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If it doesn't sell by this coming weekend, I plan to go look at or possibly buy a 1986 F250 regcab 4x4 with 4spd and the good ol' 6.9. The drive home will be interstate so I figure it's a good time to check my mileage out and see if something is unusual. I'm not sure of the gearing and I know that has alot to do with it but I would like to get an idea of what to look for. I plan on running around 65-70 all the way home ( if I don't get run over first, I75 in GA). thanks for the info and sorry if it's been asked before, I looked and couldn't find any info on mileage.
 

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i have 4.10's and it will cruise at about 65 thats where it likes it. but the only time i am on the interstate is for 3 miles....so i go about 75ish....and it will cut fuel at 77. thats with 33's

i get about 20mpg but most of that is about 55mph out in the country. around town about 15 and interstate i figure about 15. but its a freshmotor and injectors and ip...
 

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Joe Boyer
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I have a 90 7.3 dually non turbo the best I ever got was 16 on the highway in town 11. Geared 4.10
 

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Grumpy Old Member
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Mine is a 4x2 with 3:55 rear gears, 4 spd manual and 4th is OD, running 55 I can get in the low 20s normal running is 17-18, the worst I got was 13 pulling 12,000 lbs of hay running 75 & 80 on the interstate.
 

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seems these engines vary quite a bit with milage......does that have to do alot with the injection components and how tired the engine is??

i have seen a truck like mine getting 12 all the time.....but mine does good...
 

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Still Diggin
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It seems around here how tired the injection system is can vary mpg a lot. My 94 factory turbo has 4:10, 5 speed, and 4x4. Ideling around i get around 15. Out on the highway about 17/18. Thats with an easy right foot though.
 

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I've seen as low as 15mpg, and as high as 19.7mpg. This is at 60mph or there abouts as a max speed. These trucks love it below 60mph, trust me. I did two tanks at 60mpg, and it neted me 17.8mpg, and then I dropped to 55mph for the next two tanks, and drove real slow, and I got 19.7mpg. That is all backroads twisty/hilly roads with no real flat ground straight ways.

Anything over 60mph, and your fuel milage will drop real fast.....at 70mph, I wouldnt be suprised if it got 12mpg if it had 3.54's in it, with 4.10's I couldnt even image the fuel milage. These front ends arent really aerodynamic, more like bricks.
 

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FordGuy100 said:
Anything over 60mph, and your fuel milage will drop real fast.....at 70mph, I wouldnt be suprised if it got 12mpg if it had 3.54's in it, with 4.10's I couldnt even image the fuel milage. These front ends arent really aerodynamic, more like bricks.
Explains why I get about 13 and on a good tank, I pull 14. Maybe Ill try the 60mph max and see what happens...
 

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I used to have a '86 F250 w/ 6.9L. Had 3.55 gears, and got 15mpg (uncorrected for the 255 rubber vs the stock 325s).

It liked 60mph+ the best.
 

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mine likes 75 but i got nearly 18mpg at that speed. But a little slower is good. i can cruise at 90 but thats like 10psi of boost and 1000* on the pyro. i have 37's and 4.10 gears with a 5spd. In town i normally see less than 15 though right foot pending.
 

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I have been reading this thread. The one thing I’m seeing is no one is telling what the RPM are for their best mileage. Gears are OK but the tire size can change the gearing. I would think what is needed the most is RPM+MPH=MPG
 

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What will net you the best speed will the something like 60mph at 1400rpm's. I have no idea how you would get this done, probably a ZF and a gear vendors, but my guess would be this would net the best fuel economy. I would also think that the best fuel economy from your truck would come from being in the highest gear, and being a couple hundred rpm's above lugging it, remember these are some real low torque monsters, so if you could cruies 60mph at 1400rpm's it would be just about perfect.
 

