Do I use less petrol at lower revs?

[swee'pea99]

You can start off in a low gear and accelerate quickly, then change up the gears. Or you can start in a high gear, and accelerate slowly. Or you can start in a high gear, get off the saddle, have initial acceleration be slow but quite quickly build up a good turn of speed. You should do how you feel is right for you, using the least amount of energy. Same with a car, just do what feels right.

I guess that's the nub of the matter. Any driver knows how to make changes so it 'feels right'. Question is, does that, in fact, minimise fuel consumption (for any given distance travelled)?

As a corollary to that, presumably, if instead of driving so it 'feels right', you gun the engine and use higher revs, intuition says you're probably using more petrol. How about doing the opposite? If you drive in a higher gear, at lower revs, do you also use more petrol? Or do you, in fact, use less?

 

[snorri]

I had that problem when I owned a Renault. Since selling the thing and buying a different make it has gone away.

I have had it with different makes, it has been the bane of my life of driving, never had a Renault.

 

[tyred]

As a corollary to that, presumably, if instead of driving so it 'feels right', you gun the engine and use higher revs, intuition says you're probably using more petrol. How about doing the opposite? If you drive in a higher gear, at lower revs, do you also use more petrol? Or do you, in fact, use less?

An interesting thing I always noted when I had a MK1 Golf 1.6 diesel is that the the 3 mile journey from the main road to my house is gradually climbing on 3 miles of slightly twisty country road and the Golf seemed perfectly happy to drive up this hill at around 40mph in 5th gear without any obvious complaint but the temperature gauge always rose slightly whereas if I drove at 40mph in 4th gear, the temperature gauge never moved from where it normally sat.

I can only conclude it was using slightly more diesel in the higher gear as that extra heat had to come from somewhere???

 

[gbb]

I have always done this, can't break the habit, but every time my cars have required to have brake problems fixed they tell me the problem has arisen due to lack of use of the brakes, and I am urged to use the brakes more.
What I save on road fuel I spend on brake repairs, you can't win!

While its difficult to quantify how braking differs person to person, car to car, my driving style is to roll up to junctions etc, if i use the brakes its sparingly, its always been this way for me.
In 38ish years of driving, i replace cars every 3 to 4 years..i really cant remember the last time i had to replace brake pads, i think it was on my Maestro, maybe 20 years ago. But i don't get braking problems, or problems associated with lack of use.
Its not a comment of any use really, just a comparison from another POV.

Actually, thinking about it, i did suffer a bit with a sticky handbrake on my Focus, always when it was wet....just musing.

 

[snorri]

While its difficult to quantify how braking differs person to person, car to car, my driving style is to roll up to junctions etc, if i use the brakes its sparingly, its always been this way for me. But i don't get braking problems, or problems associated with lack of use.
Its not a comment of any use really, just a comparison from another POV..

Interesting gbb, the problems I get, according to those who are suppose to know, are pads not coming away from the drums/discs after heavy braking and discs/drums rusting. I've lost count of the times different garage folk have told me to use the brakes more. This is beginning to sound like a car forum, horror of horrors.

 

[mustang1]

I guess that's the nub of the matter. Any driver knows how to make changes so it 'feels right'. Question is, does that, in fact, minimise fuel consumption (for any given distance travelled)?

As a corollary to that, presumably, if instead of driving so it 'feels right', you gun the engine and use higher revs, intuition says you're probably using more petrol. How about doing the opposite? If you drive in a higher gear, at lower revs, do you also use more petrol? Or do you, in fact, use less?

Depends on engine load, not just rpm. 30mph in first gear = using too much fuel. 20mph in 7th gear = using too much fuel. But let's talk about bikes for a sec... what feels right for you... do you need a power meter to give more accurate results? Possibly, but I think w/o using power meter the rider wont be too far off (that's my hunch anyway). I guess it's the same for a car. I see your point however, and agree with it.

 

[asterix]

They need to invent a car that if you drive it in reverse the tank fills up.

 

[GrasB]

I think we concentrate too much on driving economically. You should drive in a way that is safe. That means being in a low enough gear to accelerate or brake briskly without needing to change gear. Sitting at 1250 rpm where the car takes half a second to respond to a throttle input or cause juddering & reduced braking power maybe economical but it means in any situation you've got several more actions to do in an unexpected situation to keep everything balanced (when you unbalance your vehicle you're 2/3 of the way to an accident).

Slowly allowing your speed to dissipate without braking is a problem, as the speed change is very slow without any indication that it's happening the person behind can end up getting caught out & rushing up to the back of you causing them to brake suddenly & quite aggressively, this proper gates back & can cause a traffic flow stall. You often get this effect at roundabouts, the traffic seems to stall about 500m from the roundabout then accelerate into it. In convoy driving this is almost always someone drifting into the roundabout slowing down with gears rather than braking & showing their intent. It doesn't need to be a slamming on the brakes thing either.

