The GMC CCKW

I learned a few things today, driving my CCKW.

Firstly, my vehicles fuel consumption is very poor,
less than 3.5 miles per US gallon

Secondly, my fuel pump would not pump fuel from the
tank, even though it was visibly about 25% full
(looking down the fuel filler).

Thirdly, If you have jerrycans, may as well have fuel in them.

Fourth, I will be removing a rear tailshaft before I go
on the hard surface again.

I am flummoxed about the fuel consumption figure, I know for sure
because I added an exact amount of fuel, new the exact distance,
made allowance for a worst case scenario of 4mpg, and still ran out
of fuel 1 mile from home.

My electric fuel pump is set below the tank, so I cant understand why
it would not pick up fuel with the tank 25% full...it did this once before
( ran out of fuel when 25% full) when I had the AC fuel pump. The only thing I can think of is that my fuel tank, which
is off a Canadian Chevrolet C60X and has a larger capacity, might have a pickup pipe higher. Or is this a problem on CCKW fuel tanks as well ?
I am hoping to obtain a correct CCKW tank soon

The electric fuel pump and plastic filters are certainly helpful to diagnose
fuel system failure on the side of the road.

My vehicle was unladen, but has a Carter W1 carb,it runs powerfully
and was able to climb fairly long steep hills without dropping from 4th to 3rd, which surprised me. It has new points, which are properly set.

I searched the old posts, and there was not a lot on fuel consumption, but a few mentions of less than the specified 7.5 mpg, and big improvements
with electronic ignition.

I guess I am curious if anyone has performed mpg tests and found such a low figure?

I now have a correct Zenith carb, so i will probably fit that, maybe check my CO2 %, I cant really think of any obvious reason.


I am also curious if anyone regularly drives these vehicles on well made highways with both rear tailshafts ( yes i know they must have in the war, and especially later in the USA).

I could really feel and hear the driveline winding up on the flat stretches,
i cant for the life of me believe this is not very bad for axles, tx cases
tyres and fuel consumption.

My fuel consumption has never been over 4-5 MPG so you are not that far off. Fuel effeciency is not a strong point of these vehicles. One has to take into account the weight to HP ration and realize how hard that engine has to work.
I would check the fuel intake line as it may be too short for the last 25 percent of the fuel in the tank. I ran into the same problem so made me a new line by measuring the depth of the tank, then cutting the line leaving enough clearance for the fuel to feed in. Also, if there is any trash in the tank, it has a tendency to be less dispersed when the fuel level is low and could stop up the line.
These old babys really whine. To me it is a beautiful sound. I recall as a child during WWII, I could hear the trucks coming from a distance in the miles long convoys off of US 71 and would already be standing by the roadside before they reached my home to wave at the soldiers.
Good luck with your truck and it's minor problems and remember Keep Em Rolling.

10-4 on the whine part
Fuel milage is not good either but close to 7 mpg however
Front axle is neutered in mine ( blank drive flanges installed with the orig under the seat ) that seemed to help the most
Rear axle windup is only noticable when turning sharp.But I agree with you Tony as I've been toying with the idea of disconnecting the rear rear axle and giving that a highway try.
As you know these trucks prefer being off road
When my son drives mine I can hear that whine from almost a mile away
Hope this helps
John

I wish someone made the neutered front flanges for the split axel truck. All I have seen are for the banjo.

Hi guys

A friend of mine had years ago a broken camshaftgear with his CCKW in the Ardennes,he was hauled back home with a Dodge WC-51 at 35mph, after removing all drive/axleshafts they managed 42mph.
Thats says enough i think.

My front axle has lockout hubs and steers much lighter. I dont know
about a difference in fuelconsumption, because Ive not been able
to check that accuratly.

Corné

Does anyone know under what driving conditions official( military) fuel comsumption figures are arrived at?

I presume the vehicles are unladen.

Throttle position is the main determinate of fuel consumption.

When climbing hills throttle position on CCKW is mainly wide open.

On the flat however it drives along very nicely at 30mph
( thanks Armydriver! ), with not much throttle at all.

It would make a huge difference how these test were carried out.

Because I had the truck out on a major roadway (thru suburbs with lots of traffic, stoplights, I need to move along more like 35mph, stop, idle, rev hard thru the gears, I dont suppose thats how the Army tested them.

Of course old motors use more fuel than they did when new.

"hard" driving would certainly dramatically increase fuel consumption,
just like on any vehicle.

We all know modern civilian fuel economy measurements are generally on the absolute low side of what is generally attained, I would not have thought the military would take that approach, more the other way if anything.


All the old truckies and Army guys at the clubs and shows I talk to have, or reccomend some driveshafts removed.

The problem (wind-up) was really only noticeable at speeds above 30mph, or worst, when taking bends uphill under full throttle, that is what I did not like to hear the release of mechanical wind-up occurred when the rear bogies bounce a little.

The Australian Army apparently had some system of jacking up the intermediate rear axle, I will have to find out more about it, otherwise they removed a rear tailshaft, they did not drive them on the hard surface
as 6x6 according to the very limited and local sources I have asked.

