I did a quick estimate this evening; I'm definitely not saving money compared to driving if one looks purely at the money I've spent on cycling and converts it to fuel (and mileage) that could have been provided to the car.
If I estimate my total cycling expenditure as about $1400 (both bikes, repairs, and accessories) and gas at $2.25/gal (what I paid last week), that comes out to ~622 gallons that could have been used in my car. I'm currently averaging 18.1 mpg over the 1450 miles I've put on my current car, so 622.2 gallons * 18.1 mpg = 11228 more miles that I could have driven at current tank rates if I'd spent bike money on gas money.
I've put about 1107 miles on both bikes so far, so I'd need to ride 10k more miles to break even at current tank rates. That's several years away, and that's if I don't spend a penny more on the bikes in the mean time. So no, looking at a fuel opportunity cost, I'm not nearly saving anything by riding. It's actually quite humbling to crunch the numbers.
Even if I'd only spent $60 on cycling--for example, on my $20 MTB and $40 helmet, it would mean (60/2.25) * (1450/80) = 484 miles before I broke even on such a meager cycling setup at current gas prices. When you look at things this way, it becomes apparent that you've got to find other reasons to ride besides the desire to save money, because unless you're given the bike for free or ride the hell out of it, it's far cheaper to put the bike money into the gas tank.
That's some creative accounting there. You failed to account for many other costs for the car, such as maintenance and repairs. I find that fuel is less than half the operating cost of a car, which, BTW, does not include insurance, registration, depreciation, etc. On the bike side, you include the entire depreciation cost of two bicycles. Clearly an apples to oranges comparison. You already own the car, and you already own the bicycles. An apples to apples comparison would be to compare the operating costs of each.
Unless you replace a car with a bicycle, you're going to find it difficult to justify commuting on economics alone using a $1400 setup.
But I figure the value of being able to eat what I want to be far greater than the money saved.