ZF5 speed, 3:55 rear ends, stock tires, ATS turbo, I get right at 18.7 MPG doing 100% highway driving at 65-68MPH. With that gearing my tach shows 1950 RPM at 65MPH in 5th over. Fuel mileage was exactly the same before installing turbo. Towing my 3 horse gooseneck, gooseneck flatbed or gooseneck travel trailer I get anywhere from 10 to 13 depending on the load, etc.