Why does my car get better mileage when fully laden?

Factual Questions
1

I drive a [url="http://moto.trojmiasto.pl/tapety/citroen/citroen_xsara_picasso_06_8x6.jpg"Citroen Xsara Picasso Diesel. It is a 5-seater ‘mini MPV’.

Under almost any driving conditions, fuel consumption will work out at something like 49 mpg on a trip of more than twenty miles, when it is just me driving the thing.

On quite a few occasions, I have taken trips with the car loaded up with people and/or stuff; this week it was my family and a full load of camping equipment; on these kind of journeys, a typical fuel consumption figure is 57 mpg.

This is not in any way an imaginary phenomenon

I’m still not quite understanding how this works though; I’m not consciously aware of any change in driving style - I’m a cautious driver who prefers to ease off the accelerator than use the brakes later - this is necessary when loaded, even though I do it somewhat superfluously when unloaded.

Could it just be that the extra mass helps overcome rolling friction and wind resistance, enabling the car to more or less roll down inclines where I would previously have had to apply power?

2

WAG…momentum.

3

This wouldn’t work. Yes, you might use slightly less going downhill, but you’ll use more fuel going uphill. It takes more energy to accelerate a heavier car, you can’t beat physics.

Try checking your tyre pressures. If they are a long way off, it could explain the effect.

Also, how are you calculating fuel consumption, and how many times have you done this? Do you have a trip computer, or are you using the fuel gauge? If the latter, some pumps cut out a lot sooner than others, so if you’re using the top of the gauge or miles between fillups the calculations won’t be very accurate.

4

More WAGs: the added weight reduces the frontal area of the car, reduces airflow beneath the car (and hence turbulence beneath the car) and/or smooths out airflow behind the car (since most of the weight is in back.)

Are the long trips made on fairly flat or hilly roads?

A Citroen Xsara Picasso (I just wanted to see it, and this seemed like an easy way to do it.)

5

The recent trip was from Southampton to Weymouth (UK), so a fair mixture of motorway cruising and very large hills, along with some stop-start driving at either end. With no special care, the fuel consumption figure was 56.4 mpg on the way, 57.1 mpg return.

Solely cruising on normal motorways, I cannot attain greater than 57mpg at any speed, but across hilly terrain, I can sustain 63mpg by means of careful acceleration, gear change and coasting in neutral - I cannot sustain greater than about 55mpg by the careful method unladen.

6

If you’re measuring mileage by the amount require to refill the tank, is it possible that the extra weight tilts the car enough to cause the gas tank fill tube to “burp” earlier (sothe gas station nozzle reads the tank as “full” at 1-2 gal less than normal)? This could also affect the reading of many gas gauge float designs.

If so, the car’s mpg will mysteriously decrease below average on the next (unladen) fill-up (to approximately 41 mpg). Also, if the car were fully laden for two consecutive fill-ups, the mysteriously increased mileage would be absent at the second fill-up.

7

You say your average 20 mile trip is worth about 49mpg. These trips that you’re taking with multiple people, how long are those? If these trips are like 100 miles or more, it’s probably the use of highways, and highways (when non-stop) are great for mileage. You can’t beat constant low RPM compared to a city drive.

8

The car measures the fuel consumption for me (digitally displaying either the instantaneous consumption rate or the average since the last trip reset).

On long motorway journeys, the best I can get is about 57mpg laden and about 51mph unladen.

I should mention that when unladen, the car seems almost reluctant to roll down hills and will sometimes decelerate (presumably toward some terminal velocity) even when coasting in neutral down a fairly steep hill. When fully loaded, this feeling is absent.

9

This works with gas engines, don’t know about diesels.

Warm engines are more efficient. If you drive 20 miles at a time, then a good chunk of that the engine is “warming up”. If you drive 100 miles on long trips, then more of the distance the engine is warm.

I get 40 mpg (US gallon) at 80 mph on the highway on long trips. To work and back, which is almost all highway, I get 35 mpg. Work is only 15 miles though.

10

That car’s not exactly a hauling type vehicle. How many pounds would you say equals “laden”?

Judging by your last post, your car seems to be using the mass of your load to override some sort of engine braking. This is a really WAG, but whatever system keeps your car from coasting down a hill normally would seem to be very feul inneficient.

11

Manual or automatic transmission? When you say you put the car in neutral, what do you mean?

12

You expect the normal law of thermodynamics to apply in The Kingdom Of Butter? Silly Mangetout.

13

Well, and here is my WAG- it doesn’t. Call the Citroen dudes, and I bet that the milage measurements are based upon a certain weight or load, and the figures get tweaked when you’re not at baseline.