Axle breakage is apparently very difficult to rectify, because the axle tends to twist and unwinds like wire rope, and is extremely difficult to extract from the axle housing if it breaks up,
(impossible in the case of a Dodge apparently), neccesitating a complete rear housing change.

The other thing I learned is that there is a lot of radiant heat from the
engine&gearbox combo in the drivers seat of a hard working CCKW
(, my coolant got to 150C!) ,
it was a hot day, well over 30C ( 90F), would be nice in winter!

Hi,

My CCKW gets an average on flat level ground around 8.5 mpg. On hills, city driving, around 7 mpg. This is with the canvas up, however with a load of not more than 1000lbs. I have modified the head with bigger valves, the air cleaner has been visually left the same, however, it was greatly opened up for air intake using a dry filter, electronic ignition and a super hot coil. I have over 15,000 miles on her. Yes, the front axle flanges have been removed, and I am only running the rear axle ( I removed the driveline to the center axle. Yes, I also run 8.25 tires.

Hope this helps.

With the original 270 or 302 I get about 6.5 to 7 mpg with the neutered front drive flanges. Most people I know had them made at a local machineshop. You can make them for splits too.

I tried removing the short rear drive shaft and the truck would get stuck on twisting paved roads and I had to use the front drive to get it unstuck.

Tire size on the back is super important. On a hot day in the summer after everything is not, I can PUSH my deuce in a straight line by hand on level ground. If you cannot, something is binding and may be due to unequal tires on the rear.

I also get 5.9 towing the searchlight trailer with the Clarkair dozer in it. An additional 9,400 lbs load up and down hills.

i m getting between 8- 10 if i drive carefully, and thats towing a ben hur loaded with camping gear ... uk gallons here i don't know the difference between us and uk gallons.
i never trash her through the gears just nice and slow...
she is 60+ years old.
i have a standard 270 motor,i run on 12v ,standard tyres running at correct pressure as in the manual,but exhaust has no baffle so she sounds mmm nice..
she will go to 45mph no problem but cruises nice between 35 & 40mph.
i ve never encountered mechanical wind up, but she does whine slightly..
i have changed all the axle s and transfer case with expensive modern oils reguarly grease her up too

Hi Kevin

Banjos or Timkins ?

Thanks
John

I must be pretty lucky with the mpg too. After ~10k miles I get 9 unladen, 7 with jeep in bed towing loaded ben hur. This is balls out across PA over highway and mountains.
I'm running full stock with worn (but very carefully matched ) rear tires. Split axles. Don't know what the deal is in my case. Speedo had 32k on it when we bought her. Norway did oversize the pistons .020. She does have very slow oil loss at speed, maybe 1 qt 300miles across PA at 45mph.

Try the warmed up truck and tires test and see if you can push it on level ground going straight ahead. You should be able to if you have the tires matched in the back. Also check the odometer against a known standard distance like 10 or 20 mile markers on the interstate. Not saying it can't get that mileage, just saying you need to check everything. Also, were you running with the canvas tarp on the back? That costs quite a bit too.

What will really sends your mileage down the pooper is stop and go driving AKA city driving and hills. In my experience, the load, towed or carried did not make that much difference.

hi john
i have banjo axles running on standard 7.50 x 20 bar grips
kevin

Last month when I drove to Bastogne I had a consumption of
6,5 MPG on LPG with unneutered front axle, hard working going up hill and
full speed 45mph on flat level ground.

Iam sure it does better on gasoline, but Iam not planning to try that. To painfull in the wallet

I think the tyres are pretty even know I got 8 good tyres with the truck, and 2 of them where about an 1 inch to high. And I put 2 new tyres up front. But those 2 tyres at the rears are know after only 7000miles pretty worn, almost to the canvas.

Corné

Just out of curiosity, what psi is that electric fuel pump pushing? At a certain psi the pump will push past the needle and seat and dump raw fuel into the manifold. You might look down the open carb while the pump is on but the engine isnt running and see if fuel is trickling down the throat. Also, when the engine is off, look down the throat and see if there is fuel sitting in the manifold. Alot of those aftermarket pumps push 35 or 40 psi which is too much for that poor old zenith carb.

That is a good point about the electric fuel pump maybe pushing out too much pressure.

After a fuel system rebuild, I now have the AC manual fuel pump back on, however, whilst I was moving to my new shed, began loosing power on steep hills again, this is what happened before, and why I put the electric pump on.

Ran fine except on steep hills, last time I thought I had traced the fault, by tightening the glass bowl on the pump up very tight, which cured the problem, at that time.

It is a bit of a nuisance, and not as easy to test drive as before, new shed in very built up area, old shed had big hill nearby.

regards tony

Hey Tony
Check your float level if you've got fuel starve on an incline that's usually where the problems is,providing the fuel tank is full enough
Also the later CCKWs had the 6 valve fuel pumps as oppossed to the 2 valve set ups (there must have been a reason Uncle Sam changed 'em ) If you have a glass top you definitly have a 2 valve pump.
Cheers
John