If you want to go crazy with the accounting, (and get the new bike sooner ) then I suggest you also add the depreciation of your vehicle over its useful life. So if you have a brand new vehicle that cost 25k and you plan to keep it for 150,000 miles with a 3,000 residual value, you can tack on another (25-3)/150 = .146 per mile ($44 per 300 miles). If you bought a used car just use (price paid - residual value) / (miles when sold - miles when purchased).

You aren't really saving this money since cars depreciate more based on time than mileage, but hey, you will get your "free" bike sooner!