What's the ideal speed for max mpg?

[Sippo]

88mph... then you can go back in time to a point where fuel is cheap, fill up for peanuts and not have to worry

Might need a small mod to the car first

Where we're going we don't need roads...

[wilsy7]

5th at around 1650-1700rpm seems great on mine, instantaneous never seems to drop below 60-70mpg, car just seems to drive itself nearly, very little throttle needed

[Silvergamer]

I managed to get 71.5 MPG average over 150 miles.

That was mostly in 6th gear at 1500 rpm at about 60MPH.

And that was in a 2.0 140PS CR TDi.

2 things i've noticed over the years that kill MPG is Air Con and funnily enough Cruise Control.

[jrw]

Air con makes little or no difference especially on diesel. Its another urban myth. Only on an inadequately engined car would it make a difference..such as a 1litre car with barely enough power to pull it along without the air con being on.

[Coffin Dodger]

Just done my annual holiday calculation, we did 1191.3 miles at an average of 47.27 mpg, well impressed, speeds varied from 30-110 mph (where allowed) on all types of roads and traffic conditions. 114.6 litres of fuel used, (25.20 Gallons for us oldies) spent £156.68.

By the way, isn't motorway fuel expensive! saw £140.9 per litre in places wow! I usually pay £135.9 p/lt and being a tight Scot grudge that.

However, I really rate the Fabia vRS Tdi, where else can you have so much fun for so little outlay?

[vindaloo]

On a 1.9TDi a speed where you're in top gear and the engine's doing 1500-1900 revs. Any more than 1900-2000 and MPG goes down on mine.

On a sound/feel/resonance level though, the car's far better around 2500-3000 RPM.

So less than 1900 for economy and 2500+ for feel good and responsiveness.

J.

[Kiwibacon]

Air con makes little or no difference especially on diesel. Its another urban myth. Only on an inadequately engined car would it make a difference..such as a 1litre car with barely enough power to pull it along without the air con being on.

Extra engine load is extra engine load. It doesn't matter what size the engine is, it's still going to cost you more fuel.

[999pooch]

Air con makes little or no difference especially on diesel. Its another urban myth. Only on an inadequately engined car would it make a difference..such as a 1litre car with barely enough power to pull it along without the air con being on.

It doesnt matter the size if the engine, but more the size (load) of the ac compressor.

Typically ac systems in cars use 8-10% more fuel than those without.

Many sources of info on the tinterweb, but a good link below -

http://www.airconco.com/news/does-in-car-air-conditioning-consume-fuel-if-so-how-much-648.html

[Phil-E]

On a 1.9TDi a speed where you're in top gear and the engine's doing 1500-1900 revs. Any more than 1900-2000 and MPG goes down on mine.

On a sound/feel/resonance level though, the car's far better around 2500-3000 RPM.

So less than 1900 for economy and 2500+ for feel good and responsiveness.

J.

3000rpm is well into triple figures on my car but it does feel very effortless at that speed and like it has a lot more to give. Even when we've driven for prolonged periods at those kind of speeds the economy has been very very suprising.

And with A/C. I'd rather be comfortble and have mist free windows than save the little bit of fuel having it turned off though.

Phil

[jrw]

Extra engine load is extra engine load. It doesn't matter what size the engine is, it's still going to cost you more fuel.

It doesnt matter the size if the engine, but more the size (load) of the ac compressor.

Typically ac systems in cars use 8-10% more fuel than those without.

Many sources of info on the tinterweb, but a good link below -

http://www.airconco....w-much-648.html

Rubbish.

Have you never tested it?

I can do a run to work and back, one with a/c and one without and they are identical within +/- 1MPG which is obviously due to outside influences and not the 'extra load' of the compressor. This has been the case for my 100bhp 1.9PD Polo, 130bhp 1.9PD Bora and 2.0CR Octy.

Diesels have the torque to cope with the extra load with no effect, small petrols, however it does make a massive difference as I have noticed on hire cars I have had in the past. If a/c used more fuel on my car i would know about it.

Having windows open, low tyre pressure, roof bars, roof box, bike carriers all make much more difference than a/c i am afraid.

[Kiwibacon]

Rubbish.

Have you never tested it?

I can do a run to work and back, one with a/c and one without and they are identical within +/- 1MPG which is obviously due to outside influences and not the 'extra load' of the compressor. This has been the case for my 100bhp 1.9PD Polo, 130bhp 1.9PD Bora and 2.0CR Octy.

Diesels have the torque to cope with the extra load with no effect, small petrols, however it does make a massive difference as I have noticed on hire cars I have had in the past. If a/c used more fuel on my car i would know about it.

