I have a 2003 Silverado with the 5-speed Allison and the 8.1 Vortex. I get 7.5 mpg pulling my 14,000 Montana 5th Wheel. When I am not pulling the 5th Wheel, my mileage gets up to 10-11 mpg. I use the Tow/Haul mode when pulling. I do not use Tow/haul when I am not pulling. It seems like the transmission is telling the engine that it is always pulling. Both engine and transmission are totally stock with no modification. Any recommendations?

What do you expect from a 500 cuin gas engine grossing over 20,000 lbs.?

7.5mpg at that weight is EXCELLENT mileage for a gasser. My E350 got 7.5mpg towing ~6-8,000lb. My Dmax gets 11mpg at 17,000 combined weight at 75 going cross country (10,000 load)

You might consider EFILive tuning and a free-flowing exhaust to pick up some mileage though. Keep your tires inflated at high pressure as well.

I have a 2003 Silverado with the 5-speed Allison and the 8.1 Vortex. I get 7.5 mpg pulling my 14,000 Montana 5th Wheel. When I am not pulling the 5th Wheel, my mileage gets up to 10-11 mpg. I use the Tow/Haul mode when pulling. I do not use Tow/haul when I am not pulling. It seems like the transmission is telling the engine that it is always pulling. Both engine and transmission are totally stock with no modification. Any recommendations?
Welcome to the diesel place!
A full synthetic fluid change would be the first thing I did.You may pick up a tiny bit of mpg,esp on short trips where the cold synthetic lube flows easier.Dont expect much,though.With the weight your towing it would add some protection too.Your mpg seems normal to me.My 8100 gets about 11 mpg empty as well.I havent towed heavy yet to see how it will do.

Your #'s sound the same as mine but I am pulling 10,000#. Like to hear that big block "put the power down". I have a few posts on this page about some mods that will REALLY wake up your 8.1 feel free to pm me if you want.

Mine gets 12-13 empty, not shabby with 3.73 gears and 35" tires... towing Gn trailer full of cows (about 7500lbs) get 9-11.

Truck alone is close to 7K lbs

My mileage and tow load are the same as yours. I can get close to 14mpg on a trip unloaded, about 400 miles,.

Pulling 16,000 lbs on my gn loaded with antique tractors i avarage about 12 mpg high 9-10 back hilly roads , have been able to hit 22 mpg unloaded on the highway if i stay in the 18,19 hundred rpm range

Thanks for the great information. It's great to hear from professionals that figure out mileage and not just report the highest number the computer has ever shown.
Thanks for letting me ask my question.

Empty. I can get just over 14 mpg at 60 mph. I have 4.10's and 265's on my truck.No mods to the engine.

DEWFPO

I get about the same as you towing a 37 foot Holiday Rambler Presidential fifth wheel. I get 14 empty, my truck is a 2500 HD 2 wheel drive with 4.10 gears. I put a K & N air intake system on it and it made a big difference with the power it has.

a free-flowing exhaust to pick up some mileage

Hi all This I my 1st post. I just did that with the exhaust and it was the best thing Ive done with the truck. Its amazing how the factory muffler cuts your power and mileage. I went from 6 to 8 in town mpg to 8 to 10.
I have a 02 gmc 8.1 with the ally 3.73 gears.
Just wanted to share my info on the exhaust. Tim

I've owned 3-8.1s and one 6.0, my newest 06-8.1-CC-ally6-3.7's, gets 14 unloaded highway, 11-12 mix, haven't towed yet, but expect to get 9-10 same as the last 2 8.1's. My 6.0 didn't do much better, maybe 1mpg better mix, but worse at highway (13), but was a lighter x-cab. I don't worry about it though, run the numbers. At 15K mi/yr, $2.50 gas, 2mpg difference = $625/yr. Not a huge number to have a do-all vehicle. And here in NJ, diesel costs 40-50c more right now, which makes fuel costs about a wash (assuming 15mpg), plus the 6-8K added purchase cost. Either way, It simply costs us more than our sedan brothers to do what we need to do. I like to say 'gotta pay to play'. In the coming years, DOD and VVT will help gas engines do better. This tech just hasn't made it to the big gas towing market yet, it's coming. gm's just too busy right now making the first new trucks for the mass market, and we are the minority. My guess is the new diesels with the particulate filter will crash and burn (blame the EPA, not the manufacturers), making a big gasser return sooner than later (with new tech.).

Those numbers are real good. I get 8.5mpg out of my 5.7 vortec towing a 10,000 pound enclosed trailer.

I have a 03 8.1 Alli 2wd. I pull a 30ft 5th wheel. On hilly ore in the mountains I get between 7 and 9. On a flat HWY I get almost 10. My fiver weighs 10,00lbs.