 

[gbb]

[QUOTE 2780614, member: 9609"]What a complete load of tosh - People who drive economically drive slower and hence they are much safer to themselves and every other road user. Plus the savings are enormous - I have improved from 40 to nearly 60mpg in the same vehicle, (and as I enjoy driving I welcome the extra time behind the wheel)

Driving slower does not impair your brakes (vacuum reservoir holds enough energy) and even if it did, driving slower means your brakes don't have to work so hard, I'm more worried about the boy racer who has over heated his brakes and sudden fading is a possibility.
Whilst driving for economy I can't really think of any situations where being able to accelerate quickly is major safety thing... better to have the foot ready on the brake pedal.
And as for the unobservant idiots who suddenly need to brake sharply because they aren't concentrating - they are the one causing the problems.[/quote]

As always, there is a crumb of truth in any arguement, GrasBs point maybe true in specific circumstances...BUT, in 38 years of driving, in all styles, from speeding knobber in my younger days to steadyish economical driver nowadays (and perhaps occasional knobber nowadays, i aint perfict...if i thought about it, the times ive had to accelerate in a hurry to get out of a situation are....virtually nil, in 38 years.
GrasB has a point, but it doesnt relate to my experiences...i suspect mine are typical of most people.

 

[GrasB]

[QUOTE 2780614, member: 9609"]People who drive economically drive slower and hence they are much safer to themselves and every other road user. Plus the savings are enormous - I have improved from 40 to nearly 60mpg in the same vehicle, (and as I enjoy driving I welcome the extra time behind the wheel)[/quote]
Driving slower doesn't mean driving at stupidly low rpm, causing problems with vehicle dynamics.

Driving slower does not impair your brakes (vacuum reservoir holds enough energy) and even if it did, driving slower means your brakes don't have to work so hard, I'm more worried about the boy racer who has over heated his brakes and sudden fading is a possibility.

No but an engine & near idle will resist the effort to brake causing increases is stopping distances. So instead of simply coming off the throttle with your left foot & applying the brake with your right foot, you've got do one of the following
- come off the throttle & apply the brake with your left foot while dipping the clutch with your right foot
- immediately yank the 'box out of gear under heavy load as the engine & brakes fight each other making it much more difficult than normal

Whilst driving for economy I can't really think of any situations where being able to accelerate quickly is major safety thing... better to have the foot ready on the brake pedal.

If one needs to turn aggressively torquing down the chassis has DRAMATIC increases in the levels of grip you have. To do that you need to have the engine response to produce a reasonable amount of force.
In addition to this accelerating for a very short time period makes space for someone to get them selves out of a minor problem where if I got on the brakes multiple people would be involved in a minor issue. Failure to see this these situations shows a lack of road craft.

And as for the unobservant idiots who suddenly need to brake sharply because they aren't concentrating - they are the one causing the problems.

Small changes are hard to see, this is known. By slowing down without showing any intent you force the person to pay a lot of attention to the vehicle to see the subtle changes. It ties up concentration time & observation time which would be better served in general environment observations.

 

[Matthew_T]

They need to invent a car that if you drive it in reverse the tank fills up.

You have been watching too much Ferris Bueller.

 

[gbb]

Driving slower doesn't mean driving at stupidly low rpm, causing problems with vehicle dynamics.


No but an engine & near idle will resist the effort to brake causing increases is stopping distances. So instead of simply coming off the throttle with your left foot & applying the brake with your right foot, you've got do one of the following
- come off the throttle & apply the brake with your left foot while dipping the clutch with your right foot
- immediately yank the 'box out of gear under heavy load as the engine & brakes fight each other making it much more difficult than normal


If one needs to turn aggressively torquing down the chassis has DRAMATIC increases in the levels of grip you have. To do that you need to have the engine response to produce a reasonable amount of force.
In addition to this accelerating for a very short time period makes space for someone to get them selves out of a minor problem where if I got on the brakes multiple people would be involved in a minor issue. Failure to see this these situations shows a lack of road craft.


Small changes are hard to see, this is known. By slowing down without showing any intent you force the person to pay a lot of attention to the vehicle to see the subtle changes. It ties up concentration time & observation time which would be better served in general environment observations.

Sorry GrasB..NONE of that relates to my driving experiences of 38 years. In some obscurely minute set of circumstances..or on the race track, it may be relevent...but not, or never knowingly has been in my world.
I don't doubt if i could be bothered to learn about vehicle dynamics, what you're explaining may well be true....but it just doesnt relate to normal everyday driving. 99% of people wont know what your're talking about, i'll be one of them...but we've all managed perfectly well in our state of blissful unawareness..

 

[Canrider]

[QUOTE 2776853, member: 9609"]Whilst cycling, would changing up a gear early save energy ?[/quote]
I think that depends on how we define 'early'..

 

[Canrider]

an engine & near idle will resist the effort to brake causing increases is stopping distances

For my benefit, could you point out who in this thread is advocating driving with the engine at just above idle, as I've not seen them?

 

[Dan B]

So instead of simply coming off the throttle with your left foot & applying the brake with your right foot, you've got do one of the following
- come off the throttle & apply the brake with your left foot while dipping the clutch with your right foot

Having real trouble visualising how you'd do either of these things unless you were driving with your legs crossed, which surely can't be good for safety?