14

A similar phenomenon has happened to me twice when I was moving and had a 1990 Jeep Cherokee filled to the brim with heavy personal effects. Both times I went from 19 mpg and a range of around 300 miles to 37 or 38 mpg and a range of close to 600 miles. I’m absolutely positive that neither times was there a forgotton fill-up in there as I was completely aware and more than a little concerned that the gas gauge was going down so slowly. Both times I was light-footing it because of the weight but I never was a lead foot in that vehicle and the 19 mpg had previously been pretty consistent.

One of the trips was from Texas to Colorado with a strong tailwind and the other from Colorado to Texas across mostly open plain.

15

Manual transmission; either dropping into neutral or (if I can’t be bothered, or it’s a short hill) fully depressing the clutch pedal.

The car, although reasonably aerodynamic in shape, is quite tall and displaces a fair amount of air as it moves; it obviously wouldn’t slow to a stop on a gentle slope (or else I would suspect a fault with the brakes or some such), but it will often slow down (presumably due to wind resistance) when rolling down a gentle slope unladen.

The consumption figure is, as far as I can tell, a reasonably accurate one with no ‘fudging’; all instruments are digital and the readout can be set to show such things as momentary fuel consumption, average (trip) consumption, trip mileage, projected miles until empty and total (volume of) fuel used this trip. (some of these figures being calculated based on some of the other ones, which are measured).

It is difficult to estimate the ‘laden’ weight I’m talking about, but it consisted of two adults, two children and every other available space (including the rear footwells and the space where I removed the middle rear passenger seat) packed to bursting with camping equipment (some of it quite heavy - such as the tent, cooker, gas bottle, airbeds, food). I have no idea what this would be in pounds.
The handling was very noticeably different.

16

The readout measures estimate milage, I’ll bet- and the weight is screwing it up.

Look, it goes against at least one law of Physics for your car to perform better when carrying more weight. You want to bet a gauge against Newton? :dubious:

Really, look up that instrument in the manual, or call the Citroen dudes. Try it.

17

I will add that it’s generally considered bad driving practice to coast down hills in neutral.

18

It doesn’t measure the weight; how could this be the case?
It measures the distance travelled, and the amount of fuel being used (I think it also measures the approximate amount left in the tank, but there must be an instantaneous measurement of fuel flow because when the display is in momentary fuel consumption mode, it reacts quite responsively to changes (it isn’t just fudging it from the engine revs either, as, for example, maintaining revs by depressing the accelerator as you start to ascend a hill registers a reduction in mpg).

These sort of threads seem to go this way every time (remember the one where the laws of physics wouldn’t allow a cat to jump more than three feet in the air, and the one where attaching helium balloons to cancel most of your body weight wouldn’t allow you to jump any higher?

I think you’ve probably oversimplified the problem, or rather ignored some of the variables; chiefly wind resistance, rolling friction and the efficiency curve of the engine; as I said, when driving the car unladen, it is sometimes necessary, on the way down a gentle slope, to apply engine power in order to keep the car from decelerating to an unacceptably low speed, even coasting in neutral, whereas descending the same slope when fully loaded, the car will accelerate down the hill in neutral.
Now of course it takes more energy to get the car up to the top of the slope to begin with, but this is an entirely unrelated factor.

In a frictionless, airless environment, starting from the top of a hill, an unpowered car would accelerate downhill and would reach a point equally high on the way up at the other side, regardless of load.

Add in friction and air resistance though, and the same unpowered vehicle would accelerate down the hill (more slowly due to the resistance) and would roll back up the other side to a point considerably lower than the starting point, BUT a heavier car will go further than a lighter one.

There are a range of factors at play here, each with its own curve; what I believe is the case is that when all the graphs are combined, the plot would be a curve with a dip in the middle, representing the weight at which maximum fuel efficiency is achieved and that the normal unladen weight of the vehicle marks a point that isn’t at the lowest part of the dip.

19

Well… some new facts have emerged; I’m not going to abandon my wild and contorted ideas above just yet, but on the way home from work tonight, I thought I’d try driving my car as if I knew it was heavily loaded, when in fact it isn’t.

It was only a short journey (10 miles), so perhaps not a fair test as yet, but I did notice one thing that is interesting and possibly quite significant; when I know the stopping distance is going to be greater, I tend to hang back more from the car in front; this typically means I end up not having to brake nearly so much on the way down hills, which in turn means that driving up the other side requires a lot less power; the fuel consumption on this short test averaged at a whopping 65.7mpg.

This might in fact be the most significant factor, however, I’m still of the view that for a vehicle of given size, taking into account rolling friction, engine efficiency curves and wind resistance, there will be an optimum weight for fuel economy and that both above and below this optimum weight, fuel consumption will be greater.

20

I think you’ve cracked it Mangetout. Diesel engines are very torquey, so are less effected by heavier loads than petrol engines. So the difference in driving style could make easily make more difference than the extra load.

Could it be that you tend to drive slightly slower when loaded? Just 5MPH less will significatly reduce air resistance, and the engine at slightly lower revs might be burning the fuel more efficientely.