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Fuel Consumption at Higher Speeds

Autobild1
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.

Autobild3 Autobild4 Autobild5
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!)

Comments

Joseph Willemssen

Their fuel efficiency versus speed curves are dramatically different than the ones posted in this article; they show mileage as being roughly constant from 30 to 60 mph, and only tailing off above that level.

Actually, the charts are very similar -- it's just that the ones referenced in the post don't have data points below 50 mph. If you chart the data from the TEDB, you'll see the same downward slope for the vehicles they tested from about 50-60 mph+. The data before that are rather mixed and depend on the vehicle tested.

http://cta.ornl.gov/data/tedb25/Spreadsheets/Table4_24.xls

Shmoe

Its really pathetic when you have to lie about your Prius fuel consumption. Just tell the truth. That graph obviously shows how poor the Prius really is.

Joseph Willemssen

Hey, jtantare. Why are you using multiple screen names? I see you "commenting" at 8:10 am as "LOL" and now you are posting as "Shmoe".

Perhaps you should stick to one name, otherwise people might suspect you for being a troll.

Joseph Willemssen

Joe, I can't find the TDI data on that site. Can you point me in the right direction?

Here you go.
http://www.greenhybrid.com/compare/mileage/all-nonhybrids.html

Unfortunately they don't have the data separated out, but I was able to extract it, as most people used "TDI" in their vehicle description or name.

Joseph Willemssen

Posted by: Because I can | May 22, 2006 3:36:27 PM

I see. People who mess around like that tend to have a short shelf life around here. FYI.

odograph

There is also real-world data here:

http://www.fueleconomy.gov/mpg/MPG.do?action=browseList

nij

Wow... this is one long post.

Come on Toyota, you already build some of the best diesels on the block! Put one in the prius (even if its just for europe) and silence this crowd!!

Heck car prices and fuel prices are already crazy high in the EU compared to US. You'll sell millions!!

Michael Glason

I agree with odograph! A real equal comparison would be Gasoline-hybrid vs. Diesel-hybrid, however we already can see with early Opel numbers who really wins that one too :)

Michael Glason

Whoops , sorry nij. :) Misread poster....again.

anne

@Chingy:

Can you post the well-to-wheel efficiency analysis for the Prius vs. TDI?

You are right about the point that the article's main objective was to show that speeding increases fuel consumption. But presenting the consumption of different vehicles in one article, leads to the inevitable: people start comparing. It's second nature for every one of us. And presenting diesel and gasoline in 1 graph without pointing out that you cannot really compare diesel and gasoline is worse. The crowd reads these articles superficially, and they will draw the wrong conclusions. That's why I wrote that comment. Nothing not off-base imho.

We're all trying to cut back on CO2 emissions. One litre of gasoline produces 2.4 kg of CO2, one litre of diesel 2.7 kg. So on the criterium 'amount of CO2 emitted per kilometre' a gasoline car performs better than the graphs suggest. That's why I find them misleading.

@Ash:

The Prius uses an Atkinson cycle engine, which is much more efficient than a regular gasoline engine. As I was told, it would be nearly impossible to build a non-hybrid with an Atkinson engine AND adhere to emission standards. So yes, the electric motor does help highway consumption, but indirectly.


@LOL, Shmoe:

Rent a Prius and find out for yourself.

Leszek Pawlowicz

Joe,

The problem I have with the German data vs. the TEDB is that the German curves show no signs of flattening as the speeds drop below 60 mph, and in some cases even seem to start steepening in the 50-60 mph range. The TEDB data does show scatter, but most of the data for individual cars shows either a flattening in the 45-60 mph range, or else a far smaller slope than what the Germans show. And if you average both the German and TEDB data, the discrepancy is even more dramatic. Given the difference in measurement methodologies, it may not be surprising that there's such a large difference. But given the greater level of technical expertise behind the TEDB data, I'd believe their data more than the Germans.

Bottom line is still the same, though - traveling faster than you really need to makes your car burn more gas per mile.

Chingy

Anne,

I can't find a study that specific. But, you should consider this: if your numbers on CO2 are correct, a liter of diesel puts out 12-13% more than gasoline. But, the average diesel engine is 30% more efficient than a gasoline version, hence less CO2 per mile driven. That's what most of the diesel well-to-wheel studies are about. They also include the fact that diesel needs less refining than gasoline, so less CO2 emitted during refining.

Yes, Joe is right about diesel generally having worse emissions than gas (other than CO2), but this will change in the next two years.

