My 8.1 kicks my 6.0 in milage and every other way alsoThe real question is why is an 8.1L/4L80E swap not more common. It is a much better truck engine than an absolutely gutless 6.0L or 5.3L. An 8.1L is on par with a 6.0L as for fuel mileage and has another ~200 ft/lbs of torque down at 2,000 rpm where you spend 95% of your time driving at.