If we take the tandem in, we save $5-6/day in gas.

If I ride but my wife drives in, the gas savings evaporates.

But I did sell my car, so we save about $50/mo on insurance, plus an average of another $50/mo in maintenance--car was old and needing fairly regular attention/maintenance/repair, but paid off. No gas savings, though, since we work in the same building and usually drive in together on those days when neither of us bike.

We cancelled $30/mo worth of fitness center that we weren't using.

On the other side of the balance sheet, we've upgraded her ergos on the tandem, gone through a bunch of tubes, a set of new tires, panniers, etc. Plus, I bought a motorcycle, although maintenance and insurance on it are negligible compared to the car, gas likewise--it sees little use since we started commuting in together and especially bike commuting, either tandem or solo.

Bottom line: if bike commuting allows you to live car-free, you will see tremendous savings. If you are also maintaining ownership of a car--registration, tax, car payment, maintenance, gas, possibly parking rent/permit, you might break even with gas savings v. bike maintenance.

It's the other intangibles that make up the difference in the case of also owning a car: environmental benefits, being able to travel faster that auto traffic in urban conditions, physical excercise, sheer fun of riding. Hard to put a value on these, but since there's no real monetary savings if you are not going car free, it makes all the difference.