## Fuel Consumption at Higher Speeds

##### 22 May 2006
 Auto Bild test data for 8 of the cars, rendered as fuel economy (mpg US) rather than fuel consumption (l/100km). Click to enlarge.

While it may seem intuitive that driving at higher speeds consumes more fuel, exact data on just how much a difference that makes model-by-model isn’t readily available. The editors at the German weekly Auto Bild recently changed that, however, publishing (12 May 2006) a head-to-head comparison of the fuel consumption of 16 different models of cars under higher speed driving conditions.

Testing was done at the Automotive Testing Papenburg (ATP) 12.5 km high-speed circuit in Germany. The editors measured fuel consumption using on-board computers at speeds of 80, 100, 130, 150, 180, 200, 220, 250 km/h and at top speed.

The guzzler of guzzlers was the Porsche Cayenne Turbo S, which swills down an eye-popping 66.7 liters of fuel per 100 km at a top speed of 270 km/h—that’s 3.5 mpg US at 168 mph. Of more prosaic interest was the difference in performance between diesel and gasoline versions of the same cars, as well as some data gathered on the difference a bicycle carrier on the roof can make, or driving with the top down.

 BMW 5 series diesel and gasoline. Click to enlarge. The downsized VW twin-charged TSI vs the larger diesel. Click to enlarge. GM Opel Zafiras. Click to enlarge. Prius data is included on each chart as a reference.

Adding a rooftop bicycle carrier to the BMW 530i Touring model increased fuel consumption by 11% at 100 km/h (62 mph). That gap increased to 26% at 180 km/h (112 mph).

Driving a Mercedes SLK 200 K with the top off imposed a 5.8% penalty on fuel consumption at 100 km/h.

(A hat-tip to George Minko!)

one point being, if most of your driving is spent at 80 mph (i've heard this is the case for most commuters in the Atlanta area. very little congestion, but very long commute times at high speeds), then there is no economic or ghg-based reason to choose a prius over any other similarly aerodynamic car.

if your driving is primarily at low speeds, a prius will greatly improve your fuel economy over vehicles in the same size range.

the other point being, decreasing your speed from 80mph to 50 mph will, on average increase fuel economy by 20-70% (more for SUVs and trucks, I would imagine), unless you drive an overpowered and aerodynamic car like the mercedes c180k. this is why the US had a nationwide 55 mph speed limit that was put in place in the '70s. and why some countries (japan, new zealand) still have such universal limits.

i don't personally want a return of the 55 mph limit, but i understand how it would be helpful. it could possibly be a more effective way to reduce our imports of foreign oil than all of the corn ethanol legislation so far and it would certainly do more for our oil importation than drilling in the ANWR would. it'd also be terribly unpopular and further encourage people to buy SUVs and trucks.

Brilliant! Greatly appreciate the effort converting all the figures into Excel charts - really useful at-a-glance info.

As I recall, most drivers ignored the 55 mph speed limit.

Most of the in and out expressways at Atlanta are marked at 65. If you are smart, you will keep up with the traffic flow at 80 mph. If you don't, you will constantly get cut off and sooner or later, someone will hit you.

Under these conditions, it's impossibe for the Police to do anything. They don't even try. In fact, if they are dumb enough to get on the expressway for anything other than an accident response, they cause trouble.

I had a friend who commuted back and forth to downtown Atlanta for five years. After they let him out of the mental hospital he found another job out of town.

Note that the Opel Zafira is in-between a station wagon and a minivan. The current VW Golf is a mid-size car. With diesel engines, both achieve better economy than any of the gasoline-powered cars - including the Prius hybrid - at all speeds from 50 to 100 mph. Too bad the US shot itself in the foot on diesels back in the 70s. European carmakers and consumers have benefitted greatly from the competition between these engine types.

Presumably, the diesel models tested by Auto Bild also had manual transmissions, which get 10% better fuel economy than ATs without bypass clutches. If you don't yet know how, take a few classes and learn to drive a stick. Lobby your local high school to offer this as a (for-fee) option in drivers' education class. You and/or your child will save on his or her first car, and he or she will spend less on filling up for many years.

