To formalize what Jon said about payback :
Thinking about a {your swap here} swap to save on gasoline? How long will it take to break even? I ciphered out a formula.
Let C = Cost of the conversion in dollars
G = Price of gas per gallon in dollars
P = Present Miles Per Gallon
N = New Miles per Gallon
and
M = Miles driven to break even
Then
M=C/[G*(1/P - 1/N)]
For instance if I spend $1000 (C) to boost my mileage from 17MPG(P) to 20MPG(N) and gas is $2.95 per gallon (G)
1000/[2.95*(1/17-1/20)] = 38418 miles to payback.
If I went from 8MPG to 11MPG (same 3MPG difference)
1000/[2.95*(1/8-1/11)] = 8994 miles to payback.
Too much like math? I put the formula into an Excel spreadsheet so you can just plug in the numbers.
http://www.chuckthetruck.com/payback.xls
