Can you drive too slowly ( MPG wise ) ?????

[Tampon]

The Omega I had before said in the handbook the most effient speed for the car was 57 mph, but that was a automatic 4 speed.

I have a Vectra 1.8 5 speed manual now, I drive between 60-65mph on 15 mile journey round m25 ( 2100-2500rpm ) as indicted on speedo.

I was thinking, if I went slower, it should raise the mpg, but does it get to a point when the mpg won't go any higher or would leaving it in fifth on idle ( say 20mph, accelerator left alone ) give the best mpg and anything above that a reduction ?

[CraigyMc]

Tampon said:

The Omega I had before said in the handbook the most effient speed for the car was 57 mph, but that was a automatic 4 speed.

I have a Vectra 1.8 5 speed manual now, I drive between 60-65mph on 15 mile journey round m25 ( 2100-2500rpm ) as indicted on speedo.

I was thinking, if I went slower, it should raise the mpg, but does it get to a point when the mpg won't go any higher or would leaving it in fifth on idle ( say 20mph, accelerator left alone ) give the best mpg and anything above that a reduction ?

Yes, there's a middle ground. That's to say - driving about in 1st gear isn't the most fuel efficent way of trundling about.

My car has quite a long 6th gear - it's about 1200rpm at 51mph, which is as low as you can go on flat ground before it is really unhappy.

I think however, that the best economy on my car is in 5th gear, at about 40mph.

There's no easy way to prove that.

C

[Petrolhead_Rich]

Tampon said:

The Omega I had before said in the handbook the most effient speed for the car was 57 mph, but that was a automatic 4 speed.

I have a Vectra 1.8 5 speed manual now, I drive between 60-65mph on 15 mile journey round m25 ( 2100-2500rpm ) as indicted on speedo.

I was thinking, if I went slower, it should raise the mpg, but does it get to a point when the mpg won't go any higher or would leaving it in fifth on idle ( say 20mph, accelerator left alone ) give the best mpg and anything above that a reduction ?

at very low revs the engine works harder to keep turning, it continues turning by pumping masses of fuel in, most economical speed range is usually between 1500-2500 (depending on the engine)

35-40MPH my car returns 45-55MPG
50-65 I get 30-40MPG
65-75 I get 25-35MPG

but if I drove round at 35-40 to save fuel I would have to hang myself with the shame and guilt of holding everyone up and being a tt!

[Ozzie Osmond]

You would need,

• the engine in its torque band (not too many or too few rpm)
• higher gear (for efficiency)

and, crucially

• modest vehicle speed - because wind resistance rises as the cube of speed!

So you're looking at about 40 mph in 5th. Nasty

[martin84]

Ozzie Osmond said:

So you're looking at about 45 mph in 5th. Nasty

Think of the money you'd save though!

You'd never have any time to spend it as it'd take you a long time to get anywhere but ker-ching!!

[christofmccracke]

I asked this question to one of my logistics lecture about hgvs and she said the most economical speed varies from vehicle to vehicle in regards to drag and the engine in the vehicle

[Pints]

christofmccracke said:

I asked this question to one of my logistics lecture about hgvs and she said the most economical speed varies from vehicle to vehicle in regards to drag and the engine in the vehicle

And not to forget the vehicle's gear ratios.

[varsas]

Petrolhead_Rich said:

at very low revs the engine works harder to keep turning, it continues turning by pumping masses of fuel in, most economical speed range is usually between 1500-2500 (depending on the engine)

I always thought that too, but in last weeks autocar in the review of the new eco 5 series it says it has a special harmonic damper that lets the engine turn at very low revs without vibrating...but why would you want to do that? Does what you have said not apply to diesels or is autocar wrong? It also says the eco gear change says to change up to 6th at 1500rpm, which matches what I thought and you say so why the need for the damper?

Autocar said:

the engine has a special centrifugal pendulum vibration absorber, which allows it to pull improbably low revs in high gears without provoking a grumbling vibro-massage through seat

[StottyZr]

Highest gear the car has with the engine just above labouring revs. Its not difficult to work out the most efficient speed

I once had fun trying to explain to a friend he wasn't saving fuel by driving at 25 in third. He seemed to think that driving slowly saved fuel and wouldn't believe 40 in fifth on the same road would be more fuel efficient.

