Just use 50 cents/mile to calculate driving costs. That's what the federal government pays for reimbursement and it includes gas as well as maintenance, wear-and-tear. If you have to pay for parking when you drive, you can add that as well.

I know that I save money bike commuting, even though I spend a fair amount of those savings on bike gear. However, I would rather spend my money on bike gear than gasoline and car costs. If you don't buy a lot of unnecessary bike gear, you can save quite a bit. Here are examples:

-- Gasoline and maintenance savings @ 50 cents/mile
-- Parking
-- Insurance (my company reduced my rate $50/year when I showed my mileage reductions)

I also tend to spend extra money when I drive because I more frequently go out to lunch, go shopping during lunch hour. If I sold my truck, my savings would be huge but I don't want to do that. I like having the option to drive when weather is crappy, doctor's appointments, restocking clothes and supplies in my office, bringing home laundry from work, etc.

My commute was 22 miles for 4.5 years but increased to 30 miles in October. We got our gasoline bill the other day and it totalled about $250 for the month, and all of the charges were for my wife's car. I showed her the bill and said: "Look how much money I'm saving us by riding to work. Not a single one of these gas charges were for my truck." Of course, I turned around and spent $125 on a new light system, but that should last me a long time and I can recoup some of those costs by selling my old one.