Here we go again. Comparing diesel and gasoline. For the xth time: diesel IS NOT gasoline, gasoline IS NOT diesel.

One litre of diesel contains more energy, costs more oil to produce and produces more CO2. So when you have a gasoline and diesel car with the same mileage, the gasoline car is about 12% more efficient (from a technical perspective) or 12% less polluting (from a greenhouse perspective). From the economical perspective things are different, but this site is called GREEN car congress :-).

Second detail that should be taken into account is the transmission. Were this cars with manual or automatic transmissions? I could not make this up from the article, but since this was a European magazine, I suspect all cars were manuals (except for the Prius of course). An automatic gearbox adds about 10% to fuel consumption.

Never trust facts until you've got them all.

Separate comment.

The graph shows the fuel consumption of the Prius at 36 mpg at 60 mph. In European figures that's something like 6.5 l/100 km. This is completely wrong. Every Prius owner knows that. My worst fuel consumption on a tank of gas was about 5.5 l/100 km, on holiday to France. Air conditioner always on, average speed 110-113 km/h.

I don't know about the other cars, could be these figures are incorrect too, but do not take these figures at face value. Does anyone know what's wrong with these figures?

Typo: average speed was 110-130 km/h

I question their numbers. This article claims that the Prius gets about 37 MPG at 60 MPH. I have never gotten less than 42 MPG at 60 MPH in my Prius (2004, 56,000 miles) and only that low with bad weather and cold temperatures. My normal highway MPG is just under 50 MPG and that's averaging 65 MPH and with the AC on. These testing organizations should get an education on how to set up a car for good efficiency.

JeffD

on a 300 mile round trip last weekend, all freeway, my prius averaged exactly 49.1 mpg with the cruise control set at 70. how on earth were these people able to get mid 30's !?!?!?

Don't trust these measurements. I agree with the above comments : consumption figures for the Prius are pure nonsense (I am a Prius owner). This AutoBild magazine is the German equivalent of the French AutoPlus, a cheap car magazine which is certainly not much more valuable than the paper it is made with. Kind of "absolute zero" on the automobile press scale. It has become usual to laugh at it every time it reveals the "true cost of ownership" or "true consumption" of cars, here in France.

Still, it's nice to make clear that driving very fast is not "green". :-)

it would be interesting to do a full economic analysis of the cost of speed. For example, by driving faster, you spend less time on the road and more time doing something else you may consider more valuable. For example, if your time is worth $60 per hour either in terms of billable or personal value, then if you can shave off three minutes on your transit time, you have paid for a gallon of gas. Comparing costs of all types of transport (public etc.) using public transportation is quite expensive. Coming from suburban Boston, you typically lose about three hours per day between getting to train station, train, subway, walking. since transit time has zero productivity, those hours are expensive on a personal and professional level. using a$60 per hour rate, in this scenario, public transportation costs you $180 per day versus$100 plus or minus you will lose with car transport. the model gets even worse on evening and weekend times when the schedule is reduced.

on the value of time, economists have done a good job figuring out how much time is worth to people and typically the premium is about two to three times what they make for a wage. A high wage earner is more likely live in the distant suburbs then a poor person. therefore the value of avoiding public transport, driving fast, and avoiding urban centers is increased.

I just made a one day, 14 hour trip in my Prius. The first half, I did 70 mph average, and got 51.5 mph. The second half, I did 80 mph and got 46.5. I use Mobile 1 synthetic, maybe that's why I got a little better than Jeffd...............

Liter for Liter comparison of diesel to gasoline is not fair and misleading. Diesel vehicles are actually 40% more effecient with fuel economy. So the diesel vehicle will not consume the same amount of fuel for the same given distance. It actually takes more energy to make gasoline than diesel. Diesel fuel and kerosene heating oil is made with byproducts of gasoline production. Where all diesel vehicles receive there bad press is from the fact that diesel vehicles are only as good as the fuel put into them. The diesel fuel of the 70s and 80s had 6000ppm sulfur content, hence the high CO2 and NOx. Even in the early 90s that was only lowered to 600ppm in most places. This alone was enough to lower most CO2 emissions to lower than that of equivalent gasoline engines. NOx emissions still are slightly higher than gasoline emissions. This can be verified by comparing any emission reports between equivalent vehicles (www.fueleconomy.gov).