[McSam]

Petrolhead_Rich said:

at very low revs the engine works harder to keep turning, it continues turning by pumping masses of fuel in, most economical speed range is usually between 1500-2500 (depending on the engine)

35-40MPH my car returns 45-55MPG
50-65 I get 30-40MPG
65-75 I get 25-35MPG

but if I drove round at 35-40 to save fuel I would have to hang myself with the shame and guilt of holding everyone up and being a tt!

Pretty much exactly this. The lowest speed you can manage while still maintaining responsiveness in the highest gear. The responsiveness is your measure of whether or not the engine is "happy" at that speed. Most cars I'd keep it around 2000rpm, which you can find the logic of easily since a lot of six-speeds are set up so that sixth is 2000rpm at motorway speeds!

Aerodynamic drag increases with the square of speed, and power required also increases with speed, so the overall factor is a cube of speed, as someone said. Thanks to this, you need 5.4 times more power to run at 70mph than you do at 40mph.

[ajsphead]

The SD1 is more economical at 90 in 5th than 30. It's partly a simple matter of throttle input. You need less to keep it at 90 than you do to try to keep it at 30 as the engine's going slower than idle speed so it accelerates a bit, then you let off, then accelerate etc. Also at 2700rpm (indicated 90) the engine is burning fuel far more efficiently than at 600rpm (indicated 30).

[frosted]

Under 50mph I should think , how boring would that be ?? If your that bothered about mpg buy a diesel and saves us another driving thinking 40 is plenty

[RenesisEvo]

Also consider that below about 40mph, the rolling resistance of the tyres, drivetrain, etc. will outweight the aerodynamic drag. So if you can keep the engine at an efficient point, the optimum will probably be the speed at which tyre/mechanical rolling resistance = aerodynamic drag. Above this speed, as mentioned, drag increases by the square of speed (so energy used increases by the cube), so going faster is only going to use more fuel.

Of course we are assuming a constant speed. If you are in a more urban or B road environment, you'll want a lower gear to give you flexibility, and plenty of engine braking (injectors shut off therefore no fuel). It's a shame you can't find brake specific fuel consumption (BSFC) data for most engines, as that tends to give you the most efficent operating point in terms of load and rpm.

There's a huge number of factors involved in working out the most efficient point, so to answer the OP in short, yes, you can drive too slowly.

[CraigyMc]

[redacted]

[kambites]

Optimal speed in all the cars I've ever owned, has been between 25 and 35mph. Never in top gear, either.

[CraigyMc]

ajsphead said:

The SD1 is more economical at 90 in 5th than 30. It's partly a simple matter of throttle input. You need less to keep it at 90 than you do to try to keep it at 30 as the engine's going slower than idle speed so it accelerates a bit, then you let off, then accelerate etc. Also at 2700rpm (indicated 90) the engine is burning fuel far more efficiently than at 600rpm (indicated 30).

Sure, but if you're in the right gear for the speed, then 30mph will be more economical than 90mph.

[dazco]

I learned at school, a fair few years ago, that wind drag is not linear. And the biggest first 'leap' comes just after 56mph.

Assuming your gearing allows a car to cruise at low revs at 55mph I would suggest this would be the optimum mpg speed.

[kambites]

dazco said:

I learned at school, a fair few years ago, that wind drag is not linear. And the biggest first 'leap' comes just after 56mph.

There are no specific "big leaps". Each individual car will have speeds at which the airflow detaches from the body in different places and creates step changes in drag, but it will be different for every car. Drag just increases exponentially (ie continuously) as you speed up. The only reason cars aren't most efficient idling along in first is that both engines and drive-train horrifically inefficient doing that.

[ajsphead]

What always seems more interesting to me is that there's no gear at which it's more economical at 30 than it is in 5th at 90. The tipping point in 5th is about 65 (1900rpm) The trip computer has a constant readout based on fuel pressure multiplied by length of injector opening time to give fuel volume. It's surprisingly accurate, although actually a bit on the stingy side when you double check it by hand.

[kambites]

ajsphead said:

What always seems more interesting to me is that there's no gear at which it's more economical at 30 than it is in 5th at 90. The tipping point in 5th is about 65 (1900rpm) The trip computer has a constant readout based on fuel pressure multiplied by length of injector opening time to give fuel volume. It's surprisingly accurate, although actually a bit on the stingy side when you double check it by hand.

That would mean that your engine is nine times as efficient at 90mph in fifth than in the best gear at 30mph. Given that it will be at most 40% efficient at 90, that means it's about 5% efficient at 30mph. What the hell is it doing with the fuel?