M818 better fuel mileage

[Wastelander]

So I've been combing through threads and posts looking for some definitive mpg increases and haven't come up with anything concrete. Figured I would try starting a thread where we can really think tank some mods up to help increase the mileage on an 800 series truck with the Cummins 250. I've heard alot of it just is what it is and get a different truck if you want fuel mileage, but that's not good enough, I know some ingenuity from members here can think up something. I'm not talking a fantasy number like 20mpg, but I think 10mpg is a realistic goal for an unloaded truck. Below I've listed some of the more common ideas for improved power and mpg etc. that I've read or heard about, I'd love to hear some qualified experience with these mods or some experienced thoughts. Thanks all, I can't wait to read the replies.

-Dual fuel line split
-Add turbo
-Bobbing the rear axle
-Bigger tires (48's or 53's)
-Front sprague disengage switch
-Bigger fuel button
-Transmission swap for a 10 speed
-Removing a few leaf springs
-Cut down on weight/streamlining (this one was funny to me, there's no streamlining a brick, and my 818 doesn't exactly have alot of unnecessary weight on it)

 

[swbradley1]

A wise old man once told me if you have to ask what the fuel economy is you can't afford to drive it.

The only thing I would want to do on your list is the duel fuel line but that's only to make it idle smoother (for me).

If it needs fuel I put some in it, if it doesn't then I don't. I'm not an OTR driver trying to squeeze another tenth out on a long haul. If you want to do the mods it's your truck, but do them because they are good mods (for you) and not because you want to put lipstick on a pig. Just MO.

 

[wreckerman893]

You can't gold plate a turd. (well you can but then you just have an expensive POS)

They are what they are....tough military vehicles built to haul mixed cargo over various terrains while being driven by people that could care less about fuel mileage and maintenance. And they have the aerodynamics of a concrete block.
Good luck with the quest and keep us informed if you find the Holy Grail.

 

[73m819]

One other thing, about 6 mpg +/- is what you get out of a 855 at higher rpm, it does not matter military or civilian, combined with crap gearing (644 rears and the worse geared transmission in a heavy truck that I have ever driven) equal low mpg, putting "Big" tires on just makes things worse (a MAJOR power suck), adding a 10 speed will help in the driving of the truck, it really needs it) but your mpg will not change much due the the 855 running in its sweet spot (power curve). The military calls for 3 mpg for a loaded 809 series (-10)

As a side note, make sure the front drive shaft and winch drive shaft clear the trans, what is needed is a OLD narrow spicer 8 or10 speed (I sorta been looking for one for my 819.

Just so you do not feel to bad about your 818 fuel mileage, my 819 gets a touch over 3 mpg.

 

[Wildchild467]

I love the sound of those 855 Cummins, but dang, the fuel mileage turns me right off. I love running my deuce in WMO and for me, that makes it worth it. i wouldn't be able to drive it as much as i do if i had to pay for straight diesel. But yea, an 855 with Jake Brakes and straight stacked would be fun to me.

Make sure your truck is tuned up properly. All respect to other peoples opinions on here and i don't disagree with their data. Just make sure your truck is in good working order. I like to say make sure you adjust your valves at the least and if you want to get fancy, have your injectors pop tested to make sure they are in good working order as well. Since you have your injectors out, maybe you can do a leak down test of your cylinders as well. Basically give the thing a tune up. I know deuces a lot more than 5 tons and people with deuces get all sorts of mileages reported. I used to get 7mpg but i had broken rings and blow by, so my engine was not the healthy. I just got done putting new sleeves, pistons, rings and heads on it and I am anxious to see what the mileage is. If i wanted to get the best mileage i would also go about 40-45 mph. That is less wind resistance (even though my top speed is only 50) and the engine is more efficient at those speeds. I don't know the exact difference in fuel mileage for me between say 40 and 50 mph, but it only makes sense that it would be better mileage at slower speeds around 1600-1800 RPM. I hope this helps.

 

[74M35A2]

If you want to double your mileage, sell the 250 engine and install the Cummins 8.3L. Mileage in the M939 series trucks goes from approx 4mpg to 10mpg when they made this change. Not sure of anything else that is able to double your mileage like that.

 

[Wildchild467]

Was the 855 medium duty like the 8.3 is?

 

[MWMULES]

Bigger tires was the first thing I did to replace the NDCC's till I got out west to the flint hill in Kansas, I know everybody thinks KS is flat but it isn't. So going up hill at 35 to 40 bob-tailed was not for me, so sold off the big tires and thanks to BRF went to G177 Goodyear's. Speed stays up, tire to road noise is down and besides changing to HMMWV high back seats was the best mod I have done so far. I am getting the parts together to do the dual fuel line split and maybe have some button work done in the future as I know a couple of owners that have gotten good results with that mod when done right.

 

[73m819]

Was the 855 medium duty like the 8.3 is?

It was/is heavy duty, the same 855 block with some updates could be had from the factory in the 600 hp range for everyday use on the highway, I have heard of 1000 hp being pulled from the 855 block.

