M1008 Fuel Mileage

So after owning several cucvs in the last few years, My most recent being an M1008 with the th400 and stock 235's. It was getting about 12mpg towing and 14mpg with normal driving in town. I did an auto overdrive swap on it from parts I had from a diesel blazer. The truck gained highway drive ability and all around better cruising. But the best I ever got with that was 17mpg. After awhile I noticed the input sprag was starting to let go in the trans so in went a NV4500 from a 93 6.5 Diesel truck. fully rebuilt, all new install parts etc. Now with the 5 speed I get a consistent 15mpg unless i'm towing then it drops down to 11-12mpg. The truck now has a 2" skyjacker and 33's with the 5 speed. I guess my question is how are some guys getting numbers like me and others claim they are getting 25mpg with a lifted M1008? A guy almost local to me with an M1008 for sale, lifted 4" on 35s, with a nv4500 swap also claims hes getting 25mpg.

Claims and facts aren't the same. Many folks exaggerate the mileage of a 6.2. My recent trip north in an 8500# SECM towing a half loaded (weight wise) M101 netted as low as 10.5 and one tank over 12. Truck has 235 tires but an SM465 and 4.10 gears. 400 turbo to SM made no nnoticeable difference but did pick up a couple of miles per hour. Sorry to all my friends here but I don't believe for a second many of the MPG claims. Perhaps if I ran 55 it may have helped but 70 mph was how I drove.

Must be exaggerations then. I do have a IP shaft seal leak when the truck is cold, it goes away after a few minutes but there's no way I'd gain 10mpg by fixing that. Every time I see people saying 20mpg or more I was starting to believe I had something wrong. I installed new bosch injectors recently which made a power and starting difference but nothing in the MPG department. Even with the NV4500 swap I still usually drive no more than 65mph.

Well I can't speak for others but I get a solid 18 doing all around driving.
Nothing over 55
Stock 1008 with 235 tires.
Almost 84k on the ticker.

If I cruise at 45 and not much stop and go, It gets right at 20
Headwind and loaded in back will knock that down of course.

I check mileage at every tankful

I do add one quart of two-cycle oil when I fill the tank.
Seems to help a lot.
Definitely smoother and quieter

It is what it is
believe it or not.

I drive my M1028, (648miles) per month on the highway, 31" tall tires, 456 gears, Banks sidewinder turbo, Gear vendors overdrive. Strangely enough it gets EXACTLY 12.84 MPG, loaded, or unloaded, driving 70-75 miles per hour.

I get about 10 MPG at most with all stock on Big Red M1008. I do tend to bury the speedometer. It is about 75-78 MPH on interstate I 81 , I 83, and I 78. It is a grade finder. It is speeding ticket proof on the interstate at this speed. But my right leg does get tired from keeping it floored. I burnt thru a tank of fuel yesterday in a couple hours of I 78 east and west driving. Could have taken a newer truck but someone else was asking when I would be back. With this truck they are glad I am gone. They hate it. That's why I like it. I never have to return it for the next guy. It just stays with me. 0600-1700 M-F. I don't miss driving it on weekends. I drive the Mule. Much faster and more efficient.

It is all about the speed in my opinion. I have 3 square bodies on the road everyday.

The Blue Bomber USAF Suburban has a speedometer which is right on according to GPS and the interstate mile markers. 6.2/TH400 if I keep it at 60-62 on the interstate full tank, as in liquid fuel sitting right at the lip of the fuel inlet, to full tank it gets 18.4-18.8 with 3.42 gears. Just think about running 65 and I loose 2 mpg. Driving it daily in town to and from work with speeds no faster than 45 mph but lots of a/c and it averages 14.5-15.4.

My M1009 with the stock sized tires that looked like they would blow apart any second never got run over 60 mph. Using GPS for speed and as my distance log it would get 22 most tanks on the highway with a best of 23.3 based on the mile markers for an interstate trip. I put 33x12.50 tires on it and never saw better than 20.3. However, I was running 60-65 most of the time then with a few tanks in the 68-70 range. All getting 19.5 or better. Then I put in a new IP. I haven't seen better than 18 since. But, I sure have more power and find it hard to stay below 70 in our 75 and 80 mph speed limit areas around here.

Colton has been driving his M1009, RED, daily for almost 2 years. 18.4 is the best ever recorded and that was run 60 or below on the highway due to a vibration issue. However, it has lots and lots more power at all speeds than my other square body trucks. It says 6.2L on the block, but I still wonder if it isn't running 6.5 pistons or something. Maybe 3.42 axles or something.

Both M1009 trucks have speedometers that are off. Mine is 12% slow on speed and 6% slow on distance. RED is 14 percent fast on speed and right on for distance. How many people even know if their speedometer and odometer are off when they make mileage claims?


My M715 with 5.87 gears, 38" tires and a NV4500 behind the HMMWV pull out 6.2 gets 14.3-15.6 every tank no matter how much work around the land it did. However, I can't drive it faster than 56 mph due to the NP200 transfer case getting hot on the highway which is right at exactly 2,000 rpm. Put a trailer on it or weight in the back and it drops to 11.1-12.4.


