Question about changing gears

titaniumx3 · 22 Sep 2012 at 00:27

When I drive I normally shift to higher gears as soon as I can and very rarely go above 2000 rpm whilst town driving.

Always been taught to do this for various reasons though I'm beginning to question if it's such a good idea as I find that for normal town driving with frequent traffic lights and moderate congestion, I'm constantly changing gears. I get the feeling I might be changing gears too much and it might be putting excessive wear on the gearbox and clutch.

Also is it actually a good thing to drive at low rpm all the time? Maybe revving the engine moderately is healthy? Also does driving at low rpm really save that much fuel?

Probably stupid questions, but would like some advice from people who know better.

Craigus · 22 Sep 2012 at 00:40

Driving at low rpm puts a lot of strain on the engine also. Especially in 4th, 5th and 6th. Is your car a diesel?

Revving highly is fine, cars aren't anywhere near as fragile nowadays as they used to be.

Biohazard · 22 Sep 2012 at 00:47

rag the nuts off it and don't shift until you've hit the limiter

titaniumx3 · 22 Sep 2012 at 00:47

Craigus said:
Driving at low rpm puts a lot of strain on the engine also. Especially in 4th, 5th and 6th. Is your car a diesel?

Revving highly is fine, cars aren't anywhere near as fragile nowadays as they used to be.

2L petrol with six gear manual transmission.

Does driving at low rpm with the engine borderline struggling really save on fuel?

Jonny ///M · 22 Sep 2012 at 00:50

I sit at 30mph, according to one of those flashy signs, in 5th @ 1500rpm which doesn't make any struggling noises unless it was up a hill. I don't think this damages much but having it nearly stall the way some taxi drivers drive can't be good for the engine?

I find the car feels more 'alive' when your north of 2k rpm though and will pull easier if needed.

Biohazard · 22 Sep 2012 at 00:50

titaniumx3 said:
2L petrol with six gear manual transmission.

Does driving at low rpm with the engine borderline struggling really save on fuel?

No, not greatly.

It also puts a great deal of stress on teh engine if you are being inappropriate with it.

rodenal · 22 Sep 2012 at 07:04

I tend to drive in whatever is the most flexible gear for the situation tbh, town driving where I have to slow right down quite a bit ilk just stick it in 3rd which will quite happily do me from about 8mph to (well over obviously) 30 - its diesel though not sure my previous petrol motors would have been the same, I certainly never drive in as high a gear as possible unless on motorway though . Even then I dont use 6th unless over circa 60mph

Matt82 · 22 Sep 2012 at 07:29

The fuel saving logic is questionable. In the higher gear you may be having to use more throttle to maintain the same speed in a higher gear.

If you're so concerned about fuel consumption, get some sort of obd device so you can monitor it properly. In my experience the smallest throttle opening possible is king when it comes to mpg

Berger · 22 Sep 2012 at 08:07

Rather than use 6th up steeper hills, it is more economical to drop to 5th or 4th in my car as you use more revs but less throttle opening to maintain speed. The car is geared long to decrease fuel consumption, but being a smaller engine needs some revs for power.

Around town I would try to drive smoothly more than anything else, anticipating the changes of lights, at roundabouts maintain speed, avoiding hard acceleration and braking.

rexehuk · 22 Sep 2012 at 09:00

1st is the most economical gear. Don't use the brakes just the handbrake.

Conscript · 22 Sep 2012 at 09:10

titaniumx3 said:
2L petrol with six gear manual transmission.

Does driving at low rpm with the engine borderline struggling really save on fuel?

Probably not as much as you think.

See for yourself. Spend the next month shifting at 2000RPM and record your MPG. Then start shifting at 3000RPM, record the MPG again and see if there's much difference. I reckon you wont see much drop in MPG and even if you do, shifting at a higher RPM will probably make the car easier and more comfortable to drive as you'll be making better use of the engine's powerband and will probably be better for your engine in the long run.

Dogbreath · 22 Sep 2012 at 09:17

Matt82 said:
The fuel saving logic is questionable. In the higher gear you may be having to use more throttle to maintain the same speed in a higher gear.

That's the whole point; lower RPM with more throttle is more economical than higher RPM and a smaller throttle opening. Lower pumping losses, higher dynamic CR.

Berger · 22 Sep 2012 at 09:52

Dogbreath said:
That's the whole point; lower RPM with more throttle is more economical than higher RPM and a smaller throttle opening. Lower pumping losses, higher dynamic CR.

So as per my comment above, there must be a crossover point where this is no longer true and it is of benefit to change down.

When using the "instantaneous" fuel consumption read out which averages the consumption over 6 seconds iirc why is it more economical to use a lower gear and increase the RPM and reduce the throttle opening on steeper hills?

Simon · 22 Sep 2012 at 10:43

Dogbreath said:
That's the whole point; lower RPM with more throttle is more economical than higher RPM and a smaller throttle opening. Lower pumping losses, higher dynamic CR.

Yes this is true. There are huge benefits of changing gear as soon as possible. Lower revs = less friction too

I have access to a lot of work done in this area and the summary is as above.

Dogbreath · 22 Sep 2012 at 11:38

Berger said:
So as per my comment above, there must be a crossover point where this is no longer true and it is of benefit to change down.

Certainly. As RPM gets lower the length of time available for heat loss to the combustion chamber and cylinder walls increases, and valve overlap may start working against you. Once these losses exceed the benefits from lower RPM it's time to drop a gear.

lordrobs · 22 Sep 2012 at 11:46

I tend to drive in as high a gear as possible and use heavy throttle but low revs when pottering about. Seems to have been the most efficient way to drive in all my cars so far.

There is no magic formula as to what is too low rpm wise, you'll feel and hear if it starts labouring.

Randomface74 · 22 Sep 2012 at 11:54

I change to:
2nd asap
3rd to accelerate
4th for cruising around town
5th for motorways etc

but Oxford roads are slow..

LeMson · 22 Sep 2012 at 12:07

town driving for me = 1st gear 3.5k about 20mph, 2nd gear until 30, then 4th to cruise

Matt82 · 22 Sep 2012 at 13:20

Simon said:
Yes this is true. There are huge benefits of changing gear as soon as possible. Lower revs = less friction too

I have access to a lot of work done in this area and the summary is as above.

But there are lots of times you need to get upto x speed and keeping the throttle open further for longer in a higher gear isn't the best for economy.

Basically there are no hard and fast rules.

Hypermilers will know more, their info on pulse and glide sounds logical

mjt · 22 Sep 2012 at 17:05

I change up early too.
Pootling around town at 30-40 will mean 4th. 50+ will usually be fifth.

1st and 2nd are usually quite stiff, and there's a lovely clunk into 2nd (they all do this).
4th gear is very slick and it's a nice shift.

FWIW, no one in China drives above 2000rpm. It's very odd. Audi drove 5 new A6s all around China never going above 2000rpm to test them purely for the chinese market.