All of the advice above seems to cover everything I would do to help MPG when I am in that mood. I would think the changing gears may well indeed be a little early regardless of which gear you are in the bigger idea is not the rpm but how far in you have to press the accelerator to maintain the speed or rate of acceleration, having to push pedal in most of the way in 6th having changed up too early will use more fuel than if you left it in 4th or 5th and used the feather light touch on the pedal.

Actually no. Higher gears are always better for fuel economy.

Your engine is more efficient at higher load and the frictional losses are lower at lower rpm. For fuel economy you need to be in the highest gear you can smoothly run in.

Actually no. Higher gears are always better for fuel economy.

Your engine is more efficient at higher load and the frictional losses are lower at lower rpm. For fuel economy you need to be in the highest gear you can smoothly run in.

wrong, as fubar just explained...he is spot on.

wrong, as fubar just explained...he is spot on.

To explain this properly I need to go into the detail on a few details in engine operation and tuning. It's not simple so bear with me.

The accelerator on all modern diesels is completely electronic. The throttle pedal input works on a 3D map in the ECU. One axis of the map is engine rpm, the other pedal position and the result (the map surface) is the target injection amount.

What this means is the pedal position has no direct relationship with how much fuel the engine is trying to inject for each combustion event.

Here is an example of a drivers wish map. It is in no way linear.

http://t1.gstatic.co...rkgtpiV3bUKW5RO

The efficiency of an engine is expressed as BSFC (brake specific fuel consumption). The units in metric are grams per kilowatt hour. This is a measure of how many grams of fuel the engine requires to deliver each kilowatt hour of energy at the crank. These values vary thoughout the operating range and are dependent on both rpm and load. Lower numbers are better in that you burn less fuel to get the same energy output.

The BSFC values plotted out for any engine over speed and load form a map which looks a lot like a topographical map seperated into islands of constant BSFC. Where you would normally be working your way downhill on a topo map you find yourself working towards the lowest BSFC values where the engine has it's highest efficiency.

Here is a BSFC map for a VW TDi engine. I don't know exactly which one, but all diesel BSFC maps are similar shape.

http://i404.photobuc...TDI/BSFCmap.png

The vertical axis on this map is BMEP (brake mean effective pressure) which is another way of expressing engine load. The top curve is actually the engines maximum torque curve and below that is the engines efficiency at lower loads.

You see the best efficiency point on this engine is around 17 bar BMEP and around 2200rpm. Basically at the rpm of maximum torque and with about 80-90% load.

This is where the engine is most efficient and where you would intend it to run all the time if you needed that amount of power.

So that is nice and easy, the most efficient point bar none is 80-90% load and 2200rpm.

But what if you want to run lower load? In that case you need to know roughly how much power you need. Once you know how much power you need you can plot constant power lines through the BSFC map and find where your lowest fuel consumption point is along the power curve you are at.

Here is another VW TDi BSFC map which has blue constant power curves laid over it (the blue arcs). It's from the older ALH engine. I have included this one to show how similar they are.

http://cr4.globalspe...CC650E40A3F.bmp

So on the map above lets say we are cruising and require 30hp total crank power. If you follow the 30hp blue line up to find the lowest fuel consumption, you will end up at almost full torque at about 1500rpm.

If you required 70hp then following the blue 70hp line you end up with full torque at 2,500rpm.

The outcome of this is pretty clear. Maximum efficiency is always found with the highest gear and lowest rpm you can realisticly and smoothly run. Anytime you have a choice of gears, you need to be in the one that will give you the highest engine load, which is the highest one. Particularly when accelerating.

(Apologies for the links, some of these pictures wouldn't embed).

Edited 11 May, 201214 yr by Kiwibacon

He is right about cruising in the highest gear possible, but it's a pain in the arse because if you don't downshift every time you want to accelerate, you will end up using more fuel.

He is right about cruising in the highest gear possible, but it's a pain in the arse because if you don't downshift every time you want to accelerate, you will end up using more fuel.

No, accelerating in a higher gear uses a lot less fuel than a lower gear. Plot out some acceleration lines on those BSFC maps and you'll see that lower gears (and the resulting higher rpm) are making the engine run in areas where it is a lot less efficient.

Pedal position means nothing. You need to keep the engine load up and the rpm down. Stay in the highest gear you can smoothly run and you'll use the least fuel.

Hmmm, I like to bang up through the gears and into 6th as quick as possible (worked in my old Skud), I'll try not to for my next few tanks...

I find in mine that not using 6th on the A roads improves the mpg, 6th seems a bit to long for under 60mph i think anyway

Also i get around 47-50 according to maxidot combined on 60 mile round trip commute to work on A roads with a few villages and school traffic in the morning, but have noticed in the past if i push it hard on the motorway it does drop

I have had two 1.9 Octy Estates. Both were fantastic on fuel. 60mpg easily achievable on any kind of run. 50mpg achievable bimbling around.

My VRS is relatively poor in comparison especially when cold (it takes forever to warm up too). 40 and a bit on local bimbling and normally around 53 on gentle motorway runs.

The VRS is a great car though. It's just the old 1.9 spoils you for mpg.

Thanks for the lesson Kiwibacon...

It looks like I just need to slow down a little or stop moaning, the reason for the question was mainly to make sure I wasn't getting less mpg for a reason ie. something wrong with the car. All good it would appear.

The commute last night (due to friday night traffic) was 50.4mpg - Mondays with no traffic was 48.

Again, thanks all!