The 265/75R16 is the only sized tire I have ever run. It took me a year to find the right truck, so I was not going to fuss over the 3.73 gearing even though as a tow pig it would be better suited with a 4.30 or 4.10 gear set.

Based on what I have learned from the experiences of others, going with a larger tire must be met in kind with a gear set change or the appropriate gear set already installed.

I don't see myself getting any better fuel mileage out of a 22 year old diesel towing a 5th wheel. So, based on my experience and the lessons learned from others, I would answer your question with a "yes" when fuel efficiency is the primary point of data logging and or measurement.

 

I’m late to the party but want to reiterate a couple of points.

Your MPG is actually better than you think. As exbxtoy said, you will need to use a corrected mileage value when calculating the MPG. Distance actually travelled is further than reported on your odo.

I have the same issue but on a smaller scale. My tires are about an 1” oversized (32”?) and I do not correct for it. In the past I have seen around 10mpg average when towing my enclosed car trailer. Empty I’ll see 15-17, the 17 will be under longer more steady driving conditions.

Your tire size is also not helping anything when it comes to rpm during tow. It’s forcing you to downshift sooner. 3.73 is already a little high geared for towing in OD, add to that the tire factor and you’re rolling low rpm on the highway.

Now top that off with the 369 which will want extra rpm. It will also force you to downshift sooner to keep the turbo happy. I’d say you’re rarely in the efficiency zone of the turbo while towing. But I bet that thing is fun to drive!

 

I did think about the odo reading vs actual miles on the final (320 mile) leg of the way home due to tire size. Odometer read 300 miles, but I used the 320 miles to calc the 10mpg. Using this 6.25% difference and applying it to the other sets of data with mileage reported (which didn't take into account GPS route distance) , the mpg for the different parts of my trip would be more like the following:

San Jose, CA to Bend, OR - 80 Eco Tune - Not Towing - 65-75 MPH = 12.75mpg
Bend, OR to Weed, CA - Modified Stock Tune but Mostly 40Tow Tune - Towing 3K lbs - 55-70mph = 11.7mpg
Weed, CA to San Jose, CA - 80 Tow Tune - Towing 3k lbs - 55-70mph = 10mpg

And oh yes, the truck is VERY fun to drive ! = )

Perhaps a compound turbo to help this big one on the low end would be a good idea, haha. Not needed I know.

I appreciate everyones input. Happy to know I am doing average or even a little above average on the mpg.

Consider my expectations adjusted.

Looking forward to towing this trailer loaded to MT in August. I can report back here on that trip as well if anyone is interested. I likely stay more reasonable with my towing speeds too.

 

I have learned that enclosed trailers and speed make a bigger difference on MPG than weight.

If I can keep it down to 55 I get decent mileage. That slow on a 4r100 I'm out of OD the whole time.
(3.73 and stock-ish tires)
I recently did 300 miles on country roads (55-60 towing 10k) and got about 15 mpg.

On the interstate I try to keep it at 70mph in OD because the RPM's are above 2k. I can pull most hills around here under 1100 and without downshifting. Almost always 10 mpg.

 

I use my APPS around 2700 RPM on hill climbs for max GPH

 

i have an e99 f350 manual trans, drw with 3.73 gears on stock tires. i have never considered larger tires as i wish i had 4.10 gears again. in my old '95 psd, with 4.10's i did get a little better mpg's towing but less empty. the best my drw has ever gotten was 18 mpg, in stock form under the hood, with a dp tuner in 80 dd mode. that was maybe 3 times, in the absolute perfect conditions, which is rare around here at 70 to 75 mph. normally ( on good days) the drw only gets 15/16 mpg empty, at 70/ 75 with or with out ac on. so the interesting part about towing is the different sized trailers i have or have had and towed. i have an 8.5x12 single axle utility trailer that only weighs 1300 lbs empty. it has a lift ramp gate made out of steel mesh and some 2x2 square tube. if that gate is up, it drops mpg's to 12/13 ( with a 0 turn lawn mower on it) and you can literally feel the wind tugging on the gate no matter which vehicle i tow it with. i had a 8.5 x 22 flat bed weighed in at 2500 ish lbs, (according to the manufacturer) with stow away ramps would drop me to 13/14 with nothing on it. on 4.5 hr trips, they both drop my mpg's , at 75 mph down the innerstate. now i do have a drw, more drag both in body shape and tires. i recently added a topper and where i do like the old man topper for keeping things dry i can feel the side wind grab the truck more, not a surprise either.

