Fuel cost. I average just 20 mpg if I drive - and though my bike commute is just 3 miles each way, my car commute is 5 miles each way.
2. Lease cost. This one is cheating a bit - but I add it in for a better feel-good factor. Cycling lets me keep the car mileage under the lease limit - if I exceed it, the charges are pretty steep. Because I cycle, we can use the car without worrying about the lease mileage. I cannot factor in maintenance on the car because its free in the three year lease plan.

There is a third factor that will soon appear once my wife passes her drive test - we will be able to stay a one-car family I'm planning to sell my Magna and buy a nice Topeak bike seat for my son from craigslist - if I manage to do that, I'll drop him to school on the bike and push costs even lower.

Mind you, as multiple people said above - I'm mainly cycling because its fun - the money saved is a bonus