Break out the calculator and cipher the payback miles (brain's a little rusty but you'll let me know if I'm wrong):
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 $2.25 per gallon (G)
1000/[2.25*(1/17-1/20)] = 50370 miles to payback
but if I went from a paltry 3MPG to 6MPG (same 3MPG difference)
1000/[2.25*(1/3-1/6)] = 2667 miles to payback
From your description, a rear end swap might be more cost-effective.
