Brake discs

Using brakes wastes energy (turning it to heat) but not fuel - if you need to decelerate you need to dissipate the energy somehow and it makes no difference to your fuel consumption if you go down the gears (although it may in fact use more fuel) than using the brakes and dipping the clutch at the right moment.

For example - stopping at traffic lights from 60mph in 5th in a modern car with fuel injection and a fuel cutoff - your car will need to inject fuel to prevent the engine stalling every time you change down (and if you heel / toe you'll inject even more fuel) so you can either change down 5-4, 4-3,

Should lights change you can make one downchange to appropriate gear and then continue.

Tim S Kemp ( snipped-for-privacy@timkemp.karoo.co.uk) gurgled happily, sounding much like they were saying :

Of course it wastes fuel. Where else does the energy come from?

If you lift and decelerate using engine braking earlier, then you use less fuel maintaining the speed unnecessarily.

Think. Evaluate. Write, Re-read, Post.

The fuel is wasted by accelerating to a speed faster than is required. Whether you brake to a lower spead or go down through the gears makes no difference - you have to dissipate the same amount of energy. The only way you can reduce it is to drive slower.

But you have to burn fuel to get up to the speed where you need brakes to then stop. Each time you use the brakes this energy comes from the fuel - there's nowhere else it can come from.

Pity that wasn't :- Think. Write. Evaluate. Re-read. Post.

Cos it would be a cracking acronym :)

Paul Giverin ( snipped-for-privacy@giverin.co.uk) gurgled happily, sounding much like they were saying :

It's bloody difficult to evaluate before you're written, and only marginally easier to evaluate before re-reading, anyway... Of course, by that stage, you've re-read, evaluated - and still post the original anyway, even though you now know it's a load of bollocks.

Just save a lot of grief.

Write, Post, That's it.

Exactly what I said.

The message from "Tim S Kemp" contains these words:

But that's what you do by allowing speed to bleed away slowly towards an obstruction - instead of a long flat steady speed using a steady amount of fuel then brake at the end, your speed instead is dropping all the time - using hardly any fuel in the process.

Maybe.

I happen to have an injector hooked up to a laptop soundcard in my car. On a Nissan micra K11, with a CG10DE, the only time the injector pulse width goes to zero is when it cuts out for ~8 revs, when the RPM rises over 6K.

The bottom line is engine braking. (no engine braking under 1100RPM, as that's the idle.)

The curved line above that is minimum ever injected fuel. Interestingly showing the throttle letting through air much more easily at under about

I'm unsure if the minimum on time line is simply an artefact of the software - it knows that the injector only starts to open then, so when another function calls for injected fuel of '0', it returns this time.

Or if it actually does inject a small amount of fuel at all times.

Changing down repeatedly, using engine breaking will wear the clutch more.