towing mileage

[coloradoguide]

I'm towing a 31 foot pull behind travel trailer that weighs 8700lbs, and am getting around 8 miles to the gallon. This is through PA on route 80, and I was runnin' about 70MPH. Is this unusually bad mileage, or was it just the hills and the speed? Would a chip or box help? Also, I am thinking of putting 235/85/16 michelins on this truck. Would gearing favor towing as downshift out of overdrive would happen at lower engine speeds? Truck is a 2003 duramax crew cab with a K&N filter and a stack pipe exhaust. Thanks.

[Turbine Doc]

Colorado you might want to post this in the 6.6 forum, I'd say you have a calculation error, maybe spedo is off do you have gps to check by or do a measure mile, 8 seems low to me my 6.5 gets 11-12 pulling 12K trailer

[Colorado Kid]

I'll agree that 8 mpg is worse than expected. I tow a 5th wheel of similar wieght and have never done worse than 11 MPG at the same speed, even westbound across Wyoming.