What determines fuel consumption?

The message from Marc contains these words:

Thank you for that enlightening comment.

The message from "Nick Finnigan" contains these words:

Certainly seems a lower speed band than I'd been led to believe in the past. Perhaps there was another set of mpg figures at a lower speed!

The message from "neutron" contains these words:

I could to 30 in fifth in my Maestro diesel - and still can in my TD Montego. Doesn't mind a bit on flat ground.

Not travelling at a constant 30mph in a normal car you don't. Unless you've got a _really_ strange car. Go and try it.

cheers, clive

True but most cars I have owned are perfectly happy in 4th at 30mph.

Darn, possibly got it a little bit wrong.

Roughly speaking, fuel use on a petrol car is proportional to air flow (otherwise it would be running rich or lean).

With the throttle position the same, the air flow will be about the same - it's just that there will be more air/fuel in the cylinders for each bang in the higher gear (lower revs).

Or something like that. If anybody can give a better explanation of what is happening please do!

So how do old mechanical diesel pumps work? Obviously in the lower gear you're squirting less fuel in per rev - is this done by the driver not pressing the pedal as hard?

cheers, clive

As I said, you could have stopped after the first comma.

The effecient speed is not 56mph , that is simply the figure that looks best out of the three artifical statutory required ones.

Yes it's approx 43 mph

it

Which wasn't the statment made , was it?

Not for historical for statutory , all car manufacturers are required to publish three consumption figure, they are meant for comparisons betwnn models , not a measurment on how efficient the car is.

Why would it make any difference? If numptys are determined to read the infomation incorrectyl there is no point giving them any more to addle their brains

It is a totally artificial figure as it performed when stationary on a rolling road, hence no air resistance.

Ian

A naive assumption, I would just believe that BL/British Leyland/Whatever they were called that week, were lying.

heh, 56 mph, or 90 km/h.. we've been shafted by Johnny Foreigner again :-p

Well i'm not convinced if the old Clio dci 100's fuel computer is "that" accurate, but after a bit of experimenting on the motorway the other day and taking an average over a few miles it was indicating around 99.99 mpg (ie over 100 mpg as the clock only reads to 99.99) at a steady 50, maintaining speed on the flat, however this dropped to 54mpg at a constant 80 mph! One hell of a differance for a few mph! Needless to say i soon cranked the speed back up to 80 plus as it's so damned boring pootling along at 50 holding all the HGVs up!!

Dilbert Dorkins

Your arsey replies are starting to bore me.

Looks like you choose which pollutant to minimise and then choose your speed according to Table 3. The best speed for low emissions seems to be somewhere in the range 60 to 90 KM/h, which is 37 to 56 MPH. Which one relates directly to fuel consumption I'm not entirely sure.

I note an error in the wording in the paragraph under that table. A decrease in speed from 110 to 100 Km/h leads to a 'decrease' in NOx of 6% and PM10 of

Ian

Stop reading them then!

It also looks like driving too slowly is very bad for the environment. 20 mph limits in suburban streets look like a good way of increasing pollution significantly.

Ian

Mine is mid 50s and up until drag gets bad

below 50 is awful!