80 MPH is what's really hurting you.

If you truly are going 500 miles on the weekend do a test. Go 50-100 miles at 70 (cruise set), and reset the mileage counter when you start. Do the same at 65. You already know what you get at 80, no need to test.

My 3.0L ecodiesel gets 24-26 mpg empty and 14-16 mpg towing my 20' Trancat, but I don't go over 65-70. In fact my 3L diesel gets better mileage towing than my F150 got empty!

 

I've been extremely happy with my 2019 F150 4x4 EcoBoost gas truck. It's got the highest rated towing package of 13,000lbs. I get 19mpg regularly on the highway and 9-10 pulling my 27ft Seahunt Gamefish. I don't pull it 500 miles or anything but pulling a 21ft bay boat I got 13mpg regularly when towing to Florida.

 

My 3.0L diesel BMW x5 gets 36mpg empty and 13mpg pulling my 26ft bay boat with oversized TTop 6800# at 70mph. Drops to 10mpg at 80mph.

I get 18mpg towing the 21ft bass boat at 70mph.

My towing mileage drops off big time after 70mph.

If you have to do 80mph then you already know your answer.

 

I tow about an 8500lb boat/trailer set up to Destin from Galveston and average 13.4 mpg and empty on the highway get 22 mph. I have an ‘08 2500 Duramax 4x4 all stock with 250k miles. With a newer model that has more horsepower and more gears in tranny I would expect a bit better. Mine is the six speed Allison.

 

The average tow speed is 70ish mph.

 

Already knew Grem was gonna chime in about his Beamer lmao

i get 14-15mpg hauling a 22 Xtreme down the highway at 75mph. Duramax 2500… 13ish if I got a good a head wind. If I kept it at 65-70 I’d get a bit better I’m sure. My dad gets every bit of 8-10mpg with the same boat behind a 5.3 Suburban

Get around the same, maybe a tiny less towing a 26 foot RV with my truck

Empty on the highway, same as HappyPlace… 22-24mpg. LBZ with the 6 speed Allison

 

3.0L baby duramax gets 25+mpg empty and 11-14mpg pulling a 5500lbs camper at 65-70mph. Like several have said your issue is your speed.

 

Lil 3.0 ain't it bub. All gas no brakes isn't gonna help you none either. A Chevy dura is probably your best bet for mileage. The question that's asked the most for hauling anything over 5000# is what's the most reliable truck to haul with. Vs what truck gets the best mpg while towing. Horrible MPG is just apart of the game. Slow down a bit and you might gain 2-4. Me personally if I need a truck to haul heavy trailers, it's the 6.7 HO Cummins all the way.

 

I know I would get better mpg if I slowed down. That isn’t really the question. The slower you go with any vehicle the better the mpgs will be.

 

Your question was somewhat asking why you get poor economy towing with your 3.0 Duramax. Then you said 80mph. That is your problem. Like you can see from the rest of, fuel economy takes a dump at anything over 70mph, and that's even with a 3/4 ton big boy Duramax hauling a pirogue (as Grem likes to think my boat is). If you slow her down to 65-70, I bet you'd see a pretty significant improvement in MPG. I know if I slowed it down I'd probably see 16-17mpg instead of 14-15mpg.. But, driving that slow on the interstate bugs me.. Especially when the truck is fully capable of controlling the trailer/boat at regular highway speed. At 75mph with the boat my truck stays locked in 6th gear up and down hills and bridges without even thinking and still maintains 75mph.

She ain't pretty, but she does honest work, and does it reliably

 

I guess I sort of get your point. Yes I am unhappy with my mpgs towing. I know slowing down would improve them. So would buying a smaller more aerodynamic boat. So would removing my front end replacement, so would removing my mud tires, and on and on.

My question is if I want to tow my Majek 22 xtreme down the interstate at 80 mph with a ranch hand on the front and some mud tires which vehicle is going to get the best mpgs?
I know it ain’t my 3.0 duramax because my 6.7 power stroke beats it. I have a 20 6.7 Cummins but I haven‘t tried it yet. I had an ecoboost and it was the biggest pos I ever owned. Bought it new and got rid of it after four months. Towing an 18’ utility trailer with two ATVs I was getting 4-5 mpgs. Darn thing wouldn’t make it 150 miles on a tank of gas towing.

 

Sounds like you just need to keep towing with the 6.7 and just drive whatever speed makes you/your wallet the happiest... If you're concerned about range more than cost/mile, I'd recommend replacing the stock fuel tank with a bigger one from Titan or S&B. I've got a 60 gallon replacement tank in my 6.7, so even getting 10-12 mpg towing, I can still go 600+ miles between fill-ups.

 

My 6.7 power stroke is a flat bed work truck. It isn’t really practical to use for fishing trips with the family. My 3.0 has a bed cover and everyone can put their luggage etc under the bed cover, out of the weather, locked up, and safe. With a flatbed and four people in the truck everything is riding out in the open.

Basically I am looking at replacing my 3.0 which is my family truck with something else that will get better mpg towing small loads. It doesn’t need to tow heavy. I have the flatbed powerstroke and a Mack with a lowboy if it is really heavy.

 

IMO the only truck/SUV that's going to get better MPG towing than your 1/2 ton diesel will be a 3/4 ton diesel (assuming you'll keep the oversized tires and driving speed). There may be a new 1/2 ton diesel/gasser that's marginally better than the one you have, but not enough to be worth the swap.

 

Speed is your issue. Drag (the primary force your engine has to overcome) is a square function of the velocity. Therefore, the drag at 80 mph is about twice as much as it is at 56 mph. If you're driving into a 15 mph headwind, the drag is an extra 1.4 times higher at 80 (effectively 95 mph). Physics is not kind to fuel mileage. I try not to be in a hurry and drive 60-65 mph to save fuel. Lots safer, too.

 

Again I agree 100% but that is not the question. It is like if someone was asking for advice on warm boots because their feet get cold hunting in Colorado and the replies say don’t hunt in Colorado where it is cold. That doesn’t help the people that are going there regardless.

Anyway I bought a truck last weekend. Not new it is a 5.9 Cummins diesel with a manual transmission. I drove it 1,200 miles over the past two days. Half that without a trailer and half with the Majek. Driving was 80mph the vast majority of the way. Empty it averaged 19.4 mpg and with the boat it averaged 13.2 mpg. I am pleased with it. Empty mileage isn’t as good as the eco diesel but loaded the mileage is much better. My family thinks I am nuts but I enjoy driving this 18 year old truck more than 2 year old high country truck with all the latest gadgets. I still have the high country I will give it a few weeks before I decide to let it go.

 

My point was that slowing down will help all engines perform better when towing. Generally speaking, the more torque the engine can deliver, the better it will perform when towing. The more horsepower, the more quickly you can accelerate. Sounds like the Cummins performed pretty well which isn't surprising.