Calc the mileage by bike and the mpg by car * avg cost per gallon to get direct savings. Then you get into the amortization and maint. Assuming you put 3k less per year on the car, that's one fewer oil change. Determine the residual value of the car pre-commuting. Now calculate the additional time elapsed after the car is fully amortized. The amortization by month is now savings. Determine that you can avoid replacing the car altogether and then you can drop insurance and registration values into the savings bucket. You will probably get close to break even with the additional amortization time.