Why compare apples and oranges? Because each of my trucks is different and each has a pattern I have figured out. That is what that truck does with the current set up. If I change something like tires, speed, IP or load, then the pattern changes. Even the in theory exactly the same trucks are different by a good bit. The M1008 trucks are no different. Each will vary in performance from another truck seemingly exactly the same. Speed is the one constant I have found though. The faster I go, the more fuel I use. Figure out your pattern, insure the truck is staying in that pattern for maintenance reasons and if you want better mpg, drive slower on the highway. Any kind of stop and go driving will knock just about any 6.2 square body down to the low teens mpg wise too. These are heavy trucks that need a lot of energy to get rolling.

thanks for the feedback guys. I never thought of the speedos being off. I do use a garmin 64st to calculate my mileage as I've never owned a squarebody with a speedo that was 100% correct. The more people give input here, it seems like no one is actually achieving this 25mpg milestone with an M1008 and 20mpg maybe possible with stock tires and slower driving. I have a tach in the truck that usually sits around 2100-2200rpm at the 62-65mph mark. I usually drive 62mph as that is 100km/h.

I've seen alot of wishful thinking mileage during my research and have decided that anything over 20mpg is unreachable unless you have a 2wd 1/2 ton pickup with the right gearing and a 700r4. My 87 suburban 4inch/37 tires th400 get 14 mpg calculated. My m1008 4 inch/33 tires th400 gets 15-16 max. My 83 k20 stock with 700r4 tranny (from factory) gets 19 mpg on freeway and I have a canopy and 500lbs in bed at that time.

Of course you can add a NV4500 or a newer O/D auto and you might gain some MPG, but the bottom line is you will still have that extra weight and drag of a lift/bigger tires, etc.

The only time I could see someone netting over 20 mpg even close to 30 mpg would be with a cummins swap.

Don't pay attention to the old window EPA Mpg stickers from the 80's. That is way off, as they are testing in ideal conditions and maybe even going down hill slightly. Our roads are never ideal.

Here is some data I have -

1986 M1028
Stock 6.2L diesel
Stock TH400 transmission
Stock NP208 transfer case
Stock 235/85R16 tires

110 fill ups
1296.4 gallons
$4,457.22
Across 18,499.7 miles
High 17.16 MPG
Low 10.84 MPG
Average 14.57 MPG

I'd say highway is almost all either 55 MPH at 2800 RPM (or thereabouts)
City is speed limit (25, 40, 45, etc), stop and go, not driving crazy.

wow, now thats some record keeping.

welpro222 said:

wow, now thats some record keeping.

Click to expand...

Doesn't everybody do that?

I should note that when I get fuel, I fill up the tank unless noted. I think a lot of stations in Arizona have B20 biodiesel, so there may be a lot of that in there.

I get about 15-16mpg with a 700R4 OD and 37" tires. That is cruising about 60-65 on mountain highways. Also the truck has 4.88 gears.

i have 3.73 gearing in my m1008, hmmm a 700r4 in my m1008 might not be a bad idea?

16 mpg hwy
12 city
5 inch lift,39 inch tires, 4.56 gears, 4l80e

I get about 10-14 depending on how I drive. I do very little stop and go being out in the middle of nowhere.
But I run that truck hard though, 65+ is very common for me. I run 285/75-r16 federal mid tires on all four corners and aluminum wheels. I have seen 15-18 with very easy driving running 45-55 mph if I keep my foot out of it which I don't usually don't do. Also I have been getting the 10-14 since I had the IP rebuilt, new tank, lift pump, and spin on filter. The truck has plenty of power for what it is and loves to go fast. I find it hard not run 60+ in the green machine. (Maybe I should listen to slower music?) Which is why I'm getting low mpgs. But I can say at 75 you can almost watch the gauge fall. These big brick shaped trucks like their go go juice at those high speeds.

I get whatever I get, I love driving my M1009 so much that I don't even check my fuel mileage anymore. I had a 2003 VW Jetta tdi that got 50+ highway and 45 city but it was no where as fun to drive as my CUCV.

I was thinking bigger 33" tires mite help me get better MPG on the highway. But after reading this thread it seems that theory is out the window ....(M1028, Banks turbo, Gear vendors O.D., stock 4.56 gears, 31" tires)..

I run stock except for a 3.5 inch lift and 37 inch tires, 12 in the city and 15-16 on the highway at 60-62 MPH on my GPS, 52-54 on my speedo.

33x12.50 tires cost mpg's. A few people have put on 33x10.50 tires. They don't loose as much mpg.

Something else brought up that I didn't mention was the biodiesel and ULSDiesel. My mpg drop off for my M1009 happened at the same time I got 33x12.5 tires and that was within a month of biodiesel being everywhere I go. Ultra low sulfur happened a bit before that and I think it cost some too. Just drive and enjoy the simplicity.