Friday, June 12, 2009

What’s my fuel efficiency?

People are finally realizing that we are disrupting the climactic conditions that allowed our species to evolve from hunter gatherers to sophisticated agricultural societies. As a result, there is increasing interest in regulating carbon dioxide. CO2 emissions are quite complex and indirect in some cases, but automotive transport should be one of the easier places to measure carbon emission, use, and efficiency. For example, many newer cars come with dashboard display units that tell you what your fuel efficiency is. Or so we are led to believe.

A year and a half ago, I blogged on the fuel efficiency of my old motorcycle, which was comparable to Jim and Callan’s Priuses. This autumn, I started tracking the fuel efficiency of our family car, a small turbodiesel European wagon. I did this the same way I did for the bike- dividing the fuel purchased by the distance driven to get the liters per 100 kilometers- the standard efficiency measure for metric driving.

As I was doing this, I started to notice that the numbers I was calculating did not agree with the dashboard display. While the fuel use method is known to be imprecise (it is hard to fill the tank to exactly the same level every time), it should be accurate over the long term. So I started recording both numbers. And after half a dozen refills, I have the following data:

Derivation methodmeansigma
Calculated l/100k6.30.5
Reported l/100k5.60.1

Figure 1. Blue line is calculated efficiency, Pink line is values reported on dashboard computer.

Figure 2. Same data, converted into miles per gallon for the sake of unscientific Americans

With an average of 800 km between refills, the average fuel difference is about 5.5 liters per refill, or about 11%.

I checked the computer distance traveled against the odometer- the numbers are the same. The six refills came from four different service stations operated by two different companies (Shell and BP). No obvious difference. At this point, my working hypothesis is that the computer underestimates its fuel use, but I’m open to suggestions for how to test this hypothesis. In the mean time, if anyone else out there want's to check a different car model, it would be interesting to see what y'all come up with.


Silver Fox said...

It's been my impression from watching the mpg numbers generated by a Dodge diesel truck, that actual miles per gallon is lower than reported mpg. I don't have any hard data, though, and we don't own that vehicle for checking it. Haven't compared Prius reported numbers to reality.

Chris Phoenix said...

I checked my (2001) Prius's mileage a while ago. IIRC, the numbers agreed within 5% or so, and sometimes the actual fuel usage was better than the reported (though not by much).

OTOH, I recently replaced the battery, at 145,000 miles. $2500 (USD). Ouch. Works out to a couple cents per mile - not much worse than the tire wear.

On the third hand, I'm still on my original set of brake pads. Hooray for regenerative braking! And my mechanic can't believe how clean my oil is 7000 miles after an oil change.

All in all, I'm hoping to drive it to 250,000 miles.


Dr Chucky said...

I'm willing to give it a shot. I'll run the 03 Corolla dry, then fill up with standard. I'll do 5 tanks of that, 5 of E10-type fuel, and maybe 3 premium. It'll need some collaboration from The Missus, but I'm sure she'll be up for it.

C W Magee said...

With Diesel, there are fewer choices.

Schlupp said...

There is another sinister interpretation: Antipodean fuel stations conspire to charge you for more than they give you, which lets you overestimate your fuel consumption. We know they have to conspire, as opposed to individual dishonnesty, because you did not notice any difference between the stations.

Remotely Mars said...

I have data on our Prius and Forester for many, many refills. I'll make the graphs when the Prius returns home with its data.

ScienceWoman said...

I've noticed that the Prius on-board display consistently overestimates its fuel efficiency (i.e.., gives me more MPG) as compared to what I get when I divide miles driven per gallon of gas at the station. I've been recording it since purchase, but haven't bothered to graph it yet.

Lockwood said...

According to the sidebar at The Accretionary Wedge, you have participated in this geology carnival in the past. We have a new edition going up soon, around the theme of "When and where would you most like to visit in person to witness something first-hand?" Are you going to be able to join our time warp? (more details at the link) We're trying to get The AW back on it's feet, and would love to have you participate. Sorry for the short notice, but late submissions will be added on as they arrive.

C W Magee said...

If you wanna guest blog that info here, you are welcome to do so.

Tag! Just remember to convert to liters/100km so that the rest of the world can read it.

If? I drive a time machine for a living!