Topic: Feature: What do fuel economy figures mean?

[Autos_Editor]

Why don’t real-world fuel consumption figures always match the published ones? Assistant Editor Jil McIntosh explains the factors that can affect your fuel mileage, and a surprising look at how cars are really tested for fuel economy.
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[Rupert]

If you are only going to the pump once or twice, you should make sure that it is the same pump and face the same way on the same concrete pad, when possible. Pad slope and click off point can make a big difference. Over many fill-ups differences even out.

[libraman]

This site should use Ms. McIntosh's methods if an when it decides to do another 50 litre challenge. The last 2 were a complete waste of time. They accepted the cars from dealers filled with gas (what grade, who knows), "estimated" burning off 5 litres from those with 55 litre tanks (how accurately, who knows), accepted that a 50 litre tank only holds 50 litres when this is not actually the case, did not refill the tanks at the end to verify the number of litres used.  I don't know why they keep this challenge on the website. I have noticed that the comments critical of it have been removed.

Peace

[tpl]

If you are only going to the pump once or twice, you should make sure that it is the same pump and face the same way on the same concrete pad, when possible. Pad slope and click off point can make a big difference. Over many fill-ups differences even out.

Quite right. I do this with both the GTI which has a computer and the Fit which does not.   I find that the GTI computer and the pump method agree well enough.

[Canada Stig]

Generally, I find the new EPA methods used in the US to be fairly accurate, while transport Canada's numbers seem to be overly optimistic to the point of almost being fraud. 

For example, my Honda Odyssey doesn't get anywhere near the city/highway numbers published by transport canada, but gets almost the exact numbers published by the EPA under the new method in the USA (17/25 city/highway miles per US gallon). 

I average between 19 and 21 miles per US gallon in the car, mixed driving, depending on winter or summer (snow tires and winter fuel do lower gas mileage) - and can get 24-25 miles per US gallon on a long highway trip (or, about 9.5-10l/100km - way below the 7.8l/100km rated by transport Canada).

[Peter R.]

A few years ago, the EPA in the U.S. changed the way they did their mpg testing .Their calculations on their average mpg went down about 10% from their previous mpg's on all veh's.

When I see the mpg sticker on the window of a new car I automatically take off 10% from the est. Eg: 33 mpg on the hwy , I take off at least 3 mpg, down to about 30 mpg and I'm a lot closer to the actual every day mileage.

[aknutson]

Excellent article and a good read! As always though, I come away wondering what I can really expect from manufacturer figures when trying to budget for a new car. Perhaps most buyers are fortunate (or ignorant) enough not to question how much their new car will cost them monthly for their expected travel, but for those who are interested in these things it becomes difficult to plan ahead. I would also be interested to know why the gas mileage is expected to differ significantly after the 6,000 Km mark.

[Bubba]

I would like automakers to provide fuel consumption numbers for constant speeds of 50 km/h and 100 km/h.

[Toe]

My experience over a number of vehicles has been that the highway figure is pure fantasy and that the city figure is a reasonable representation for what I can expect in daily driving.  This continues to hold true for both my '07 Vibe (05spd)  and '09 Montana (extended).

For strictly city stop-n-go driving, I figure the sky's the limit as I have been stuck in traffic that resulted in a 35 km trip taking 3-1/2 hours.  There is no such thing as fuel economy in those circumstances, unless you're driving a hybrid.  But it's still better than being jammed into a TTC bus.

[safristi]

 Fuel economy figures are like GREEN alligators.....they bite ya in the arse when yer dreaming of buyin' that 45MPG SUV.............................

[vdk]

I would like automakers to provide fuel consumption numbers for constant speeds of 50 km/h and 100 km/h.

Some cars have that. Instant fuel economy.

[Bubba]

I would like automakers to provide fuel consumption numbers for constant speeds of 50 km/h and 100 km/h.

Some cars have that. Instant fuel economy.

I'd like to have those numbers before I purchase a car.

[safristi]

  How NUMBED dy'a wanna bee............ . how.pusillanimous

[Sival]

My experience over a number of vehicles has been that the highway figure is pure fantasy and that the city figure is a reasonable representation for what I can expect in daily driving.  This continues to hold true for both my '07 Vibe (05spd)  and '09 Montana (extended).