i have towed a 36' GN with a 6" taller roof ( 9' 6" to ac on height) that weighed in at 13,200 loaded 9 to 9.5 mpg, a 34' fifthweel toyhauler with a 12'6" height that weighed apprx 11,500 loaded 8-9 mpg, a genesis supreme overnighter toyhualer,11' tall , 8500 ish lbs loaded (31' oal but only 12' of camper side wall) 9.5 to 10.2 !! mpg ( felt like doing the happy dance at every fill up ) and my current 26' bumper hitch toyhauler (30' oal) , apprx 10,500 loaded and also 12'6" tall. the 36' GN towed the easiest with the best mpg's vs the fiver and current bumper hitch. the genesis surpreme overnighter toyhauler towed the easiest overall with the best towing mpg's 9.5 to 10.2 mpg. i live at 1440' elevation but for some reason towing that fiver across wyoming at 75 mph, hovering between 5500' to 7000' yielded the best mpg's. saw 3 fill ups at 12 mpg from pine bluffs to rock srpings wyoming on 2 diff trips, no idea why, must have been the favorable wind but the largest hills/pass's.

i say all this as wind resistance is the #1 killer on mpg's in my case. i swapped to a hydra tuner, 205/30 hybrid inj's and a 366/73 .91exh turbo. didn't loose any empty mpg's at 75 mph but the turbo was too big for my application and had a tendency to lug under 2k rpm which reduced mpg's. in 6th gear at 60-65 mph thats 1750 to 1800 rpm and that turbo just wasn't in its efficiency range and would lug the engine and raise egt's. i have since switched to a 363/68 .91 exh turbo and that brought the lower rpm mpg's back. i recently made a trip to mt rushmore, towing the current toyhauler which was 9 hrs each way. hovered around 8.5 to 9 mpg never exceeded 25 psi at most egt's well under 1000* with a few peaks to 1200* on tall hills. return trip was a diff story, only one fill up dipping to 7.20 mpg. that is an all time record for poor mpg's for myself but not by a huge amount. i ran into 40 mph gusts and wound up on a very hilly stretch of road for 2.5 hrs, one that campers have no business on, imo. when i say hilly, according to the road signs there were 4-6% grades i pulled along with the other hills. when the wind turned and went dead on into a head wind the truck required apprx 10 psi more boost to maintain. actually saw 30 ish psi several times along that stretch or road. i do not dwell on mpg's but i do watch them just because.

in your case, only imo, i think those larger tires and that turbo are hurting your mpg's but all in all, your not that far off imo.

fifthwheel



genesis surpreme overnighter and best mpg's




current toyhauler. i thought the sloped forehead would help cut wind, i now believe that it produces more down force then cuts wind. just my opinion though.

 

my 2006 went from 10-12 mpg towing to about 5! Any ideas what could cause that?

 

You might have better luck asking about a 6.0 in the proper forum. But, a few here have experience with the platform.

 

The engine is working to overcome the air drag of your truck and the enclosed trailer and the drag increases with the square of your speed. At 75 mph it is 4x as great as at 37.5 mph. Most truckers with their big rigs get 6-7 mpg but the really good drivers get 9-10 mpg with the same rig on the same route.

Head winds or cross winds (two thirds the vector force of a head wind) are also a factor. With my previous truck and a big slide in cabover camper in the bed I would get about 13.5 mpg at 65-70 mph. With a 10 mph headwind if I was driving at that 70 mph the fuel economy would drop to 11 mpg. I could either slow down or plan on stopping sooner for more diesel.

Tuners can change the power curve but they will not increase real world miles per gallon. Most of the ones I have looked at do not mention improved fuel economy as they know better and don't want angry customers.