Does my cargo van get better gas mileage when fully loaded?

Factual Questions
1

I don’t have any hard evidence, just judging on my gas gauge.
I drove 45 miles, 95% of it was freeway with gently rolling hills. In an empty 1-ton cargo van.

Filled my van with fuel, then loaded it up with about 3,000 pounds of various things and drove back the exact same way.

The way there i used a little over 1/4 of a tank, on the way home i used a little less than a quarter.

It seems that once i get the van up to 65MPH, it takes very little effort for the vehicle to maintain that speed fully loaded, compared to when i was empty. Does adding weight to a vehicle actually increase fuel efficiency?

2

Possibilities:
Overall, it’s downhill on the return trip.
With 3,000 lbs on board, you’re gentler with both gas and brake.

3

Also, you’re carrying far more weight for the same frontal area so you’re less affected by air resistance.

4

Was the tank full when you started out on the first leg of the trip? If not, it is quite possible that the gas gauge is not equally accurate throughout its entire range.

IOW, Going from half full on the gauge to one quarter may actually be the same as going from full to seven eights.

5

On the one hand, you might be right, a cargo van has a TON of air resistance and by loading it up with stuff you do give it a lot of momentum that will help it push through all that wind without a lot of help from the engine.

On the other hand, often times those quarter marks aren’t delineated in a way that quite represents the tank very well. That is, if you put 40 gallons in the tank, you might use 12 gallons when you hit the first quarter mark, 25 by the second one, 33 by the 3rd one and only have 7 gallons left when you get to the last one. Those are random numbers, but how many times have you heard someone say 'The second half of my tank always seems to go faster than the first half). I assume the bottom half of the tank is smaller than the top half but the float just tells the gauge how far down it is.

To get an actual reading, fill up the tank, drive it then fill up again. That’s how much gas you used when the truck was empty. Now load all the stuff in, drive back and fill it again and see how much you used when it’s full. With only 65 miles to work with, I’m going to guess the difference won’t be much.

6

That would only be helpful when coasting, when the foot is off the accelerator, or just depressed less. So that would only help in very specific driving conditions that don’t come up that much. Most of the time when you slow down, you use thr brakes to stop at a light.

7

what runner coach said, it must be downhill on net on the way back, and your weight helped you down the down slopes.

8

A head wind vs a tail wind can effect your fuel mileage too.

9

Correct in the sense that aerodynamic drag force is a smaller percentage of the total force the engine must deal with.

But incorrect in the sense that aerodynamic drag force is actually smaller. Rolling resistance will be larger due to the extra mass, and thus gas mileage over the same stretch of road with the same wind will be lower.

10

Driving a van is strange, it’s like driving a kite. The wind kicks it all over the place and when it’s empty there’s no weight to it, relative to it’s size. The OP said he was doing almost all highway driving which means he got up to speed and just had to maintain it. In a van, that means keeping your foot on the accelerator to compensate for wind resistance (ignoring traffic).

Adding mass to it will help push through all that air*. It’ll take a lot more energy to get it started and stopped and slightly more to maintain it though. You’ll also expend less energy keeping the vehicle straight since the wind won’t kick it side to side as much.

Honestly, though, my vote is that you used more gas with the thing fully loaded, I can keep doing this, but it is hard to argue that you moved more stuff with less energy. Even if you did, 65 miles, one time is a pretty miniscule sample size. Anecdotal even. I’d chalk it up to inaccuracies built into your fuel gauge.
*put a .25# weight on wheels and roll it towards you and try to stop it by blowing on it. Now put a 1# weight, of the same size and shape, on wheels and do the same thing. the 1# weight will be much harder to stop.

11

Physics fail. To the back of the class with you.

At most an increase in mass increases inertia, but the added inertia (which tends to maintain speed) is only gained at the expense of the extra energy required to get it up to speed in the first place.

On the flat, the amount of energy required to maintain speed is purely a matter of air and mechanical resistance. Mechanical resistance will increase with mass. Wind resistance won’t vary at all with mass.*

It is correct that a heavier van will roll further than a lighter van, faced with the same resistance. However, this is (a) irrelevant for a van at a constant speed and (b) offset by the greater amount of energy required to get it up to speed in the first place.

*Perhaps subject to a slight change due to the van sitting slightly lower on its suspension with more weight but whether that would increase or decrease wind resistance is a moot point

12

Vehicle fuel gauges are notoriously imprecise. If there was enough room on them, they’d read “Roughly 3/4,” “Approximately 1/2,” and “More or less 1/4.” :slight_smile:

Combined with the unknowns of uphill/downhill and headwind/tailwind, this means you really don’t have a good idea of comparative gas mileage for the trip described.

13

To begin with 65 miles is too short to really run a true experiment. The energy needed to bring 3000 extra pounds up to speed would be much greater than for the empty van. You might get a better aerodynamic shape when the back end lowers from the weight, but drag is the least of your energy usage. As observed above unless you are traveling downhill all of the way back, you are going to expend more energy on the return trip. This will cause you to use more fuel. You might violate the speed limits but you cannot violate the laws of physics.

14

Sorry my last post was probably a bit harsh given that I agree with **Joey’s **overall conclusion. However, my point is that saying added mass helps the van push through the air is really neither here nor there since what the mass giveth later is only what it tooketh away to begin with.

15

There was no point discussing physics, the gauge is simply not accurate enough to the discussion.

Especially as he filled it up… then he used “two quarters”.

