predator wont let YOU tune it enough... so thats a no go.
as for gas mileage i still get 18-19mpg around town and i got 25mpg during a road trip going abotu 65 the whole way... i got 30mpg with the stock gear running 70 in 95* degree heat in July of last year tho...
the gas mileage hit is worth the yank that you get when you step on it in 1st and 2nd gear alone lol... and it makes passing on the highway MUCH easier... dont have to downshift into 4th, just roll into the gas...
i will say that if you dont have sticky tires like i did, i was running 245/45R17 Kumho Ecsta Supra 712's, youll be doing a lot of spinning the tires... makes for great fun lol... the 315/35R17 sumitomos fixed that tho