Fueling around

Since I do a lot of highway commuting between home and work and Ottawa and Montreal, I am able to obtain pretty reliable information on my fuel consumption (it is between 6.9 and 7.5 L/100Km - I do a quick compute in my head every time I fill up).

Various factors will affect fuel consumption including weather, road conditions, and the type of driving I do (city versus highway).

One of the things we are constantly told is that a car burns 10% more fuel for every 10Km/h you are above 100Km/h. So if you are doing 120, you burn 20% more fuel than if you were doing 100. I can categorically state that my experience contradicts this idea.

Websites promoting this idea often display charts showing diminishing fuel efficiency over 100Km/h. The stated reason is a combination of engine efficiency and drag.

The door to door trip from my home in Ottawa to Montreal is 170Km, about 165 Km of that is straight highway. The door to door trip from My home in Ottawa to my work is 32Km - 30Km of which is straight highway.

I have done the trip from Ottawa to Montreal at speeds as high as 140Km/h (and yes, I eventually got caught and I don't do that anymore, I stay under 120Km/h).

The first time I did the round trip at 140, I was expecting to see a fairly serious drop in fuel efficiency - I didn't. Nor have I ever seen a change in fuel efficiency based on my driving speed (whether 100Km/h or 140Km/h). - it always remains around 7.0L/100Km. I would notice if my fuel efficiency went to 8.5L/100Km or 10L/100Km (projected when I was doing 140), but I don't notice if it is 7.2 one time 7.4 the next and 7.0 the next (I do notice, but it is not seriously outside my expectations for normal variance and error).

I do notice a drop in fuel efficiency if I have a wind perpendicular to the car.

I also noticed a 10-15% drop in fuel efficiency when I fill up with an ethanol blended fuel (10% ethanol). It drops to about 8.2L/100Km. This is not surprising since ethanol provides less energy per kg than gasoline. The 10/90 mix should provide about 5% less energy than 100% gas, but, my experience does not square with this.

Note: none of this is scientifically documented. It is based on my empirical observation since September 2004. However, my empirical observations do not jive with the commonly held opinions about fuel efficiency. My baseline fuel consumption is fairly consistent within about 5% - so I should notice a 10, 20, 30 or 40% increase in fuel consumption. I do not.

The only explanation I have is that those claiming a figure of 10% per 10Km/h over 100Km/h are being disingenuous about the way they are representing fuel consumption. I agree that fuel consumption increases by 10% for every 10K I travel above 100, if and only if, you are measuring fuel consumption per hour. But, you also travel 10% further in that hour. I should also note that at high speeds (I am not sure if 100Km/h counts as high speed or not), drag increases with the square of the velocity. Eventually you will reach point where the drag is pushing back against the object and you reach terminal velocity.

As far as I see, there is little or no change in fuel consumption per unit distance traveled. I know what the equations for calculating drag look like, and I have no reason to expect they do not work as expected. I simply repeat that my empirical observation does not accord with the conventional wisdom. At sea level, the speed of sound is 1225Km/h, 120Km/h is 10% of that. I have serious doubts that 10% the speed of sound counts as high velocity.

I should point out that drag is an important consideration, and I am sure that if I was travelling fast enough, it would become a problem, but modern cars are very aerodynamic. As well, there is also the issue of wind. If you were driving 100Km/h into a 20Km/h headwind, you should experience a 20% decrease in fuel efficiency (according to the oft quoted wisdom). I doubt you will because the car is engineered to be aerodynamically efficient pushing aside air at speeds above 100Km/h.

As mentioned, I do notice a decrease in fuel efficiency when I have a cross wind (mostly perpendicular to the car) from the North (I can’t recall many Southerly winds in this part of Canada).

I do not encourage or condone speeding. There are many risks associated with it. It is not simply a question of keeping your car on the road and avoiding other cars and obstacles (like deer, moose and raccoon families). Your tires are also rated for a certain speed. Above that speed, they run the risk of tearing themselves apart from centripetal forces - small nicks and abrasions increase that risk.

If someone wants to point out why my experience is at odds with conventional wisdom (and official pronouncements), please let me know. I put 46-47L into my car (a Mazda 3) every 650Km or so. I tend to use the same gas station and the same pump.

If you don't have any ideas, you might be interested in knowing that in the US, it is apparently perfectly legal for news organizations to disseminate misleading, distorted and intentionally false information. Accurate and objective information is not a requirement. I suppose that no legal requirements exist in Canada either as to the accuracy of information (so long as no one is libeled).

Long ramble, but I finally got it off my chest.

For those who have dug in and read through all this go reward yourself with some eye candy (there is NO need to install the Japanese language pack).

Image nabbed from here.

[edited on 12-October-2006 to fixed broken hyperlink.]


Good post. You have everything figured out to the nth degree unlke us...we seldom even look at the gas except hopefully before it's empty! We do drive an SUV Nissan Pathfinder because we do a lot of off road and winter conditions into ski country,and it's not great on gas...but it IS our only car, a 98 at that.

I guess temperature and precipitation wouldn't affect what you're talking about here? I can see how wind can, not to mention the storms and blizzards you guys get there!
ghee said…
You are a Walking Calculator!!
whoaahhh! thats great!

gasoline is sooo expensive.even here in Japan.You use much of it coz of your location.

Does the company provide your expenses?
What about engine capacity? How would that affect any differences? It's a trade-off, if it's worth it to pay as much or more to get somewhere faster because you've driven further in the same time, then that might be okay.
Sounds like there are a lot of factors here to consider.
Richard said…
MOI:Efficiency is simply the ratio of the amount of energy expended and useful work performed. The more energy expended for less useful work, the lower the efficiency. The less energy expended doing useful work, the greater the efficiency. When your car is simply idling, your efficiency is zero. As you drive, the faster you go, the greater you efficiency - up to a point. At some point, the engine will be doing more work to push aside the air than in transporting you and the efficiency will drop. This is pretty much what they are trying to capture in that 10% more gas burned per 10Km above 100Km/h. The problem is that my empirical observation does not bear this out. While I do not dispute the validity of the premise, my experience is that it does not hold true (at least for my car) at highway speeds. Maybe if I had a boxy car …

When it is very cold (like -40C), I find my fuel efficiency drops (likely caused by incomplete combustion), as well, if it is very humid and damp (raining heavily) it also seems to drop. Of course, when it is raining, there are other factors involved as well, the wet roads, the rain on the car. A dirty air filter will also affect efficiency.

ghee: No, my company does not subsidize my driving, neither does the government. According to the government, driving to work is pleasurable driving. If I was a contractor then I could deduct the cost of driving as an expense, but not if I am a full time employee.
I was thinking that too, that you should get a boxy Jeep! Too bad you can't be self-employed and contract yourself out so you can deduct expenses. I know, government benefits are too good in all the other areas to give up...same with teaching.

BTW, they were very cool moving snakes eye candy!

Popular Posts