Commuting by bike?

Yeah Paul I’ve often wondered the same things, and have done the math somewhat, though not quite as thoroughly as you did there.    Bottom line, though (literally the bottom line, of any write-up of such math-doing), should be…

“Benefits of getting more-frequent exercise .... PRICELESS.”    

My Giant OCR3 cost me about $500 two years ago, and I've spent at least $500 since then on tires, brakes, lights, a new wheel ( :), etc, etc.

My commute is about 7 miles one way, 14 miles roundtrip. If I average four or five days a week on the bike, I biked about 416 days, or 6000 miles. So the cost of the bike has been about 16 cents per mile.

My shiny new (to me) '99 Impreza RS gets about 24 mpg around town, maybe slightly better if I stop enjoying it so much. I paid $8000 for the car with 96k on it and hope to drive it at least 80,000 miles. So the total cost of the car is 10 cents + 13.7 cents = 23.7 cents per mile.

If the car lasts to 250k, the cost is 5.2+13.7 cents = 19 cents per mile, at current gas prices.

I like seeing the attempt at the analysis, but Hmm - seems like you're not doing the analysis in the same way on car as bike. You predict a lifespan for the car, but just total up cost-per-mile as of now on the bike. Unless the bike has reached its demise (in which case, you gotta buy a better bike next time - it should last more than 6K!!) then you should also predict the lifespan of the bike. Then of course you gotta factor in repairs and new tires and other maintenance on the car since you're doing it on the bike, and we all know cars ain't free to keep running (to say nothing of predicting inflation on fuel costs and assuming a net present value on future fuel costs etc...).

I'll get right on it, Jim.

Any experienced bikers want to comment? I don't know how long the Giant will last, but it needs new headset bearings, the bottom bracket makes a racket, the brakes are thrashed, and the front rim is wearing out......

I think Jim's right, both about amortizing the cost of the bike over its lifespan, and buying better quality components. My commuter bike is a 14 year old custom steel bike with top quallity components (Dura Ace) and Mavic ceramic rims (bomber) which cost $2100 then. I've replaced wheels (once) and cassettes (twice) and use a chain and set of tires per year, but it's still going strong. It sees around 3,500 miles per year. $500 road bikes are made for people who leave the bike in the garage most of the time.

I think the cost per mile on a bike can be close to or exceed the cost per mile for a car, but a lot of that is because people tend to go a lot longer distances in a car, so the cost gets spread out over more miles. When you look at it on a cost per year basis, most bike commuters don't spend nearly as much on their bike as on a car. Bike components like tires and chains and lights don't last as long as car components. but there's no insurance, and major overhauls are a lot lot less. There are long strings of calcs on this on usenet groups with results all over the map. One "problem" is that if you have both a bike and a car, one or the other is "extra cost" over what you have to pay anyway just to own the first vehicle. I prefer to think of the car as the added cost, and at least I get a discount on my car insurance by not using it for commuting.
I have a Bianchi road bike I got used for $400 and spent another $400 or so fixing up over a couple years and outfitting with rack, pannier, etc. Recently spent $30 for lifetime use of Wright Bros Cycle shop space and tools in Fremont, so changing brake and drivetrain components and truing wheels is just the cost of parts. I disagree that a custom bike is going to last longer than a production road bike. The custom part is mostly for fit and style (and some of them are fairly fragile machines). There are plenty of decent steel production frames that will last as long as you can find or make components to fit them, and you can make all kinds of changes to stems, bars, seats, etc., to customize fit if the frame is even close to what you need.
I guess the moral of the story is to ride more so it will cost less.

I disagree that a custom bike is going to last longer than a production road bike.

Sorry if I seemed to imply that spending extra to get a custom frame would increase the longevity - I've had plenty of nice steel frames that would probably have lasted just as long. It was reassuring to watch this bike being built, however, and see that the builder got maximum penetration of brazing material into the lugs and bottom bracket (I've seen "name brand" frames where this was not the case).

For commuting purposes, most robotically TIG-welded steel or aluminum frames will last for a decade or more. I think it's wiser to spend the money on places where your body contacts the bike (saddle, bars/stem, pedals) and high wear areas like rims and hubs.