You will not save much money by leaving your car parked in the driveway and taking the bus. Yes, the IRS allows you to deduct based on a formula that includes depreciation – because it’s the only way to give you any credit for having your personal vehicle tied up for business use. It does not under any circumstance mean that depreciation is mostly a function of miles driven – because it is definitely NOT; depreciation has more to do with age than use.
The last time I did this, I ran the numbers and estimated that depreciation due to age is roughly ten times the depreciation due to miles in a high-mileage scenario.
The summary is: in most cities, you will not save much money, if any, by leaving your car at home and taking the bus or train to work – unless you’re unlucky enough to have to pay a lot of money to park. And, of course, you have to have unbundled parking costs (pay per day rather than per month).
The converse of this, though, is: You will save a surprisingly large amount of money by going from two cars to one car. Insurance. Registration. Car payments. Most of the depreciation bill. Maintenance (like depreciation, most maintenance is a function of time rather than miles).
Alternatively, if your company opens up an office in one of the few parts of the suburbs to which even I can’t tolerate the bus commute, you face spending a LOT more money going back up to two cars. That’s where to focus the energy – not on the “leave your car at home today and save N bucks” argument – because N is likely too small to be worth the trouble.
For my trip, for instance, google doesn’t have cost figures (must not be hooked up to Capital Metro’s farebox) – but I can give an estimate from my own commute calculator which shows that the bus trip cost $1.00 round-trip (allocate 50 cents each way) compared to $1.32 for the car (66 cents each way). That means that I can save 16 cents by spending an hour and forty-five minutes on the bus instead of the 15-30 minute drive, which is only a good deal if the value of my time is at or below 15 cents / hour.