 

[NDT]

My M915A1 (essentially the same engine turbocharged to 400 hp) cannot break out of 7 mpg bobtail at 55 mph (2100 RPM). It has no transfer case drag etc. So 10 mpg is hopeless for this engine at freeway speeds.

 

[fuzzytoaster]

Can confirm NDT's post. My m915a1 got 6.5 mpg bobtailing at 60 mph and 4.5 mpg flat towing a 5 ton at 55 mph. I have since stepped up to 395s on it but have not taken it for a long enough drive to get accurate mileage. The mileage is the mileage on these beasts.

 

[ross165123]

Now this won't help with fuel milage but there is a guy who runs 50/50 mix off diesel and wmo. It could help of set the cost for some all that he did was put a m35 pump in the tank.I'll try to find the thread for ya

You could also try an electric rad fan

 

[tim292stro]

For a large truck, with a truck drivetrain, you're going to get low MPG. Look at it this way though: the 1980's Chevy Crew Cab I bought that has a stock 454, got 7.25MPG empty (okay it had a flatbed carrying the original bed, but that bed was empty too ). My 40' Transit bus gets 8MPG...


Most of the aerodynamic things you would have to do would make the truck useless as an M818, or at least unrecognizable... Driving the truck at freeway speeds will increase the drag, you aren't getting around this, so slow down (remember the trucks were built for the expectation they would "get there", not "get there yesterday" like the ones built today are). A tire set with lower rolling resistance and narrower tread face will provide less wind resistance, but also worse traction off-road (serious rutting likely).

Running the engine in the upper bounds of the power band will drink fuel, you'd want to be in the lower 1/3 of the power band for high speed diesels, this entire power band is between approximately 1600-2000 RPM (the lower 1/3 would then be 1600-1750 roughly). The only historical engines I'm aware of that worked well above this is the good 'ol Detroit 2-strokes (which my bus has, 6V92TA). A turbo will help if you see altitudes over 2000', mostly it'll keep you from incompletely burning fuel when the air thins out. You lose about 5% of your total power for every 1000' of altitude increase with no turbo.

Check your fan clutch (or add one if constant direct drive) - a truck radiator fan will use >15HP. If your engine doesn't need all the airflow, the fan can be idled or run at a lower speed (two speed fan drives are out there if you want to adapt them).

Spinning drive line parts that aren't in use wastes energy - if you're doing mostly on-road driving the only axle getting power should be the front-tandem. Some trucks don't easily allow you to disconnect the front-steer axle and rear-tandem axle, and if the wheels spin the differential in the axle housing, that also wastes energy. If you have to remove a drive cap and put in a dummy cap to keep oil in on two axles, that can save you an MPG or so.

Air - diesels are lean burn, that means they need more air than fuel in the cylinders to ensure complete burn. If you're doing any driving that belches black smoke, you're spewing unburned fuel out the stack, and that's where your MPGs are going. Make sure your air intake filter is not adding any more that 6"H[SUP]2[/SUP]O worth of resistance at max power (modern elements are available that flow better and take less space in the housing without affecting the look). Likewise, your exhaust shouldn't put more than 5"H[SUP]2[/SUP]O back-pressure on the engine at max power.

Most of the weight on your truck is frame, engine, transmission and transfer case, and finally axles. Not much you can do there...

 

[73m819]

Driving a 915 pulling a lite box trailer, I got right at 6 mpg doing 70 with a bit of work, tried to suck the bottom, after that 65 with work was all I could do.

 

[red]

I'd start with either pulling the drive shafts to the 2 extra axles or pulling the axle shafts and making blank caps for them, reduce the drag.

From there its getting the rpm's down and the power up so the truck can cruise easier. Dual fuel mod, turbo, fan clutch, air filter mod, fuel adjustments. For the rpm's go with 11" s (actual highway tires if you want to spend the money on new). Then either a trans swap with a better overdrive, or swap to the T1138 Tcase from the M939 series.

With the rpm's below 1800 and those upgrades done, 8-10mpg should be capable.

 

[Wastelander]

I'd start with either pulling the drive shafts to the 2 extra axles or pulling the axle shafts and making blank caps for them, reduce the drag.

From there its getting the rpm's down and the power up so the truck can cruise easier. Dual fuel mod, turbo, fan clutch, air filter mod, fuel adjustments. For the rpm's go with 11" s (actual highway tires if you want to spend the money on new). Then either a trans swap with a better overdrive, or swap to the T1138 Tcase from the M939 series.

With the rpm's below 1800 and those upgrades done, 8-10mpg should be capable.

Thanks Red, this is pretty much what I was looking for. Speed isn't a huge concern for me, although I'd like to be able to cruise at 65 which is why I thought some bigger tires would be best. I live in Spokane, WA and drive the truck once a year to southern CA and back, other than that its a fun truck up here for me that I make a little money with through the winter pulling people out of the snow.

I'm doing the dual fuel line mod, adding a turbo, putting the Michelin 53's on, and the transmission is the only place I was stuck at, wasn't sure what would work best for the eventual swap. I'll look into the fuel and air filter mods, as well as the fan clutch, much appreciated.