I own a 3-axle '74 Peterbilt 359 that currently pulls a 38' bottom dump. I am averaging 4.0 to 4.5 MPG. While I've driven many years I haven't owned my own truck only until 3 years ago. To be blunt my mileage is a lot to be desired. I usually spent a lot more on fuel than other bottom dump owner/operators I work with. Because this truck is a classic I want to (eventually) restore and operate it but am considering getting something newer that gets better mileage. The truck is super dependable, always starts, doesn't use or leak oil and gets down the road with the best of them. I've had zero down time in the time I've owned it yet have seen newer trucks sitting along the highway.

Point being I'm asking if there's a way to improve the mileage on a truck manufactured at a time when fuel was cheap and fuel economy wasn't really a concern for the trucker.

Specs: 350 SC Cummins, 13 speed, SQHD's with a ratio I believe at 4:11 or 4:33. It's geared fairly low. Tires are 24.5's with a wheelbase of 220".
I realize I could put some drop-ins in the SQHD's to a higher gearing but then I lose on performance and power. I am considering selling or leasing the bottom dump and putting a 10-12 yard dump box on it. I also realize running it as a dump it won't be quite so bad on MPG.

Any comments or ideas...?

Thanks in advance.

Rod

I wouldn't suggest putting a dump body on that tractor. The frame just won't take it.

Dump trucks converted to tractors, such as for heavy haul, do fine. But tractors converted to dump trucks have a way of breaking in half. They need double frame rails.

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Z-Lady's devoted hubby.

Thanks for the reply, Mack E-6. Yes, I'm aware a frame can break in half, usually right behind the cab and was considering double-framing it before putting on a dump box. This truck was a water truck for years with the same weight on the frame but used differently as obviously the tank didn't move where a dump box would.

Thanks again...

Firts thing is find someone that knows how to set up a pump.
That's gonna be a trick today.
Yours might be set fine but you can't know that unless a good pump mech goes over it.

the real problem with tractor to dump conversions is the brakes. You are currently set up to stop 80k with at least 5 axles, you would be asking the same tractor to stop 80k with only 3 axles on the ground. I've had plenty of folks double frame and add a couple of leafs to the front springs and think they would be fine....they usually end up running over something. It can be done correctly, but if you spent the money needed you would be better off to buy a FACTORY dump.

Not sure about your state, but I know of no place where he is allowed to carry 80k on 3 axles. Here in Mich, you would be limited to 54k on a 3 axle rig, and you would have to have the correct steer axle and tires to make that pencil out.

But, to get back to the question of how to improve mileage, he has some strikes against him. The Small Cam Cummins was not noted for it's fuel mileage to begin with. It is an older engine and turns a higher rpm than current ones do, and more firing cycles means more shots of fuel going into the engine. he has relatively high gearing to go along with the engine Rpm, and has 24.5 tires, which are about the worst ones imaginable for fuel mileage.

I would make a couple of suggestions here. If it hasn;t been dole already, he needs to have the overhead run and make sure that everything there is set to spec as it should be. he needs to chaeck the pump and ensure that someone hasn;t done some homemade settings on it prior to him owning the truck.

Then, he needs to get onto the Pittsburgh Diesel site and look up his engine. they have a lot of experience with the older Cummins engines and if there is a solution to how to get an improvement, they are going to know about it. The electronic improvements they do to the computers won;t work for him, so it's going to have to be set up the old fashioned way. Somewhere on their site they had a collection of past magazine articles the owner had written, and there was a lot of Cummins info in there.

Or, he can just keep it as a toy and get something more up to date to do the work, and drive that for a living while playing with the old truck on th weekends.

Here are my suggestions:

Make sure the overhead is right. By right I mean set at the correct temperature, using an inch pound torque wrench on the injectors, and setting the intake, and especially the exhaust valves, a little loose. By loose I mean .001 /.002 wider than called for.

The SC Cummins will get better fuel mileage with the pump hotter that factory spec. A little extra rail pressure and a 2300 RPM setting will give extra power for pulling hills with less gear changing, thus saving fuel.

Get into the throttle easy. You don't need to hear the injectors rattle off and see black smoke every gear change.

Use progressive gear changes. Don't bump the govenor when empty or in the low side of the transmission.

When topping a hill back out of the throttle and let the weight and momentum of the truck get you up to full speed instead of holding the pedal down.

Keep the air filter clean.

Keep all tires at the maximum pressure on the sidewall. If the steering tries to wander, back the steers off to 105-110.

This is very important. Make sure the tractor and trailer axles are aligned. The Peterbilt 4 spring, if this is what you have, will wear the bushings and cause an out of alignment condition. This not only wears tires quickly, it will cost anywhere between .25 -.75 MPG to turn the tires as they scrub down the road.

Keep the 5th wheel greased good. You will scrub the steer tires off trying to make turns. Even changing lanes will cause a tire scrub with a dry 5th wheel. Tire scrub uses more fuel. Plus, the truck drives so much better with a well greased 5th wheel.

You will not get great mileage with the ULSD. Our trucks have noticed .50 -.75 MPG decrease in fuel mileage with it, but, you should be able to achieve 5.0 to 5.25 MPG since you have very little wind resistance.

Again, don't overlook the suspension. I had 4 spring Pete suspension with a '75 model 350 SC. The scrubbing tires from a worn suspension cost me a fortune before I found the problem. I had replaced, rebuilt, or adjusted everything in the fuel system, and the whole time is was the axle alignment eating my fuel and tires.

I am sure you know most if not all of these things, but there may be someone here that this will help.

These are my opinions based upon personal experience, and are not hear-say.

Take a look at the "Storage trailer" section of the forum. I am posting a link in there to the articles concerning Cummins engines that the owner of Diesel Injection of Pittsburgh has written that cover a lot of the older engines.

You're happy with this truck, so I'd tune it up and keep it. Why buy a new truck just to save a little bit on juice? If you do buy new, you'll spend how many tens of thousands? That money can buy you plenty of green stuff for your old gal. And don't forget depreciation on your new queen if you get her. That will also buy you plenty of tankfuls.

I wouldn't worry about it too much. You love this thing, so you should keep it. Get it tuned and start driving like stranger advises. Maybe you can pick up a lighter bed someday, but that's all I'd do with a switcheroo. You can even give 'er a nice paint job and new chrome wheels with the money you'll save. Put that money in a 401-K or some other retirement fund. Your truck does the job just fine.

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--Paved Dudley--

Yeah I drive a Swift truck
And that means one thing
It means she's slow
It's a typical company truck
It's just all show and no go

I'm gettin' passed by Yella
And even Overnite
I'm gettin' passed by ever' body in sight
46 days on the road
And I'm not gettin' home tonight

No, my hometown's nowhere in sight
And if you think I'm pissed off
You're right
46 days on the road
And I'm not gettin' home tonight


Love pissing off those trucking company insiders.

I just did some research on Pittsburg Power's site and have some numbers for those that care.

At a cost of $2400 for their computer upgrade and their CLAIMS of .3 mpg improvement it would take 80,000 gallons of fuel to pay for that computer. I used $3.359 a gallon cost for my figures. This is going to be roughly 4 yrs for a break even return on investment of that $2400. Hardly worthwhile in my opinion. HOW EVER if you were to see a full 1 mpg gain like they say has happened then you are looking at less than 2 yrs for ROI. Much more like it.
But if you are looking solely for power up grade then you probably won't care about your mpgs and cost for the upgrade.

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