Having windows open, low tyre pressure, roof bars, roof box, bike carriers all make much more difference than a/c i am afraid.

Exactly how much power AC sucks depends hugely on the ambient conditions. When it's clutched in and working hard it's pulling about 3kw straight from the crank. 3kw on a diesel with BSFC of 250g/kwh is 750g of diesel (almost a litre) per hour.

If your situation isn't stinking hot then the AC compressor keeps kicking itself out when the evaporator gets too cold to prevent it icing up (2 deg is a common cut-out temp). Then it cycles in and out as needed to maintain the set temperature.

In the worst case it'll be sucking another litre of diesel per hour, in the best case when it's almost never kicking in the result will be hard to notice in fuel consumption. But in no way is it accurate to say a more torquey engine suffers no penalty from air-conditioning use.

[Phil-E]

I find that with the A/C running round town the economy really takes a nose dive and often see 25-30mpg in start/stop. This is nearer 35-40mpg with the "econ" button pressed.

Can't compare what difference it makes while on the move and we rarely turn "econ" on on motorway runs and just leave the climate to do it's thing.

I think the difference is less noticeable at higher speeds and while cruising etc.

I do remember though when we hired a Yaris in Australia (1.0l I think) and turning the A/C on and off even on the highway you could actually feel the it and there was a noticicable drop in performance (not that there was much to begin with). We tried running with the A/C off but 40c and crazy humidity it was unbearable and just left it on!

Phil

[gregoir]

Turning my climate control off has no noticeable effect on the trip mpg. I get 50mpg whether I pootle along, in 6th, on my cross country commute or drive at a true 70mph. Faster than that the mpg suffers.

Never found a better (slower) speed-I'd have to start driving around in lower gears which gulps the juice.

[countryboy]

The best speed is that at which the car, will pull comfortably in top gear at the slowest speed it will do so, usually around the peak torque speed as that will, within a little, coincide with the lowest specific fuel consumption and near to lowest absolute consumption.

Ian

[KenONeill]

Exactly how much power AC sucks depends hugely on the ambient conditions. When it's clutched in and working hard it's pulling about 3kw straight from the crank. 3kw on a diesel with BSFC of 250g/kwh is 750g of diesel (almost a litre) per hour.

If your situation isn't stinking hot then the AC compressor keeps kicking itself out when the evaporator gets too cold to prevent it icing up (2 deg is a common cut-out temp). Then it cycles in and out as needed to maintain the set temperature.

In the worst case it'll be sucking another litre of diesel per hour, in the best case when it's almost never kicking in the result will be hard to notice in fuel consumption. But in no way is it accurate to say a more torquey engine suffers no penalty from air-conditioning use.

Complete ball****s - The 3kwh is true, but the extra demand is proportional to what demands the driver is making on the engine rather than to the SFC.

[Phil_P]

The best speed is that at which the car, will pull comfortably in top gear at the slowest speed it will do so, usually around the peak torque speed as that will, within a little, coincide with the lowest specific fuel consumption and near to lowest absolute consumption.

Ian

That was my thought - best consumption/efficiency should coincide with peak torque, all other tings being equal.

[gadgetman]

On a 1.9TDi a speed where you're in top gear and the engine's doing 1500-1900 revs. Any more than 1900-2000 and MPG goes down on mine.

On a sound/feel/resonance level though, the car's far better around 2500-3000 RPM.

So less than 1900 for economy and 2500+ for feel good and responsiveness.

J.

Been testing the ideal MPG lately and found it's not so much speed but the ideal RPM which then gives you a speed.

The works car, loaded, seems to like 1900 on motorways. Havent tried round town. Be easier if it had cruise :(

[Kiwibacon]

Complete ball****s - The 3kwh is true, but the extra demand is proportional to what demands the driver is making on the engine rather than to the SFC.

Care to back that up?

3kwh is 3kwh. That is the demand and the engine supplies. You'll have to expand and explain this "extra demand" and "demands the driver is making". Especially the bit where it somehow over-rides SFC. I'm open to the laws of physics being bent today, there is an olympics on.

[KenONeill]

Care to back that up?

3kwh is 3kwh. That is the demand and the engine supplies. You'll have to expand and explain this "extra demand" and "demands the driver is making". Especially the bit where it somehow over-rides SFC. I'm open to the laws of physics being bent today, there is an olympics on.

SFC varies with engine revs, but you already knew that. So does actual power output, but you knew that too. What you seem to have omitted from consideration is that steady speed driving means steady revs driving, and at some road speed/gear combinations the engine is actually making more power than is needed to overcome mechanical and aerodynamic drag. The reason the car is not accelerating is that both the engine revs and gear ratio are constant.

