Love your bike, but for one reason or another can't give up your car? You can use this calculator to figure out the cost of using either on a per-mile and per-commute basis, and how much you'll save if you switch to biking more often. Plus, if you input the cost of your bike, it'll tell you how much use you need to get from it before it pays itself off on a pure cost-savings basis.
So, here's the thing: if you have to own a car (for things like distance travel, families, hauling, etc), then you have certain necessary costs that exist regardless of whether you're using the car or not. These are the required costs for 1) owning and 2) legally operating a car in your area. Among other things, these obligatory costs include the price of the car (plus any financing), your insurance premium, and any government requirements and taxes (like license plate renewals in the US). Since these costs have to occur for the car to remain available and road-legal, they are considered forgone for the purposes of these comparisons, and won't be included in the calculations.
It should be noted that if you are in the unusual position (in America) of having only a bike and considering whether you should obtain a car, that the vast bulk of the expense of owning a car are these sunk costs. For example, even paid off, over 70% of the day-to-day operation cost of my car is down to insurance premiums and license plate renewals. If you're considering owning a car, fill out the Car column with the theoretical data for the vehicle you want, and then remember that the numbers generated in the results may be as little as 30% of the actual cost of maintaining that vehicle, not counting what you pay for it.
Unless noted otherwise, all fields per column are required before results will calculate.