Saturday, June 02, 2007

Drive Slower, Save Fuel

Back in October Bean-Sprouts challenged readers to keep their car tyres inflated to save fuel. But it turned out that, despite all the "tips" telling you to do just that, tyre pressure makes no appreciable difference to fuel economy. Edmunds.com, who manage CNN.com's automotive websites, tested the tips and found out what really works. I was delighted to learn that air conditioning makes no appreciable difference either. Hurrah!

So what does make a difference to car fuel economy? Well, accelerating gently, driving slower and turning off the engine when stationary for more than a minute. So this June the challenge is to drive slower to save fuel.
How much slower? I'm going to leave that up to you. If you tend to drive at "speed limit plus ten", you might try driving at the speed limit. It could save more than money - it might save your license or even your life. If you tend to stick to the speed limit, you could try driving at "speed limit minus ten percent". It's up to you.
Don't forget to vote in the poll in the right-hand sidebar.

13 comments:

Hedgewizard said...

Aircon makes no difference? Really? Have you got a link to that Mel - it'd be splendid news... sounds too good to be true though.

Regarding the speed thing, I don't doubt it for a minute. The trouble is, there's a trade-off between speed and safety unless you've got the road to yourself; if you try to travel at slower-than-average speeds then other road users begin to want to get past you.

It's important to realize that the tests on speed vs efficiency are conducted on race tracks, in top gear. As a matter of fact the "sweet spot" for fuel efficiency is somewhere between 40mph and 60mph depending on your engine and driving conditions, so pootling along at 25mph in a 30mph zone is actually less efficient than driving at 30mph, unless the extra speed is making you brake and accelerate more!

Melanie Rimmer said...

All the data are from http://www.edmunds.com/advice/fueleconomy/articles/106842/article.html#test5
The tests were conducted on real roads, but they weren't repeated. For scientific accuracy I'd want to see the tests conducted several times, with a variety of different cars, and averages taken. But I can't find such a study so this is the best I can do.

Korenwolf said...

I'd also want to see the tests done over the period of a year (differing weather / temperature conditions), I see a significant (4-8 mpg) difference between winter and summer.

Wulf said...

Gosh, Mel, your blog must be widely read. I sat behind several people yesterday afternoon who were going at speed limit - 10% or even speed limit - 30% driving through the streets of London on my way to and from a gig! ;-)

As I understand it, fuel usage is dependent on how hard the engine works, which is measured by the RPM meter; my present car (a very compact and efficient Daihatsu Cuore) has one of these on the dashboard. Somewhere hanging below 2K rpms seems to be about right as the level that gives enough energy to keep the engine going smoothly driving round town. Having the meter allows me to check that against road conditions (eg. incline) and find the appropriate gear.

Hedgewizard said...

I did a bit more reading on the A/C thing Mel - going to the USA since they have more experience of it. (http://www.nrel.gov/)

It seems that A/C reduces your cruising economy by about 1 or 2mpg, about the same as having your windows open - not too important at speeds, but a big difference when idling.

It's worth mentioning that it is, indeed, hardly significant when compared to such biggies as staying off the damned breaks, driving sensibly, and not tailgating the Hedgewizard. He hates it, yes he does!

Usha said...

I had read I think on treehugger.com, not 100% sure, that you should not accelerate too slowly either. You shouldnt let your pedals hit the floor, but not accelerate too slowly either.

Anonymous said...

Hedgewizard is quite correct, Mel. Using Aircon DOES make a substantial difference according to all of the studies that I've seen, of anything up to 10 to 20%, however this is exactly the same as the drag induced increase that opening your windows causes. At motorway speeds air con is more efficient than windows though.

The trick, therefore, seems to be to only use your aircon when it's strictly necessary, not as an automatic habit (which, living in good old blighty should be at most 3 days a year...).

You are spot on, though, about keeping to the speed limit. Driving at 75 rather than 65mph damages fuel economy by over 10%.

As Hedgewizard points out, driving smoothly is also key point.

Andrew

She Who Digs said...

Hi Mel,
As well as overall speed, it’s how you get up to speed, which can alter your fuel efficiency. My DH accelerates away much harder that I do which is why I can get more MPG from our Toyota Prius than he can, but I’m sure is the same for other non-hybrid cars too. (I’ve turned this into a private competition with him!) I’ve found that keeping the tires inflated helps too, along with not driving with the windows open! This causes the aerodynamics to be interrupted- (and makes a horrible noise) and is true of any car, particularly if you’ve also got a roof rack or box attached. I don’t know how open windows vs. air-con measures up in fuel terms, but maybe adjusting the temperature by only a few degrees, like what is promoted with home heating thermostats, could still make a difference! There is so much information out there it’s so hard to know what to believe but I do agree with you that if everyone were to do just a little, the world would be greener for it! Love your blog- how do you find the time for everything? SWDx

Anonymous said...

Ooops, posted my comment to the old blog entry. Re-entered here, with some spelling corrections :) -

I am struggling to believe that A/C makes no difference, though I notice they seem to have only tested with A/C on, or with windows down. Also this is USA based research where the engines are much bigger.

I can feel the speed difference that A/C makes on my 1.25 Fiesta, so I would think it has to be equivalent to at least a couple mph.

I try to use the windows at slow speed and the air-con if absolutely needed only at high speed.

I should probably drive less aggressively and fast but it's a hard habit to break. Easy to just use alternative transport wherever I can.

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