E55 speed vs emissions - a dilemma...

[E55BOF]

The W210 E55T (well, mine, anyway) seems to have a 'sweet spot'. Hypothetically, I recently did a 256-mile journey, which I normally do at an indicated 75 mph (yes, I'm a bad person. Stop reading now...), going (traffic permitting) rather faster than usual (my wife was unwell, and needed to be home in bed) - all right, barrelling effortlessly along at 95mph; I'm an even worse person than you thought... - except for average speed limited roadworks, where I stick to the posted lower limit, and anywhere with camera boxes, where I stay within the ACPO guidelines, just in case. To my surprise, the hypothetical fuel consumption came out at 27.6 mpg, as opposed to the usual 24-25. Is this normal for the E55, does anyone know?

And if so, this is where the dilemma comes in. I read recently that there are plans to impose lower speed limits on numerous sections of motorway in order to reduce vehicle exhaust emissions. I can reduce mine by going faster. Do I obey the law of the land, or put my moral obligation to help save the planet ahead of that law? And if I choose to do so, when I get the inevitable speeding ticket in due course, will a scrubby beard, a tank top, Green Party membership and Creeping Jesus sandals save me from damnation? Please advise....

 

[whitenemesis]

First question, does level of emissions decrease with increased mpg? A second question, how significant is 2mpg?

Third question, you drive an E55, so why give a f*ck?

 

[paul_c]

A car is normally most fuel efficient at the lowest rpm it can achieve in top gear. Was there a gear change in between 75mph and 95mph?

If no gearchanges, then the lower the speed, the lower the fuel consumption (and emissions), due to power required being a square of the speed, due to pushing the air out the way.

 

[V12]

Your obc is lying to you. I've never driven a MB or any car for that matter that is more efficient at 95mph than 75mph.

The increase in drag between 75-95mph is huge.

I can't remember which vehicle was tested (not a mb) but it was shown that the difference between a constant 60 and 70mph speed resulted in a 10% increase in consumption and then the difference between 70 to 80mph constant speed was another 25%.

 

[E55BOF]

The less fuel you burn over a given distance, the fewer emissions your car puts out. The car was in top as usual; no gearchange involved. I know that higher speeds burn more fuel, and am aware of the relationship between speed and drag (whether form, interference, or skin friction).

No, 2-3 mpg is not significant, and I don't really give a stuff about my fuel consumption, as I drive only about 7-8000 miles a year in the E55, but having felt forced into cruising at a higher speed than normal, I thought I'd see what difference it made to the fuel consumption.

There is no obc on the old bus; it was all measured at the fuel pumps and by the mileometer. I know the mileage is correct (unless half-a-dozen cars over the past twenty years all had the same mileometer error), and I doubt whether the petrol station pump delivered lots more fuel than I was charged for.

Tyre pressures were the same as usual, the car was not freshly washed and polished, road conditions were much as usual (except that this time it was colder than usual). There is usually a services stop on the journey, and there was none on this trip, but that's at most a mile or so, and I can't see it would make much difference.

It would seem reasonable that a powerful car would show less difference in fuel consumption between higher speeds than a less powerful one much nearer to full throttle at the same speeds, but still, we all know that the faster you go, the more fuel you consume.

We are all in agreement, then, that I could not have used less fuel at the higher speed. Only... I did. Ergo, my measurements must have been wrong. Only...they weren't. I'm baffled; it seems so is everyone else. Sadly, I'm not likely to try it again, so I shall remain baffled...

E55BOF

 

[paul_c]

Its probably down to variations of a couple of litres or so when you fill up, unless you spend 10 mins with the pump clicking and clicking until you can visually see the tank is brimmed, to the top of the filler neck.

 

[V12]

In that case you must have been travelling north to south

 

[E55BOF]

Its probably down to variations of a couple of litres or so when you fill up, unless you spend 10 mins with the pump clicking and clicking until you can visually see the tank is brimmed, to the top of the filler neck.

Thanks for the thought, Paul, but that's not the answer. The distance is correct. If I had travelled at 75 mph, I could accept that as a possibility; cruising at my usual 75mph, I would use approximately 46 litres (10 gallons) over that distance, so a couple of litres more or less "lost" in the filling-up MIGHT account for the small (some 10%) difference.

BUT... According to received wisdom, I should use much MORE fuel at the higher speed - at a guess, at least 25% - yet I used less.

So either I worked it out wrongly (and I didn't), or the fill-up pump was faulty in my favour (unlikely), or the fairies have been filling up my tank for me :bannana:, or I actually used less fuel.

Oh BUM! I can't leave it there. I'm going to have to do it all over again.....

 

[Bryan Allman]

road conditions were much as usual (except that this time it was colder than usual).

E55BOF

Colder temperature, better combustion, less fuel at a given speed ?

 

[clk208]

Head wind or following wind might have an impact I guess..