The guage is not linear like that , the 4th quarter may be a very different amount of fuel compared to the third quarter… its really just a rough indication of fuel levels. The gauge shows depth,roughly, while the tank has a rounded or tapered or sloped top so that an accurate measure of depth is poor presentation of volume…

16

What they said. Higher mass means higher energy requirements, both to accelerate the vehicle and to maintain a constant speed. Higher energy requirements, that is, assuming everything else about your drive was exactly the same.

Because (as multiple people have said) your 1/4-tank increments aren’t necessarily well-calibrated, it’s most likely that you really did use more gas.

However, that’s not the only possible explanation. You probably had different grade profiles and/or wind speeds, which could affect the losses. Also, as running coach and JoeyP allude to, the feel of the loaded v. unloaded van on the road could cause you, the driver, to use the accelerator differently. If you were on and off the throttle all the time on the unloaded trip, but were more smoth on the loaded trip, that driver difference could be enough to account for a significant fuel economy benefit.

17

Yeah, as soon as I read the post title the word “No.” immediately came to mind.

Check out this cracked.com article regarding gas gauges.

18

Vehicle fuel economy is notoriously difficult to measure. Even the Big Guys (car companies/EPA) have a difficult time doing it. The real world has so many variables that official, legally-binding fuel economy tests aren’t even done out there; they’re done in a lab, with the vehicle on a dyno. Everything about the test is controlled, in order to make the test as repeatable as possible:

-the vehicle is held at a controlled room temperature (something like 24 hours) before the test to ensure that its starting temperature is the same every time.

-the tires are filled to a specified pressure with a NIST-traceable tire pressure gauge.

-the vehicle is placed on a chassis dynamometer (wheels on rollers with computer-controlled drag torque) using a vehicle mover (the car isn’t started at all yet). The combined aero/wheel drag characteristics for the vehicle are entered into the dyno’s computer. Those drag characteristics are measured with a carefully-controlled, real-world coast down test. This is done on a long, flat stretch of road (e.g. an airport runway); the coast-down test must be run in both directions to eliminate any effects from slope, and there’s an anemometer on site (or in the best case, on the vehicle) that accounts for local wind variations. Incidentally, this coast-down test is where Hyundai screwed up a few years ago, resulting in overly optimistic MPG ratings on new-car window stickers (and a whole bunch of owner compensation).

-The driver starts the car and puts it through a very specific driving cycle (either highway cycle or city cycle). He watches a computer screen next to his window, which shows his vehicle speed along with error bands; he has to keep his speed inside those error bands for the duration of the test, or else the test gets voided. The bands are pretty narrow, and it takes some experience to be able to complete a whole test successfully.

-The dashboard fuel gauge doesn’t matter. In fact, they don’t even measure fuel quantity directly; they measure the carbon output from the tailpipe over the duration of the test, and infer fuel mass from that.

So, back to the OP’s van. There are so many variables out in the real world (including a fuel gauge with coarse gradations and unknown linearity/repeatability/precision) that it’s impossible to back up any claim with a single round-trip. It’s entirely possible that the aero drag characteristics changed because of the differing ride height; it’s also possible that there was a slight wind in effect (5MPH steady wind means your airspeed is 70 going one direction, 60 in the other), and it’s also possible that the OP drove the vehicle differently on each leg of the trip. The tires may have been cold/low pressure when he began the first leg of the trip, and then much more warmed up (higher pressure) for the return leg.

Having said all that, here’s one other interesting thought:
Gasoline engines are substantially more efficient at higher loads. For a lightly-loaded van, it takes X1 engine power to get up a given hill, and it takes Y1 engine power to overcome aero drag when going down that same hill (Y1 < X1).

Now do that same hill again with an extra 3000 pounds. What happens?

-The engine is making X2 power on the uphill climb (X2 > X1), but it’s also more efficient. When going back down the hill, the extra 3000 pounds means the driver is using less gas (Y2 < Y1). The net result may in fact be less total fuel used: the total energy pissed away to aero/tire drag is the same in each case, but in the latter case, the net efficiency of the engine may be better.

This squares with my understanding of why hybrid vehicles like the Toyota Prius can be so efficient. They have regenerative braking, which helps, but the energy recovery from that is actually pretty small. An additional factor is the ability to use the battery to trim the engine output, allowing it to operate in efficient (high-load) modes even when the wheels don’t need all that power. If you’re into electronics, then you can envision this arrangement as being a bit like a switched-mode power supply, in which the Prius battery is analogous to the SMPS’ capacitor: the engine can operate in a high-efficiency/high-power mode or a low-loss/low-power mode, and the cap smooths out the output. Something similar could well be happening to the OP’s van on those hills, where the engine’s excess power output gets stored as gravitational potential energy and then released on the downslope, allowing his van’s engine to operate in a similar way.

So I think it’s theoretically possible that the OP is enjoying better fuel economy with a loaded van on his hilly drive, but I wouldn’t trust the dashboard fuel gauge to make that determination. Moreover, even if it’s true, it would only be applicable to slighly hilly highway drives. Mountain passes with steep downhills will require braking, which would ruin fuel economy. And city driving likewise requires lots of braking and acceleration, and that’s where the extra mass will absolutely kill your fuel economy.

19

Most of the fuel is spent during the acceleration of the car.

For whatever reason, you probably had a smoother drive the second time.

20

My car has a dashboard display that shows the amount of fuel used on a trip. I’ve found that the same trip can use quite different amounts of fuel, even though I use my cruise control to stay at the same speed most or all of the way. I don’t carry cargo, so the main things that could be different are the wind and the temperature. The wind obviously can affect your mileage depending on whether it’s a headwind or tailwind, and the temperature can affect how hard your AC is working, which can impact your mileage as well.