The prius seems to do best on longer trips, warming up the engine (10-15 mins) tends to reduce the MPG. I would guess the same for all cars. Maybe they did not warm the engine for their tests? I went on a long trip (200 miles) to the beach (fully loaded w/ stuff too) and got 57 MPG @ 55 MPH. I get around 43 in the winter and 50 in the summer with a 15 mile one way comute to work.

Smart cars that communicate with each other could allow drafting and save fuel.

Anne, you're completely off-base. Diesel is more efficient with well-to-wheel measurements than gasoline. Do a Google search on diesel well-to-wheel, you'll learn a lot. Diesel is, in fact, the way to go now that low-sulfur fuel is being mandated in the US. A diesel version of a Prius would get about 75mpg on average. Add to that the ability to just pour in domestically produced biodiesel (straight or from waste vegetable oil), and you have an almost perfect solution. Ethanol is the new hydrogen for the spin masters. It is a horrible substitute as transport, storage, and piping systems need to be totally re-engineered. Biodiesel demands no changes to the current infrastructure.

Eric S. Johansson, we don't care what your time is worth. Just drive a more efficient car!

I love these speed/mpg studies in theory, but the data looks bad to me. Not least because I consistently rack up 50 mpg in my Prius at 65 mph highway.

I found a bunch of reports here:

a couple guys say the "real world" number is 42 mpg at 80 mph. one guy gives a table, based on his experience:

60 MPH: 55 MPG
65 MPH: 52 MPG
70 MPH: 48 MPG
75 MPH: 44 MPG
80 MPH: 42 MPG

I guess my experience matches that - the small 65/70 difference could be hidden by head/tail winds or slight grades.

Hope this helps.

Oh, if "Automotive Testing Papenburg (ATP) 12.5 km high-speed circuit in Germany" has long sweeping turns, then I can believe their numbers for their course. The numbers might even be good for comparisongs between cars (over the equal course), but:

the numbers drawn on a curving track are not going to be directly applicable to our highway driving experience.

Anne et al. -

you are right in that diesel fuel contains 12% more energy by volume than gasoline does. Taking that into account, the energy used by the Prius and the Golf TDi is actually very similar.

Of course, in Europe, a liter of diesel is cheaper than a liter of Euro 95, because of the differential in taxation. So from an economic point of view, over here the diesel is the more attractive choice. In the US, total fuel cost would be about the same.

Too bad the test - flawed though it may be - did not include one of Opel's monovalent-plus CNG vehicles. This get good mileage and that particular fuel is dirt cheap (in Germany).

FWIW, it looks like Papenburg is a long oval:

http://www.atp-papenburg.com/D/I_01.html

The circular looking ends look like they could scrub tires on all cars and cost MPG.

"For example, if your time is worth $60 per hour either in terms of billable or personal value, then if you can shave off three minutes on your transit time, you have paid for a gallon of gas." Eric S. Johansson, you completely disregard the cost of the additional gas burned(at lower mpg), the additional emissions generated, and the additional risk of accident at faster speeds. Go ahead enjoy 3 more minutes of American Idol. BTW at GCC we value time min.$100/hr.

"Under these conditions, it's impossibe for the Police to do anything. They don't even try. In fact, if they are dumb enough to get on the expressway for anything other than an accident response, they cause trouble."

OK hon, pack up the truck we're moving to 'hot'lanta!

Ditto on the Prius. I get more than 50mpg at 60 mph. This study is total b.s.

Ya its a totaly useless test. Anyone who knows how to get fuel savings going fast knows its BEST when the road is completely straight and level or at least has extremely wide turns.

Thats because you have to get into overdrive and stay there AND you need the right overdrive gear. Most cars now adays have overdrive gears made for passing not for cruising.

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