By way of practical illustration, I left work yesterday and the Climatronic was blowing full cold in Auto. I put the trip computer into "instantaneous mpg" mode as i reached a straight, flat piece of road, and took a reading. I switched the Climate into "Econ" mode, which turns off the aircon compressor. For your treatise to hold true, the instantaneous mpg figure should have improved significantly.

[Kiwibacon]

What you seem to have omitted from consideration is that steady speed driving means steady revs driving, and at some road speed/gear combinations the engine is actually making more power than is needed to overcome mechanical and aerodynamic drag. The reason the car is not accelerating is that both the engine revs and gear ratio are constant.

This is the bit I cannot agree with (in bold above).

An engine can't put out more power than is being absorbed by the load applied.

Newtons first law (Conservation of energy) and all that.

By way of practical illustration, I left work yesterday and the Climatronic was blowing full cold in Auto. I put the trip computer into "instantaneous mpg" mode as i reached a straight, flat piece of road, and took a reading. I switched the Climate into "Econ" mode, which turns off the aircon compressor. For your treatise to hold true, the instantaneous mpg figure should have improved significantly.

When continually bombarded by a thousand variables, instantaneous fuel consumption can be a little hard to reconcile with one minor change.

Edited 31 July, 2012 by Kiwibacon

[2SkodaFamily]

With cruise control set at 80, my mpg is low 50s, say 50-51. Cruise set at 70, mpg is 57 or so. If I set cruise control to 60, I get an indicated 65mpg and more. That's about 2000 rpm on my car, which is interesting given that its the same engine (and gearbox?) as Phil's Cordoba.

I ran my car for the first 18 months I had it with climatronic set to Econ unless it was really hot. The last 3 years I've run in Auto all the time and I've seen no drop in the fuel economy as a result, but this could be that the conditions aren't really stressing the system much (i.e. crap weather)

[KenONeill]

This is the bit I cannot agree with (in bold above).

An engine can't put out more power than is being absorbed by the load applied.

Newtons first law (Conservation of energy) and all that.

So, if I'd changed down from top gear to 3rd but held the same road speed, my fuel consumption would not have roughly doubled as the fuel demand doubled to meet the higher engine speed? I think it would have, despite the extra power available (power = torque * engine speed). So clearly it's possible for an engine to make power that's not being used to overcome drag, or accelerate or lift a mass.

Why is it so difficult for you to accept that some of that power could be used to run ancilliary devices?

When continually bombarded by a thousand variables, instantaneous fuel consumption can be a little hard to reconcile with one minor change.

I'll ignore the hyperbole of "a thousand variables", and ask which variables you think varied given a level road, no other traffic, constant weather and a driver who is trying to hold a constant throttle position and observe the effects of air con on speed and mpg?

[Padrino]

It depends a lot of the way you drive. In my 2003 1.9TDI PD100 estate, if I drive "spirited" I get 50/51MPG if I go the same route with steady 62/65MPH my fuel consumption goes up like a rocket to a 64/65MPG. This is for a 90 miles drive a day, 45 miles each way: 50% motorway, 40% B road and 10% city. The time difference is between 5 an 10 minutes overall one way, but the MPG difference is impressive 15MPG. I've been travelling this stretch for over a year now so my numbers are very accurate. If I stay "calm" I go 710/730 per vented tank with no problem, otherwise 560/580 miles per tank.

[Kiwibacon]

Sorry, should have been Newtons 3rd Law.

So, if I'd changed down from top gear to 3rd but held the same road speed, my fuel consumption would not have roughly doubled as the fuel demand doubled to meet the higher engine speed? I think it would have, despite the extra power available (power = torque * engine speed). So clearly it's possible for an engine to make power that's not being used to overcome drag, or accelerate or lift a mass.

Why is it so difficult for you to accept that some of that power could be used to run ancilliary devices?

At higher rpm pumping and frictional losses are higher, that's where your extra fuel is going. If you are holding the same speed, you are putting the same power to the ground, if you were putting down more the car would accelerate. If you put down less the car would decellerate. Assuming all externals are kept the same, which they almost never are.

Power available =/= power being used.

The power used to run ancillary devices is not free. It comes from fuel in your tank. The more devices you run, the more fuel you burn.

Even your headlights use fuel from the tank. It's an amount small enough to not easily notice, but it's still there.

Air-con is the biggest single ancillary device you can run on most cars, which is why it can have the biggest effect on fuel economy.

[gadgetman]

Instantaneous mpg Ken? You mean the one that often says you're mpg is 99.9?

You need to retry using the average memory 1 and reset whilst at a steady speed and run for a few miles with air con, and reset and repeat without.

You won't see much difference