Changing Gear - Speed & Revs

jasonb

Hi all,

Just curious, something I've been thinking about the last couple of times I've been driving. Is there an optimum point to change gear? I mean, of course there is, especially in racing etc., but in average day to day driving?

I drive a 2ltr diesel ( Peugeot 406 ). Is there a particular speed ( or revs ) that I should change gear at? Is it something consistent ( like I should always change gear at say 2500 revs, no matter what gear ), or does it change depending on what gear I'm going to/from? I suppose I mean optimum in terms of good for the engine / good for performance.

Another question, does Fuel Consumption come into it as well? If I'm doing 60kmh in 3rd, am I using more / less fuel than doing 60kmh in 4th?

Like I said, just curious about this, any thoughts / advice appreciated, thanks!

J.

Max Power1

jasonb wrote: »

Hi all,

Just curious, something I've been thinking about the last couple of times I've been driving. Is there an optimum point to change gear? I mean, of course there is, especially in racing etc., but in average day to day driving?

I drive a 2ltr diesel ( Peugeot 406 ). Is there a particular speed ( or revs ) that I should change gear at? Is it something consistent ( like I should always change gear at say 2500 revs, no matter what gear ), or does it change depending on what gear I'm going to/from? I suppose I mean optimum in terms of good for the engine / good for performance.

Another question, does Fuel Consumption come into it as well? If I'm doing 60kmh in 3rd, am I using more / less fuel than doing 60kmh in 4th?

Like I said, just curious about this, any thoughts / advice appreciated, thanks!

J.

As regards to optimimum revs, it would depend on whether you are driving a diesel or petrol. Diesel would be lower - most common diesels redline between 4-5k revs, and optimum power is just before this afaik.

Once the engine is not laboring, you would normally be using less fuel doing 60kmh in 4th than in 3rd.

Anan1

It depends far too many variables to come out with a definitive figure, the incline and what you plan to do next being the first two to come to mind.

peasant

The most fuel efficient way to drive a diesel is to keep it at the revs where it has its highest torque, which in a diesel is usually at fairly low revs.

R.O.R

On most Diesel cars there is a point in the rev range where the noticable acceleration stops, but the noticable noise increases - it's usually here that I'd change gear and that should drop you to just below the power band of the engine in the next gear.

A lot of the newer cars have a gear change indicator for the most fuel effecient way of driving. I'm usually late in to 2nd (on a BMW it usually indicates a shift is needed as soon as you get past 0.05 km/h), pretty good from 2nd to 3rd to 4th, but in to 5th and 6th too early.

When trundling through town or a housing estate I'll quite often be in too high a gear (according to the car) even though the engine isn't labouring, for the speed I'm going.

Anan1

BMW ran an ad years ago (for the E30 325i I think) where they claimed that the most fuel-efficient way to attain cruising speed was through pretty much flat-out acceleration from standstill. Clever advertising, even if it wasn't true.

Umiq88

I drive a diesel but its turbo charged I'll generally keep it just below 2000rpm which is just before the turbo kicks in that way if i need to pass out or want a bit of boost she's a fair bit of power left. I find changing at around 3500 to be the best anything after that and its out of the turbo range and is giving less power and reving the enginer alot more.

For economy or motor way driving its aroun 1800rpm unless I'm in 5th and going to fast to stay below that.

As others have said there's alot of variables and its different from car to car.

Anan1

Anan1 wrote: »

BMW ran an ad years ago (for the E30 325i I think) where they claimed that the most fuel-efficient way to attain cruising speed was through pretty much flat-out acceleration from standstill. Clever advertising, even if it wasn't true.

I read this at some hypermiling website: At high loads the efficiency of the engine is at it's best so the BMW claim is pretty much true. On most of the cars the fuel mixture is enriched when the pedal is on metal so something like 90 percent throttle probably close enough the ideal. Once the cruising speed is reached change to the highest gear possible (where the engine pulls cleanly).

Having said that, slower acceleration does not harm the economy as the resulting lower average speed to the cruising speed makes up for the decreased engine efficiency.

I think the conclusion was that when aiming for good mpg you can't really hurt the economy whatever you do with the throttle but you should avoid using brakes at all cost. Reading the traffic conditions ahead and slowing down by throttle instead of brakes makes a big difference in fuel consumption.

JimmyCrackCorn

http://www.autospeed.com/A_111510/hDg34uLtp_1/cms/article.html
http://www.autospeed.com/A_111511/hDg34uLtp_1/cms/article.html

Explains where an engine is more fuel efficient with graphs and no its not driving flat out or 90pc flat out everywhere.

Anan1

samih wrote: »

and slowing down by throttle instead of brakes makes a big difference in fuel consumption.

Although the ideal for consumption is not having to slow down any more than the car would in neutral - even engine braking is a waste of kinetic energy. Not that it's of much relevance on most roads today.:)

Anan1

Anan1 wrote: »

Although the ideal for consumption is not having to slow down any more than the car would in neutral - even engine braking is a waste of kinetic energy. Not that it's of much relevance on most roads today.:)

In ideal world you would be able stay on the top gear at peak torque for ever :-)