Christ -
I agree with the honesty policy. If a person wants the car but doesn't want the mpg-mod, then you can let the market decide. If the car sits too long, reverse the mod, and call back the person that wasn't willing to pay for the extra MPG. If it's a complicated mod, you can document how to undo the mod.
I think that keeping a good car on the road is "recycling", so that's already an ethical act. Even though your choice of car may be limited due to the one that needs the fewest repairs, selecting good MPG vehicles is also a way to improve MPG.
The bigger(?) tire + speedo mod sounds cool. If gas prices rise as is currently being predicted, you're business strategy will become fashionable again. Here is one dude named fistopher who was doing serious MPG mods during the recent high gas price era :
http://ecomodder.com/forum/showthrea...1-vx-4000.html
CarloSW2