I am currently on my first RV trip with this truck. I am towing a 25' travel trailer weighing about 7000 lbs. My fuel mileage is 10.3 MPG running 62-65 MPH. This is unacceptable. The truck is a 1998.5 auto with 3.55 gears and LT265/75 tires. The truck has a shell on it and is otherwise stock with 102K miles. If i cannot find a cost effective soulution to this fuel mileage problem this truck will have to go. This truck is replacing a first gen that got 13.5 MPG while towing the same trailer to the same locations

 

Welcome to the site... You might want to fill in you signature in UserCP...

I would start with basics first...

Check fuel pressure & change fuel filter.

Check your air filter and replace it if needed.

Check you IAT / MAP sensors for sooting.

Check Tire pressures.

Check for codes.

Does the truck have gauges in it?
If not I highly suggest you get some! It will answer a lot of questions quickly with the numbers from Fuel Pressure, Boost Pressure, and Pyro Temps.

 

My trailer is a 25' and is a little heavier. Mine is also a 5 speed. When stock, I bumped out 12.5 to 16 MPG depending on the terrain. Staying below 65 made a lot of difference. If your truck came with 245's and your running 265's, you will be going about 5% faster then your speedo reads and will need to adjust your miles when figuring MPG.
Have you checked everything to be sure you have no problems with the truck? Fuel filter, tire pressure, etc... Do you have gages to tell if your LP and turbo is working properly?

 

That's better than a gas big block!

 

what are your rpm's. do you have an auto trans. I also have an 98.5 and mpg towing a 24'tt are usualy between 10.5 and 13. our 94 with 350k on it gets 19 pulling 5 horses. It is also a 5 speed. IMO the 24v can't compair with the 12v in mileage.

 

My boss gets 5.5mpg with his GMC (6.0L V-8), pulling his 30' toybox.

 

Mine gets 10 pulling 30ft/10,000..... my old gasser is not open for discussion

 

I have the same truck but have 245"s on it, have a few other goodies. I tow a 7500lbs 25ft 5er and get 13-15 imperial. Where I live you must go up a nasty hill to get out of town, hence the 13 on local trips. But 15+ is my average on a long trip and I have a heavy foot. Best tank was 17. My buddys V10 get less than 10mpg imperial and he claps and dances at anything over that empty!!!
Bob

 

That must be an interesting sight!

Unless you are towing many miles per year, and driving many more than that, you will likley loose more money in a vechile trade than you are loosing towing at 10.5 mpg. In addition to what has already bees suggested, check (or just replace) your air filter as well. Then it's time to stop and consider your load. What could you do to lighten it some what? I don't know what you pull, but if it's landscape equipment, could you take along one less heavy-duty mower each day? If it's tools, could you reduce your supply on the rig by 50% and still complete your jobs? Thjese are simply personal observations, I know that tools wind up being stored where they get used - even if it's only once a year! Perhaps there are ways to lighten what you are carrying and therefore raise your mileage a bit.

Overall, trading in a truck for a mileage imporvement is a loosing battle. With depreciation factored
in, and possible repairs needed with the new unit, you will likely loose more money trading it for another one. What other truck will pull that sort of load and get better mileage anyway?

 

i really dont know why your complaining, those numbers aren't that bad for pull 7g's...try that in a gasser and u will get 6-7 mpg.

 

13.5 towing a 8700# trailer @ 80-85 mph (auto-3.55's 2200 rpm)

these trucks seem to have a sweet spot (rpm) find it and you will improve your mpg by 2-3 mpg

going 65 in mine @ 1800 rpm i get 12-13 mpg
going 75 @ 2000 rpm it is about 11 mpg

 

yeah i gain 1-2 mpg when i tow stuff between 75-80 mph usally there are sweet spots on these trucks just have to find them and work around it.. the bigger your turbo the better milage u will get from high speed miles if that what you do more of....there was a website a while back where some guy averaged 14-16mpg towing a trailer in stock form and went and got a twin turbo kit and averaged 20-24mpg on the highway but his city miles shot way down near 9-10mpg on the same trailer. my suggestion is if you do a lot of high speed highway driving get a bigger turbo or twins and you will make up for the price of upgrading but if you do a lot of stop and go traffic the smaller more faster variable speed turbos help you out there on your fuel miles

 

Like some of the others said, slower does not mean better fuel mileage... Hell, my gasser tow rig got 2 mpg better mileage at 3,400 rpm in 3rd (drive) than it did at 2,800 rpm in 4th (OD).

Your optimum mileage with the auto is going to be in the ball park of 1,800-2,000 rpm WITH the converter locked... At 60-65 you're only in the 1,500-1,700 rpm range... This is below the peak power/torque curve on a 24v and using unnecessary fuel to maintain speed... Given there's no mechanical variables, bump it up to 70 and you WILL see an increased MPG. If the tranny's hunting a lot (locked to unlocked) you might consider a manual lock-up switch.

Replace your fuel and air filters and perform a lift pump pressure test.

Last but not least, adding an EZ makes a ridiculous difference when towing... Towing at the speeds you mentioned, you won't believe it's the same truck.

 

Gyro, what are you getting from fuel mileage when your empty