Sure Grip Rearend Value Question

yeah they are a pain in the *** man, you let off the gas and the car is like BRRRRRRRRRRRRRRRRRRR and slows way the hell down. Like i swear to god you can feel the gas being guzzled LOL. Im all for having a fast car with alot of giddy up and go, but if it gets such **** gas mileage i cant afford to ever drive it then its not doing me any good. I wanna get like at least 18mpg.