Squeezing more mileage out

[tucsonjwt]

For reference, Gear Vendor advertises a 20% improvement in fuel economy. This is only valid if you are driving over a minimum speed (say 40 mph), so it is mostly a highway driving advantage. I have a GV on my 83 454/4 spd granny tranny, and I got about 20% improvement in gas mileage when I started using it, but since I travel mostly city driving it does not do me much good. The GV effectively changes your final drive ratio by 20%, theoretically giving you "extra gears" between your normal gears. So, I have a 4:10 rear axle and have a 3.28 effective rear axle ratio with the GV engaged.
Even used GV and GV adapters are pricey (check Ebay). Consider that you need to shorten/modify your drive shaft to install one, you are looking at $3.5 - $4.5K installed new, and maybe $1K less for used. I did read somewhere that GV will rebuild one of their own units for about $850, so that might make a used unit an economical choice if you got one for a few hundred dollars (not likely though).
Also, I think with an overdrive transmission and lock up torque converter the GV is of little or no fuel economy use, but is supposed to improve performance.