For strictly city stop-n-go driving, I figure the sky's the limit as I have been stuck in traffic that resulted in a 35 km trip taking 3-1/2 hours.  There is no such thing as fuel economy in those circumstances, unless you're driving a hybrid.  But it's still better than being jammed into a TTC bus.

The highway driving figure is not fantasy, I get it in summer on the highway driving at 100 Km/h, avoiding bursts of acceleration. It helps that I have a Scangauge that is a very good indicator of instantaneous fuel consumption that can be adjusted depending on the results at the pump, so I know how to maximize fuel economy.

People just think it's fantasy because they do not drive in a conservation-minded fashion. In stop-and-go traffic, you will never see it, and when they finally get on the highway, they drive like bats out of hell at 120-130 Km/h and then wonder why they get bad fuel economy. Hint: driving at 120 Km/h will burn around 20% more gas than at 100 Km/h*, your engine or transmission doesn't matter much in this, it's air resistance and simple physics... unless you simply drive behind another vehicle and use their slipstream, which need not be dangerous, just match speed and stay 2 seconds behind them.

The way I see it, the Canadian fuel economy for the highway is representative at what your car gets at 100 Km/h constant speed on a flat highway with no wind. The city rating is closer to a 50% suburban, 50% highway commute, so most people should expect that number in their commute. If you're in a very urban setting, then all bets are off, I suggest looking at US city rating for that.

*See http://www.mpgforspeed.com/ for info.

Oh, and I support the comment about the very poor methodology of the 50-litres test. There was no rigorous accounting for any variation in tank size between the different vehicles, driving a car until you think you've burnt 5 litres of its 55-litres tank is horribly unprecise, and I noted that cars that had LESS than 50 litres in their tank size had been presumed to have 50 nonetheless, like the G5 or Cobalt, who have 49 litres but who had their fuel consumption calculated on the assumption of a 50-litre tank.

[Canada Stig]

You can also go to owner forums to find out MPG estimates.  Again, most as miles per US gallon, but you can get a good idea.  Just search "(Make, model) owners forum" on google and you'll find a few to choose from. 

MSN autos (the US one, not the Canadian one) has a great owner review section where you can get a good sense of "real world" mpg from "real" drivers.  Again, mostly miles per US gallon but find owners in climates/driving conditions similar to yours (ie: live in Ontario, find people posting from New York, Penn, Michigan, New England..) and you'll get a good sense.

Of course, your mileage may vary!

[Toe]

Well, like I said, it's my experience.  And yes, I do go like a bat out of hell on the highway...

As Jim Kenzie from the Star likes to say: speed is not the problem, it's the objective.

[coast]

Hwy mileage is not fantasy at all.  I've been able to beat it on every car I've owned in the past 30 years.  It's the mileage you get on a long trip at the speed limit. 

A lot of people whine that they can't drive 20 to 30Km/hr  over the limit with the AC on and still get the rated mileage.  What do they expect?

City mileage is much tougher as everyone (and every trip) is different.  It's just a standard amount to allow you to compare models.  If your commute doesn't have a lot of lights or traffic then you'll beat the offical numbers.  If it takes you a 1/2 hour to go amile then you won't.

[Richard Dinning]

The other factor you completely left out is time of year. My former Camry XLE 4 cyl would average about 9 L/100KM during the summer for my average weekly driving. (I'm retired so I don't commute, but at 66 my habits are pretty much the same week to week.) During the Winter that would go to about 13 L/100KM. My driving is a mix of short trips on local streets to the grocery store, the bank and the drug store and longer trips to various clubs on super highways.

All the numbers given are based on the built in computer that gives average mileage between fill ups. When I've actually calculated mileage using the fill the tank and calculate distance traveled v. litres used I find the computer reads about 1 L/100KM low, but that doesn't change the Winter Summer variation.

Similarly my 2008 Lexus RX-350 gives me about 12 L/100KM during the summer and an average of 14.5 L/100KM during the Winter.

I put this down to two factors, the gas we use in Winter is more volatile and thus has less energy per litre and the engine takes much longer to warm up in Winter. Also when you do a bunch of connected short trips, in Winter the engine cools down much faster so even a short stop at the bank means you are starting off with a cold or nearly cold engine.

Put these together and the 2 to 2.5 L/100KM worst mileage in Winter isn't surprising.