I get 14-15 at 65 with my 6.9. My 7.5 gasser would only get 12 at 60 down hill with a tail wind. I'm trying to figure out how much I want to spend on it. I found that Gear Venders does have an overdrive unit for mine that goes between the trans and transfer case. Probably means drive shaft mods though.

 

My 86 F250 351w gets 10mpg to get anything higher you would need to tow it or push it off a cliff.
Just bought a 93 7.3 5sp 4x4 if it gets 16 or better when tuned up I will be thrilled.

 

I have an ole 79 3/4 ton gasser that gets pretty milage. It gets 15 to 17 but it did take some work to get it there. It has a t-18 and 3.55's.

 

Could get better mpg out of the 351w but it was the last year before fuel injection and all the pollution control equipment is rotted.Biannual testing/inspection prevents ripping it all off and doing it right.Bet with all the vacuum leaks,air pumps,holley carb and egr removed and edelbrock manifold/carb,headers and new distributor the old girl would run cleaner and more efficent.

 

Just a tid bit of information from west coast differential (differentials.com) They don't list it on the website catalog but they told me they have a 3.23 option for the D60. (matching the option in the D44, which many of the 4x4s came with) Not a big difference over the 3.54 but every little bit can help... with 32" tires @ 70MPH would drop RPMs from 2602 to 2374.

http://www.gearvendors.com/f2wdt.html kick ***. Eventually I would like to get one, but for me they cost more than everything I have spent on my truck to date!

 

Well, it seems that some people must have found the legendary fountain of self-replicating fuel, or have a magic calculator. In 20-plus years of owning the same truck and owning it since it was practically new, I never approached those numbers no matter how much I feather-footed it or how slowly I drove it. I wonder what I'm doing wrong?

 

Jim, those 4.10 gears are costing you. C6 and 4.10s are about as bullet proof as you can get, but I never got more than 13 MPG with that setup. You need to bring the RPMs down, it remains to be seen if the original post is just a fluke of the filler nozzle shutting off too soon, or if it is in fact an average MPG. But 20 MPG is possible with a 6.9 if its setup right.

 

David: Even a steady 50 mph, on cruise control, and on flat ground did not yield more than 17-18 with some pretty sophisticated (for the early '90s) testing tools attached. And the truck had about 35K miles on it! It's not all rpms either. Air drag is a lot less at 50mph than at 60 or 70 and the engine is not working hard.

I can believe 20 is possible with a 2wd, tall gears, overdrive and careful driving. Above that I reserve judgement or, when I'm in a bad mood, shout BOVINE SCATOLOGY.

The laws of physics are at work here. There is only so much work that can be pulled from a gallon of diesel fuel in a 6K pound truck with a nearly 7 liter engine that has an antiquated and fairly inaccurate fuel system. There is a tendency for people to take a single, mpg check that shows some fantastical mpg and wave it around like a battle flag. Like you said, early nozzle shut off (the only accurate way to be consistent is to full to the brim of the filler, which everyone who owns one of these truck knows takes about 20 minutes), inaccurate speedos (normal error but also often tire size variances) or good old math errors can throw the numbers off. In a few cases, there is also a "mine's bigger" attitude at work. I'm immersed in cars and trucks 24/7, and have been for almost 40 years, so have developed a pretty good BS detector for what possible or likely and what's not. Ringalingalingaling!

Too pooped to put on my flame suit so I'll just use the flames as a depilatory and get rid of some excess nose and body hair!

 

You could get good milage out of those old gassers if they were built right. I got great milage out of mine but a guy I worked with at the time did better. He got 17 to 19. The thing was is that these were not stock. Mine was hot but his was built. He had a 351w in a half ton pick up. The motor came out of a Jeep he had ran in the mud. It had a rollercam, roller rockers, big valve light valve, springs to match, aluminum rods the works. A lagit 400 hp motor. When he put it in the truck he went back to a small carb. Just cause the motor could take the added fuel he didn't put it in it. He had a gas analizer and we could tweak the carbs so they were fuel just right. He did have to change his cam to a lower duration because it would get to lean. Basilcy biuld a hotrod motor and but don't give it the fuel.

It's like this. These numbers examples. Lets say you have a 100hp motor. Stock it takes 50% to run itself. Drag from cam, pistonings , valve train ect. When you build a hotrod motor to get power you have to increase volumetric efficiency. Reduce drag ect. Rollercam, light valves so you can run lighter valve springs which take less effort to open. Those are examples.
Now you have a motor that uses 25%. More efficient.

 

I usually get 20 when I drive to Pittsburgh (lots of hills), but I got 22 on a trip to South Carolina. Daily driving gets 16-17 if I baby it.

 

I've been reading this thread and biting my tongue, but I'm with Jim Allen on this one. 21 mpg with axle gearing that has the engine howling at anything above 55 mph on the highway just ain't gonna' do it. And that is pretty much what he described in the first post. On top of that he says straight highway driving only accounts for 10% of his driving. So 21 mpg in mostly city driving? Nope, sorry, I just can't buy that.

The NA that I just sold/traded had 3.55 grears and a C6 and it purred along at about 2500 RPMs doing 65 mph. Sounded good too - certainly not "angry" - and no matter how easy I drove it, in over a year and about 18k miles, I never got better than 17 mpg running straight highway miles - and more like 15 mpg in mixed driving most of the time. Sure taller axle gearing, or even a manual tranny will help a some, but from his description it doesn't sound like he's got the axle gears, though he may have the manual tranny.

I'm afraid this is a simple case of miscalculation due to a speedo that isn't matched to the axle ratio, or failure to get the tank full when fueling.

 

Best I've ever done running highway was 22.5 mpg on the US gallon, that was running 33's and 55 mph(3.55 gears,) filled at the same pump to the same level, with a GPS to track the mileage, with the 33's on, my odometer is reasonably accurate, but, I carry the GPS all the time anyway(BTW, with the stock size tires, both the speedometer and odometer are optimistic.) That's 27 mpg on the imperial gallon though... LOL
Running around town, and some highway mixed, typical driving for me, I average around 14-15 mpg on the US gallon, but, I allow a good amount of idling for warm up time, and I like watching the black smoke roll and listening to the turbo scream... LOL

 

I can believe the higher milage in town then on the interstate IF. He has tall gears like my 2x4 83 had and he lives in a small town. If he drives like me in town my truck never gets over 1500 Rpm because I idle around town. Take off in second gear and so on. I live in a small town. I couldn't do that in a big city.

My brother has a 2000 350 PS with 410's and he gets awful milage out on the interstate because just keeping up with traffic the motor is screaming but around town he drives easy and the motor never gets wound up tight. Thats small town living and ALOT on how a person drives.

20 to 21 can be achieved but not by me anymore. My ole 6.9 has 3.55 gears now and is a 4x4 but I am not getting of my rear end with airplain gears. I may need it one day.

 

Well, I was floored when I first took to the freeway after ditching the 4.10s for the 3.08s, then went goofy nuts when I got the E4OD setup properly. Still not sure how to finish the 4wd conversion though. I don't need to tow much either.

 

The nay-sayers are admitting in their posts that they never got better than 17 or 18 miles/gallon. If you ask me, that's awful close to 20 already. Fuel additives adding btu value to the fuel may be some of the difference.

I have one of the worst combinations, with a 89 7.3 NA, 4x4, 4.10 gears and a lift with 35 inch tires. The guy I bought the truck from said he got 12mpg. I have already made a difference I can feel by pumping the tires up to 50psi, taking the soup bowel out, and putting new bushings in the frontend. I need to fill up the tank and see what I am getting now.