18 wheeler fuel economy

What sort of mileage do multi-ton truck haulers get? is diesel better?

No doubt the number will vary widely between different makes and models and ages of trucks not to mention driving conditions and how much weight they are hauling but the link below suggests that at least for some Mack Trucks the average miles per gallon is around 4.5 - 5. The link is hard to read and take with a large grain of salt with all the caveats above.


As a fun aside I recall taking a tour of the Queen Mary in Long Beach where they told me the ship got 12 feet to the gallon! :eek: Worst mpg I have heard of yet although doubtless something out there probably does worse.

5-7 MPG is common.

Large diesel engines are pretty well suited for trucks. They have high compression ratios and good efficiency, as well as providing plenty of torque for hauling.

A new, full-aero 18-wheeler tractor-trailer at 40 tons will get close to 12 miles per gallon on level ground at highway speeds.

I believe they don’t even make gasoline-powered engines for 18-wheelers – they’are all diesel.

And while the mpg is rotten, the more important statistic to consider is the mpg per pound of freight. An 18-wheeler performs a lot better than a local delivery truck.

I drive 18 wheelers for a living and I have never seen a truck get much better than ~8 MPG. There does not seem to be a noticible difference loaded or empty in my experience.

The mileage isn’t so great but the fuel used to haul one ton one mile is. At 30 tons, of which maybe 20 is cargo and 8 mpg that’s 160 ton-miles/gal. Even if you assume a return trip empty that is still 80 ton-miles/gal. Your automobile hauling 4 people plus luggage is hauling about .45 tons and at 30 mpg that’s 13.5 ton-miles/gal. Need I add that most of the autos on the road aren’t hauling 4 people plus luggage?

There shouldn’t be. On a long haul at highway speeds, nearly all the power exerted goes to counteract air friction, which is dependent on the size and shape of the truck, not the weight.

At highway speeds that sounds right but in stop and go traffic I expect having a heavy load with you creams the gas mileage.

Yes. I have heard that a fully-laden "B Double (two trailers) will burn three or four dollars’ worth of fuel (Australian) to get back up through the gears to cruising speed if it catches a red light. Fuel is more expensive here than i the US, but I think it is still a consideration.

[Slight hijack]

This thread got me curious. According to my train expert, a diesel electric train gets 4-10 miles/gal. How can a train hauling hundreds, if not thousands, of tons get the same or better fuel economy as a truck hauling 30 tons?

Diesel-electric drive allows the engine to always run at the most economical rpm while still having great flexibility in setting the proper speed of the train.

Years ago \mMy dad asked a railroad locomotive engineer how much fuel it took to haul the daily freight from my home town to Sioux City, IA, a distance of about 55 miles. The answer was about 17 gallons or a little over 3 mi./gal.

And I think trucks come close to doing the same thing. Our trucker poster can enlighten us on this but I’m told that the truck runs at its best engine speed, say 1800 rpm, and it the rpm gets below a certain number, say 1700, the driver shifts to a lower gear and it it gets high, like 1900, the driver shifts up.

In addition wouldn’t there be less friction rolling the train wheels over the rails than the truck tires on the highway?

This is made even more efficient because big rigs typically have a lot more gears than cars do. Most I’ve seen have 15 or more as opposed to 4 or 5 on a car.

Greatly less friction, and less wind resistance too. I’d bet that if you put a bunch of semis end-to-end and ran them on rails, they’d get very good efficiency. :slight_smile:

I drove for 7 years. Here’s a few thoughts.

  1. There is no semi that gets 12 mpg unless it’s not running and falling from a cliff. If you think the truck gets 12 mpg, drive it up to North Dakota. I have oceanfront property for cheap!

  2. Diesel is the only fuel you’ll see in a rig for a long time. The energy yield in a diesel engine under tremendous load is far superior to gasoline. A fully loaded (legally) truck is 80,000 pounds in most of the US. Also, all “heavy-duty” pick-up’s and SUV’s for private, general use are equipped with diesel engines. You’re not likely to find an F-350 with a gas engine.

  3. Standard gearboxes are 9-speed, Super-10, 13-speed, and 18-speed. Eaton-Fuller is what I’ve always had.

  4. Gears are much higher because the engine produces the most torque at the low end of the RPM range (as with your very own car). It is very detrimental to a rig’s engine to run above 3,000 RPM’s. Most are “governed” at around 2800 or so. HP can run anywhere from 350 to 800. (unless there’s been a new engine introduced in the last 2 years. (Likely). Also, I never downshifted before 1300 RPM, unless I was facing a long upgrade. The turbo wants a little something to work with,

FWIW, my last 3 trucks in decending order were (these aren’t mine) Kenworth W900, Peterbuilt 379 EXHD and my favorite (this was almost better than my apartment) the Volvo 770.

That doesn’t seem right. You still need to rev up the engine to provide more power to the motors, right? Or do diesel-electric locomotives have batteries to provide extra power (and store excess power)?

I think the main difference between trucks and trains are rolling friction (tire vs. steel wheel), air resistance (train cars are essentially drafting each other), fewer hills and curves, and more constant speed (fewer acceleration/deceleration cycles).

You can determine a power output-rpm table for the engine. Then as you vary the electrical drive you keep opening or closing the prime mover throttle to run at the most economical rpm. I don’t know that this is what is done, but that’s the way I’d design it.

Sure, it takes more power to pull grades and the like but engine rpm and power output are independently controllable in a diesel-electric setup. You can hear the diesels going up Cajon pass moving relatively slowly with their engines running at the same general rpm as when they are going 70 mph on the level.

And I agree with you that rolling friction and air resistance are also big factors.

Of course, engine rpm and power output aren’t totally independent but they are independent over quite a range of rpms and power outputs.