A Conversation for Ask h2g2

What's your car's optimum speed in terms of MPG?

Post 1

winnoch2 - Biding my time..

Many cars nowadays have MPG readouts which calculate average fuel efficiency at predetermined intervals. Now assuming these devices are accurate (even if they are not, they should still be internally consistant), has anyone here watched their read-out closely to acertain which speeds their car is most effiient at?

I'm taking about motorway driving- it's a given that urban driving is hard on fuel consumption. I've read, and it may be an urban myth, that 50 mph is the most fuel efficient speed to drive at, but surely every model of car will be calibrated differently and will have a different 'sweet spot'?

I commute a lot and even a difference of 1 mpg makes a significant impact on my weekly fuel cost (as can be demonstrated with various fuel economy apps- try playing with the figures smiley - geek). I would be more than happy to alter my motorway speed anywhere between 50 and 75 mph if it saves me cold hard cash. I'm no slave to speed limits smiley - smiley


What's your car's optimum speed in terms of MPG?

Post 2

swl

Broadly speaking, go as slow as you can in as high a gear as you can. This way the engine is putting in the least amount of effort for the maximum return.


What's your car's optimum speed in terms of MPG?

Post 3

Rod

May I adapt that, swl?

In as high a gear as reasonably possible, go at the lowest speed that the engine feels & sounds comfortable at, (or just one or two mph faster) - in an automatic, of course, that may not be as easy as it sounds.


What's your car's optimum speed in terms of MPG?

Post 4

Bald Bloke

I agree with SWL and Rod.

Certainly travelling at 50mph in top gear will save you fuel on the motorway, but it is a trade off against journey time.

You can also save significant fuel by travelling in the wake of a large lorry, not too close and you need to maintain your concentration.

If I'm not in a hurry I try to run at 55-60 mph which balances with the HGV's and doesn't hold other people up.


What's your car's optimum speed in terms of MPG?

Post 5

Pastey

We drive a 1955 Landrover, fuel efficiency isn't an option smiley - laugh

There was a nice experiment done by the Top Gear guys where they filled to the brim two cars, an "efficient" small car, and a big BMW. They then drove them around the track at sensible motorway speeds for a fixed time, and then filled them to the brim again.

The bigger engine used a lot less fuel because it was strained less.

I don't remember them doing it with town speeds, stopping and starting every few minutes. But it'd be interesting to see them try driving across London, or any major city during rush hour and seeing what the actual difference is.


What's your car's optimum speed in terms of MPG?

Post 6

Bald Bloke

Around town all cars score 0 mpg when static in traffic, so small engines win as they drink less on idle.

Unless of course you have a hybrid or one of the fancy ones that auto stop starts.


What's your car's optimum speed in terms of MPG?

Post 7

Spacial

I think I remember that. But they also did a similar experiment where they drove a car from London to Edinburgh and back on one tank of fuel. Made it as well, but sensible driving.


What's your car's optimum speed in terms of MPG?

Post 8

KB

I'm not sure how much I'd trust Top Gear not to falsify their results if they thought it made better television, though.


What's your car's optimum speed in terms of MPG?

Post 9

Spacial

I don't take TG too seriously either, but on that point, it would be a bit obvious.

Anyway, here's the video:

http://www.youtube.com/watch?v=YBKlo_sfmVE


What's your car's optimum speed in terms of MPG?

Post 10

Orcus

0 mph smiley - winkeye


What's your car's optimum speed in terms of MPG?

Post 11

KB

Then you'd also get 0 mpg. smiley - laugh


What's your car's optimum speed in terms of MPG?

Post 12

Pastey

Interestingly, around town our classic Landrover gets the same mpg as a modern Range Rover.

Cars are not designed for cities, the cities are way too congested. But the cities are still trying to encourage cars in rather than focussing more on providing alternatives.


What's your car's optimum speed in terms of MPG?

Post 13

winnoch2 - Biding my time..

"Cars are not designed for cities". That'll be why service guides describe city driving as 'extreme conditions' when referring to service intervals and tasks, etc. Your average little old man/woman pootling back and forth from the shops 2 miles away every day probably have no idea that they're subjecting their cherished micra to extreme conditions, but they are smiley - laughsmiley - laugh


What's your car's optimum speed in terms of MPG?

Post 14

Orcus

I'm not convinced, personally, that that is a design flaw, merely an inevitable consequence of how a combustion engine works.

I rather suspect that small cars particular are exactly designed for city driving. After all, on a long drive you want something with a large wheel base, passenger room and luggage room ideally. People want a device for just pootling to the shop and back and car manufacturers are stupid if they do not design their products accordingly. I imagine the cars are designed in such a fashion as to minimise as much as possible the effects of these so-called extreme conditions.

Perhaps an engineer could comment on this? smiley - smiley


What's your car's optimum speed in terms of MPG?

Post 15

paulh. Everyone is a critic!

I've seen various optimum speeds mentioned -- 35, 45, etc. There may be other factors than speed that can affect fuel economy. Keeping the tires at optimum pressure, for instance, or waxing/simonizing the exterior to reduce wind drag. Keep extra weight to a minimum. On the other hand, regularly carrying one or more passengers increases the fuel efficiency per person per gallon. Cars that are designed to carry a lot of people in comfort are not a bad deal as long as they carry more than one person.

The thing is, *somebody* has to drive in the city. The city is where most people live, isn't it? They have to go through various amounts of city in order to get to the highways. I live about three miles from a highway. There are seven stop lights between my house and the highway. It's rare that I can get to the highway without hitting at least one or two red lights.

Then there's Winter. Cold air is said to be heavier than warm air, so the car has to work harder to make headway against it, and fuel economy will suffer. Wind is another factor. If you regularly drive a route that gives you a tail wind, that's likely to improve your fuel economy.


What's your car's optimum speed in terms of MPG?

Post 16

swl

<>

Ah, Ginster's pasties have made it over the pond I see.


What's your car's optimum speed in terms of MPG?

Post 17

paulh. Everyone is a critic!

smiley - laugh

Some of us get gas at Mel's Diner. smiley - winkeye


What's your car's optimum speed in terms of MPG?

Post 18

winnoch2 - Biding my time..

Ah yes, it did occur to me when posting this question that it's perhaps more relevant to think of MPG in terms of specific journeys rather than specific cars. For example the same car will guzzle fuel at a different rate if the route is full of hills, with constant gear-changes required, than if it's on a flat motorway. Different cars may also respond differently to the same route.

I now tend to reset my MPG dispay to measure regular journeys, i.e. to work from home and back, rather than try to ascertain an overall MPG for the car. I think this is more meaningful. I get 49 mpg in my newer car for my 70 mile work journey. Last car only managed 38 mpg. So i save around £3 per journey smiley - smiley


What's your car's optimum speed in terms of MPG?

Post 19

bobstafford

My average is 41 mpg
Best speed for maximum mpg is 61 mph returning 55 mpg

Strangley I was expecting less Volvo XC70 4wd 2.4 deisel estate, so I am not complaining however I cant say the mpg thingy is 100% accurate but the range to remaining fuel metervis accurate.


What's your car's optimum speed in terms of MPG?

Post 20

winnoch2 - Biding my time..

Probably shouldn't link to this, but it's not a commercial site so OK I reckon? http://www.fuel-economy.co.uk/calc.shtml Anyway this is great for playing with MPG and fuel prices to see how much your journey costs under different scenarios.


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