This is not a very useful or logical way to calculate depreciation of your bikes. It would be better to divide the net value of the bike by the projected number of years of service. For example, let's say your $1000 bike will last 10 years, and its salvage value at the end of 10 years is about $100. The net cost of the bike is $900. Divide that by 10 years, and you have $90 per year of depreciation.

If you ride 5,000 miles a year, that is

*less than two cents a mile*.

I'm far from being an accountant, but this seems like a much more meaningful way to determine depreciation costs, rather than your ever changing figures.

http://en.wikipedia.org/wiki/Depreciation