W210 320 CDi no load fuel consumption

[RFdesigner]

I'm trying to find why I can't get near the book 45mpg extra urban figures under any condition, even briefly, except perhaps dropping off a cliff.

To this end I've tried finding out how much fuel the car uses when it's doing the same revs as when it's doing 50mph, but with no load. The result

3.2litres an hour.

If I did 50miles in that time I'd be doing 66mpg.

This seems a little low, considering that the car isn't being pushed in the slightest, at 45mpg I'd be using up about 5 litres an hour, so turning the engine accounts for 60% of my fuel consumption at 50mph.

Does anyone know where I might start looking to get hold of any expected no-load fuel consumption figures.

Derek

 

[Sprint'n'Go]

I think the fuel consumption figures quoted by the manufacturers to a government set standard are achieved on a rolling road in a closed environment. This is to allow standardisation and repeatability but unfortunately means these figures are not necessarily achievable by us real people. This does not render the figures useless but does provide a benchmark by which differant vehicles can be compared.

If I am right, you might be chasing an impossible target!

 

[seabadger]

Here's a shot I took after a gentle run back from work - must have had a following wind?

Click on the thumbnail to read the data - yes it says 48.1 mpg after 54 minutes and 46 miles driven.

I admit I was pussy footing and going for my personal best.

Cheers

Seabadger

 

[malcolm E53 AMG]

The best I've seen on my 320CDi is 46mpg. That was obtained on a 40 mile run on A roads at a very low average speed of about 35mph.

Average consumption is 38 - 40 (100 miles) motorway and 26 - 35 (5 to 10 miles) local runs depending on distance, traffic and time of year.

From other posts on the forum this seems to be the norm.

Hope this helps

 

[RFdesigner]

The best I've seen over any length of drive is about 41mpg, and it doesn't improve as the speed drops. and yes that is with a following wind.

The idea of a no-load test is that there are no road conditions to vary, so each vehicle, regardless of tyres etc, should behave more or less the same.

I'm very much aware of the Euro test, and it is idealised, but at an extreme, one should be able to get near it. I also have a simulation program that calculates the mpg acceleration etc.for your car based on all the technical data and it expects the fuel economy of most cars to peak at lowest revs in the highest gear. It certainly seemed to match what I got out of my old 124. With the 124 the 56mph test gave 40mpg and the 75mph test gave 34, a 20% difference, something I'm not seeing on this car.

It's looking like I'll have to debug this one myself.

Derek

 

[Glenn Smith]

I run a 3.2 Jag and find it gives better mpg if i cruise at 80-90 than if i cruise at 60-70, most of my cars i have been getting around the published figures without any effort, i believe the latest figures are much more realistic than the old 56mph and 75mph figures

 

[jberks]

Well, the best I've seen on mine is a shade off 48 and that was being really careful on a 120 mile motorway run. 45-46 has been achievable though, but only on motorway runs.
I did once reset whilst at 50 in roadworks - it read 51mpg but soon fell off when the road cleared.
I tend to find that speed is irrelevant (below high 90's anyway) it's acceleration that counts, hence it's not easily repeatable given varying traffic conditions and how hard you press on. On a steady run to work 30 mile mway+20 flowing urban I used to see 41 regularly.
based on my 270, 48 out of a 320 is impossible.
One interesting point is how much it takes, just to turn the engine as this is the biggest factor when you drive in traffic. I can see 39 but a few mins in traffic and I'm looking at 20's.

 

[hawk20]

The best I've seen over any length of drive is about 41mpg, and it doesn't improve as the speed drops. and yes that is with a following wind.

The idea of a no-load test is that there are no road conditions to vary, so each vehicle, regardless of tyres etc, should behave more or less the same.

I'm very much aware of the Euro test, and it is idealised, but at an extreme, one should be able to get near it. I also have a simulation program that calculates the mpg acceleration etc.for your car based on all the technical data and it expects the fuel economy of most cars to peak at lowest revs in the highest gear. It certainly seemed to match what I got out of my old 124. With the 124 the 56mph test gave 40mpg and the 75mph test gave 34, a 20% difference, something I'm not seeing on this car.

It's looking like I'll have to debug this one myself.

Derek

1. The AA said, in response to a news item demanding lowing speed limits to 'save the planet', that with diesels the consumption alters little between 50 and 80 mpg. Assuming that is true, you won't find the kind of difference with speed that petrol gives.
2. J Berks is dead right. Acceleration is the killer of fuel economy. After even 50 miles check the computer predicted economy and then put foot down for a long burst and the economy drops amazingly -even after a good distance like 50 miles.
3. My S320cdi does 35-37 on long motorway trips and 27-30 pottering around locally. The govt combined figure is 35 and that is about what you get on a run. The extra urban figure has never been achieved by me however gently I try.

But on Petrol Mercedes I could always achieve the extra urban figure with enough effort. Go figure.

 

[Blobcat]

A couple of trip computer pictures following my trip through mid Wales this morning;


The overall is lower than normal as I have only been using it for short journeys recently.