Calculator/formula for FE investment payoff?

[theholycow]

I sometimes think about investing in something for FE, but I decide that it probably won't pay for itself. I don't know that for sure, though. Is there a calculator, or at least a formula, that can help?

I imagine it would require the following data entered: Cost of investment, expected MPG gain, miles driven, and cost of fuel. I could probably figure it out myself but I'm feeling lazy, and why reinvent the wheel?

[GasSavers_RoadWarrior]

Better do it on percentages, getting an extra 1mpg on a 50mpg civic has twice the payoff time to getting +1mpg on a 25mpg Mid/full sedan, and 4x the payoff time of getting it on a 12mpg truck.

[theholycow]

That doesn't tell me if it's worth my money to invest in, for example, aero hubcaps, or a new exhaust, or whatever else. I'd like to feed my data into a calculator and have it tell me how long it would take to pay off the investment, or how much I could invest, or maybe some other ways of looking at it.

I know I can figure out the formula and make it into a nice javascript calculator where you fill in what you know and it figures out the rest, but I'd rather not reinvent the wheel.

[GasSavers_BEEF]

first of all I am pretty lazy myself but here is something I scratched out.

[(miles per week)/(new or expected MPG)]-[(miles per week)/(old or existing MPG)] = gallons saved per week

(x/a)-(x/b)=c

x = miles per week
a = expected MPG
b = existing MPG
c = gallons saved

you can then multiply the gallons saved by the price of gas and see dollars saved per week. divide cost of upgrade by this figure to see how many weeks it would take to pay it off.

I know that isn't what you wanted but it is a way to do it.

just my 2 cents