Joseph Willemssen

Chingy, you should bookmark this table:

http://www.eia.doe.gov/oiaf/1605/factors.html

It's only tailpipe emissions, but gallon for gallon diesel puts out 14.4% more CO2 than gasoline.

I actually came across that table when looking to calculate CO2 emissions when sourcing from a standard fuel mix for electricity. I hadn't known there was a substantial difference in CO2 from different fossil fuels, assuming that greenhouse gas output was a direct analogue for fuel efficiency.

For example, you could look first at a Corolla rated at 33 mpg combined and the EPA estimates that that translates into 5.5 tons of CO2-equivalent output annually. A Jetta rated at 39 mpg combined is estimated to have the same greenhouse gas output.

Of course, as we've discussed, these would probably need to be normalized for real-world performance which indicates diesel doing at least as good as EPA or better, and gasoline vehicles doing a little worse, than EPA on average.

Michael Glason

Joseph,

So did we just return full circle then. Even though technically correct comparing equal volumes of fuel (dino diesel vs. gasoline), diesel has more CO2. This is then made irrelevant if you can travel further on that volume of fuel and produce the same amount of CO2 (5.5tons vs 5.6tons for TDI) emissions as an equivalent gasoline vehicle (brand switching aside). I just wish more people would give the TDI's a chance, they are a valid path to green. We just need to convince companies to give particulate filters as options.
And once again, hopefully a diesel-hybrid for North America.

anne

@Chingy:

I agree with you. The well-to-wheel of AVERAGE diesels is indeed better than the average gasoline car. But reading the posts, you can see that people are drawing conclusions for SPECIFIC cars based on these graphs. And that's what my concern is.

The Prius is not an average gasoline car, with a fuel consumption that is about 30% lower than a comparable gasoline car. Based on your data, the Prius will perform quite well in a well-to-wheel analysis against almost any modern diesel.

Until a hybrid diesel comes along. But that is speculation, and of no use when you want (need) to buy a car today.

Last thing that you point out correctly are the emissions during refinery. I totally ignored this aspect. I'm no oil buff, so I don't know much about that. It is very hard to find usable, complete, compact, digestible information on this subject.

I've read on various web sites that it costs more oil to produce a litre of diesel compared to a litre of gas. But then you have the byproducts, that can not always be converted to some useful product and are burnt, and whether gasoline perhaps produces more unusable byproducts than diesel, and all the different refining processes, and all the other nitty-gritty details.

Chris

The chart is wrong with a Jetta 1.9 liter as a comparison but may be correct with the 2.0, I don't know. I have a 2001 TDI Jetta. I get 43 to 45 MPG with A/C and cruise set at a little above 80 driving on mostly flat IH-10 roads in Texas. If I drive in town with mixed 70MPH highway travel, this goes up to as much as 46 MPG. The best I have gotten is 49 highway, traveling at a constant 72 MPG with no AC.

The test does not say what type of diesel was used. If low sulpher, the MPG will go down. The curves on the charts are plain wrong as far as their steepness goes too. My car has been shown to have optimum MPG traveling at 2500 RPM's. This puts me at about 72 MPH as I stated earlier in 5th gear. Traveling at 60 MPH will actually lower my MPG as I am not operating at my most effiecient 2500 RPM's.

The Prius is a great vehicle. I like them and would consider them. I don't like the fact that you have to replace all those batteries long term which makes the effective MPG actually lower due to the battery replacement cost. Diesels have been shown to have the longest durability of any engines. If you are in it for the long haul, a diesel will have the lower overall operating costs.

Having said this, if you were to buy an economy car from another make running on gasoline and getting in the mid to upper 30's MPG, you cost to operate would be lower until you reach about 150,000 miles; at which point, my TDI is once again at a lower cost to operate based on selling price (keep in mind that I have not included maintenance costs which once again are higher with gasoline engines)

My TDI met California emissions for GASOLINE engines, even as a diesel back in the 2000 to 2002 era. I think the newer ones do as well due to the recent drops in MPG that I have seen the newer generation have. When comparing CO2 output like I have seen posted, you have to be careful when comparing as the data will be scewed because most diesel vehicles are actually diesel passenger trucks and large rigs. They will put out more CO2 due to the fact that they are not as stringently regulated as passenger cars.

