So how does this work, in the city which is 70% highway I get at best 15.5 mpg hand calculated, which is babying it...towing a 26' boat this weekend, with a fully packed bed I got 14 mpg.

So my complaint isn't the towing mileage, but rather my completely unloaded, nothing being towed mileage which is only greater by 1.5 mpg. So what gives?

i can get down to 13mpg on the freeway if i dart in and out of traffic, make full throttle runs to pass, ect.

try setting your cruise control between 55-60mph for a length of time and you'll see what you can really get. i average 18mpg on the freeway

I drive ALOT different empty than towing. I dont even realize it, my wife was mad at me the other day because i was driving 'spirited' and she told me "you should hook the trailer up 24/7 and keep it on there, it's the only time you're not sporting around like you're at Indy"... LMAO
I chuckled, and then realized she was right, damnit.
Cruise control = mileage. If i want good mileage i HAVE to use it.
Also, i think the best fuel saving device I installed was gauges... the more boost, the more fuel its using, the higher the EGT the more fuel is getting into the cylinders.

My Highway mileage varies dramatically with speed.

Measured mileage driving 50-100 mile highway trips on fairly flat roads with a few miles of "City" driving at each end

- Steady 60-65 I can average around 19 US mpg

- Steady 65-70 I get down to 18 to 18.5

- Steady 75 it goes down to near 17

Best I've ever seen on the DIC after a good stretch of driving around 60 mph was 21.4 mpg

I don't do enough continuous City driving to get a good result, but I'd be guessing it to be around 16-17.

One other thing, if you get to running up a long grade, even empty, with a 7000 lb truck, you can easily get down under 10mpg for that stretch, and it takes a long time to average back up again.

When I first got my 3500 srw I was getting around 14 mpg around town. I started Now that I am aware of trying to get the best mpg, I can get close to 16 around town try to stay around 55. Its not fun but it works

My last highway trip empty, was 20 MPG USgal through the mountains from BC to Calgary. Loaded of course is "slightly" lower.......13000+lbs 5er is around 9MPG USgal.

i just drove about 900 miles round trip, my highway mileage is always 20.5 mpg at 70mph. i get between 16 to 17 mpg driving in town...... truck is stock...

Put on a tune, mild tunes do the best 40-60 Hp I'll get 1-2 mpg more than stock, also, try a cetane booster in you next fill up.