Most commuting calculators use the ridiculous figure from the IRS, which includes deprecation. While this makes sense for a business vehicle, it is nonsensical for a personal vehicle. I've never gotten one dollar more or less for a trade-in based on the number of miles I drove it. Sure, if you can completely get rid of a car because of bicycle commuting, you will in fact save this depreciation cost; but most people can't do this, and in fact, most existing comparisons out there include language along the lines of "this is how much you will save if you bike one day a week", implying that they also assume the person in question keeps the car, but leaves it parked in their driveway.
In addition, most calculators seem to think that riding a bike has zero cost. But, if you ride like I do, meaning you actually put tubes and tires on your rims, you do in fact consume something that costs money to replace. In addition, many peoples' bike commute utilizes a "bus boost" in one or both directions, which also usually costs money.
This calculator attempts to estimate a more realistic comparison, and is likely to be disappointing to bicycle advocates. However, using the typical comparison is highly misleading and not particularly useful in policy discussions. For instance, it becomes pretty clear that the variable cost of driving is not very expensive compared to the alternatives, which points to a major problem in current transportation economics.
For my OLD calculation (when I took the express bus in the morning to 183/Braker way back in the day in a car that only got 32 mpg), click here. And if you can make a strong argument that my default costs are wrong, please email me.