[bigsixman]

I have a 2004 GMC 2500HD 4x4 with the 8.1/ Allison and 3.73 gears. I did a Nelson Performance tune, 180* thermostat, different spark plugs and I went with 265 Michelin tires.


I pull a 24' enclosed on pretty flat going and it will get 7 1/2-10 mpg depending on the strength and direction of the wind. The truck will get 14-15 mph @ 65 mph by itself with no wind and no passengers. I have the 5 speed Allison and around here it holds 5th gear @ 65 mph when trailering. My friend has a 2006 Chevy with the 8.1/ Allison and 3.73 gears and he cannot hold 6th and we get basically the same mileage. The gearing thru the first five gears is identical in both designs. The 6th gear drops the overall rpm by about 350 rpm and it makes a difference when you can use it. The aftermarket makes a kit to convert a 5 speed to a 6 speed and it costs about $1,800.

A few years ago, he put larger aggressive wheels and tires on his truck and lost 1 1/2 mpg in the process. He put 265 tires back on it with the stock wheels and the mileage came back.