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Intrailhunter said:
I have been reading this thread. The one thing I’m seeing is no one is telling what the RPM are for their best mileage. Gears are OK but the tire size can change the gearing. I would think what is needed the most is RPM+MPH=MPG
IMO,road speed is a bigger contributor to MPG than RPM in a full size pickup,esp the 4x4s. This translates into engine load,which you can best see in the pyrometer.Keeping the pyro under 600 at all times is how you will get the best MPG,choose whatever road speed,and gear you need to,but go over 600 degrees,and MPG drops a lot.

While RPM is important,the hp it takes to go say 60 mph in an F250 doubles at 70 mph.It doesnt matter as much how much or little you rev the diesel from idle to hi idle,it needs to burn xx amount of fuel to make the HP to push the truck thru the air. Just a FYI, with my Dodge/CumminsI have gotten 14 mpg highway towing my 10K utility trailer loaded with steel ( 18,000 GCWR)down the interstate at 70mph(2100RPM). The next month I tow my travel trailer (8000lbs) 15,600GCWR at 65 mph (1950RPM),and only get 10-11 mpg.Its the wind resistance putting the load on the engine more so than RPM.

My guess is you want to run around 1800 rpm at your cruise speed for best MPG with a v8 diesel. The 12V cummins likes 1600RPM,and the 24Vs like 1800 as well.In most cases the money spent regearing the truck or adding a GV is never recouped in fuel savings,it just makes the owner feel better by not revving the truck as high.
 

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Grumpy Old Member
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Let's do the math (my math may seem strange but it works)
20 gal tank
$3.25 per gallon
15 mpg
=300 miles per tank
spend $2500.00 to get 3 more mpg
+360 miles per tank or equivelent of having 4 more gallons saving $13.00 per tank.
2500 divided by 13 = 192 fill ups to recoup cost
Nope don't work out too well!
 

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John DiMatino Well said.

It is not just one thing but a lot of little things that make up the MPG. I should had put that in my posting there were more to getting good fuel mileage then RPM+MPH=MPG. It is not so much the MPH but the drag though the air. I have seen a motor turning at more RPM’s getting better MPG. It is mostly the lugging of the motor that can get worse MPG then a motor turning higher not lugging.

John Great point on the pyro reading that is importation.

Maybe with it hard to measure the drag on a truck we would say most are trucks would have the same drag would be close to the same. With that said then maybe MPG=RPM+MPH+PYRO for a better comparison?

ChobeeMan It took me awhile but I see what you mean.

I am going to change my gearing and I want the best MPG to pull at 70MPH for my bus with GVW @ 15000-18000. Now I have 5.38 and I can’t run but 60-65 max RPM at 3200-3300. I’m looking to 4.10 but thought about 3.73 but that might be lugging the motor to much and pulling the mountains the 3.73 might be too high 2249 RPM, 4.10 is 2473 RPM for 70MPH. Gearing is importation to me at this time
 

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My truck gets 15mpg and it has 3:55 gears
 

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Howdy!, gearing has an importation to mileage, I get 20 mpg at 60, at2000,with 3,08 gear unloaded not pulling. Tonneaucover reduces air drag, but frontarea is the factor!. Correct ip timing, fresh air filter and drive a sunny day w. wind in the back helps!. Here in sweden diesel cost 8,34/gal.,90% is procentual environmental taxes!, a truckdriver make 28000$/year after tax!. Fueltheft is common. a better way is slipstreaming! behind a bigger truck!.
 

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singles club 213!!!
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well then i dont know my gearing i think its around 4:10 at 55 im running 2400 rpm and a c6 tranny i only drive in town @ 14 the best 10 usual what would be the reason for the low mileage i have turned my injectors down so it dont get over 750 egt usually i dont push it to hard unless i got a trailer
 

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wow. I thought the 7.3's and the 6.9's got better then that. i thought they got around 22 mpg. Im probally thinking of the 6.2's and 6.5's then. I think those get around 22ish.
 

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newer powerstroke 7.3s got upwards 20mpg... but not the old timers.... at least not the majority. Some rogue trucks get 18mpg regularly...
 
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