So, drive what suits you. I am very happy with my TDI with 114,000 miles driven so far. The jury is still out on the newer hybrid cars as far as longevity goes and overall operating costs. Like I said, I like hybrids too but most of my driving is highway where I believe a diesel will outshine a hybrid. The hybrid is definitely the way to go for city driving.

Chris
San Antonio, TX

MH

I Agree with Chris.

I know that this thread is only about the impact of speed on mileage, but I can not resist giving my opinion about diesels.

Typically these cars need 30% to 40% less fuel and they’re really fast and powerful. They’re highly available (at least in EU) and are cheaper then gas hybrids. This is a bad example, because most people do not need this kind of car, but it gives you a glimpse… a BMW 535d takes only 6.5s to 62, have a 413lb ft torque, 272 Hp, and still have a nice 35/40 mpg (http://www.pistonheads.com/doc.asp?c=100&i=9847). In 2007 Citroen is expecting to launch the C4 diesel hybrid with more than 80 mpg (diesel hybrids does much more sense).

Although diesels are much more efficient, they still have huge potential of improvement. As an example, I modified my 98 320d BMW ECU software, now I can get easily 70mpg and at the same time increase the engine output to 160hp (previously 136hp). This horsepower improvement is achieved solely through a higher torque (from 280Nm to 340Nm) that permits less shifting due to a lower rev threshold and also a lower manifold pressure which in turn resolves to lower fuel consumption.

Stephen

so here's a simple 2007 comparison to put this to bed. Compare mpg of the toyota avensis, and the toyota prius. Now I've sat in and driven both. They are roughly the same size car (large family cars as they are called in the EU). From toyota themselves, a prius mpg is combined 65.7 mpg UK (http://www.toyota.co.uk/vs2/pdf/PS2_63_spec.pdf I beleive US mpg is about 10 lower?). An avensis 2.0L petrol (about the same performance as a prius 0-60) comes in at combined 34.8mpg UK. An avensis 2.0 diesel comes in at 51.4mpg UK combined, but a 2.0 diesel will be a little sluggish in comparison so you'd probably opt for a 2.2D which comes in at 47.9. Lets not bother about price differences as this varies too much from country to country, but where i am in Ireland, an avensis of this power is about 35k euro, with a prius at about 30k - so much for hybrids costing the earth: here it is the cheaper option (we have huge tax on cars and the Prius attracts a significant subsidy).

So what do we learn? Well first, you gotta compare equivalent cars. Second, the hybrid powered prius, for a combination of reasons including but not exclusively the engine type, does way way better than an equivalent petrol driven car. Third, in making various comparisons, you've got to include engine size, unless all you are interested in is what is the lowest mpg thing i can get about in, in which case i suggest a push bike. Fourth, the nature of the hybrid engine can lead some to compare their 1.4 golf etc. with what looks like a 1.5 equivalent, but is in fact in performance terms about a 1.9 petrol - apples and oranges. Finally, diesel does well, but once again you need a bigger diesel to get the same performance, so the mpg stats drop.

So of course if you drive a small/compact and drive a diesel, do most of your driving on the motorway, and aren't worried about overtaking on a tight straight etc. and if you are a very careful eco driver, then you'll do best with a diesel. The Prius is not aimed at you - its aimed at the 2.0L market normal fuel inefficient driver, and persuading them to drive a hybrid is very good for the environment.

Final advice, anyone driving a 6-10 year old car who wants to do the best thing for the environment will probably do more by switching to any more modern car of the same size. Fuel consumption has improved dramatically. Of course you've got to balance that against C02 emissions in the production of the car, but I'd guess if its done 100k it's probably burning more that you'd be saving not sending it to the crusher.

Bill Haines

Lots of car MPG ratings listed on this site.

Bill Haines

Lots of car MPG ratings listed on this site.

Sniper

I agree that time saved by driving at a considerable speed is important. That was of course the reason speed was introduced into the modern world. I constantly drive at 90-95 mph on long stretches of the British motorways. Going faster would save even more time but consumption would really go through the roof and there might be a little problem with the law… (goes up in a square function which would be apparent if the graphs above were the European l/100km and not the inverse function of mpg). In my 2.5 l Mercedes Turbo Diesel it feels wonderful and I got an average of 8.8 l/100km on my last tank of diesel which I cannot complain about at all!
The solution is not to slow down. The solution is to produce the liquid from other means (algae biodiesel?). That has been the lesson of the past - people do not want to reduce their standard of living, just find a technical fix and let’s enjoy the scarcest resource we have on this earth. Our own incredible short lives!

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