varying fuel economy.. - CM
On Friday I had to pootle down the M4 so I thought that I would set cruise control and see what my mpg was.

It took me about ½ an hour to get to the M4 so the car was up to working temp.

For the 1st 5-10 miles on the m\'way the mpg readout was about 39mpg or so and this slowly improved to when I came off 50 miles later reading 46mpg. From about 10-25 miles it stuck at 41-42 mpg

I was wondering why the figures slowly improved?
varying fuel economy.. - Mark (RLBS)
perhaps you were looking at the average figure rather than the instant figure ?
varying fuel economy.. - CM
I only have an average figure on computer readout.

But this shouldn't matter as, if the first 10 miles were 39mpg, which gradually improved until I ended up after 50 miles with 46mpg, it is likely that at some stage I was doing well over 50mpg!

Is this possible in a 530d estate at 70mph? Or is it the readout over estimating and playing up?
varying fuel economy.. - Colin M
Those electronic driver information systems can be notoriously inaccurate vis mpg. The only accurate way is by monitoring your own mileage and litres put into the tank.

I've seen 10-20% errors on the system in my Audi (always in the manufacturers favour too!)

varying fuel economy.. - Mark (RLBS)
Depends - if the average is "an average since you last reset it" then probably not; on the other hand if its "an average over a rolling [say] 20 miles" then it is quite possible that you didn't do better than 46mpg but did at least that well for the last 20 miles.

'course its also possible that the thing is just plan wrong.
varying fuel economy.. - PR {P}
My GTA's average reading is always about 2mpg better than it actually is,(which I work out from brim to brim fills). This is the case with both motorway and town driving
varying fuel economy.. - Andrew-T
CM - electronic consumption figures can only be as accurate as the fuel gauge in your tank, unless you can tell the computer exactly how many litres you have added. Otherwise it has to guess, from the amount the float (or other device) has changed. On my wife's Clio, the various readouts are zeroed separately; your cruising mpg should gradually improve during a run, unless you zero it (say) 5 miles after starting.
varying fuel economy.. - Flat in Fifth
I think you'll find quite a few calculate from the fuel injection computer, ie if its calculating how much fuel to inject so as to get the engine to run properly then why not use that information to provide a readout.

Mark RLBS is correct in that it depends on whether its a true cumulative from reset point or a rolling average over the last few miles.


Don't vote it only encourages them.
varying fuel economy.. - X5
Whatever the answer, it's probably in BMWs computer programming. My X5 3.0 diesel behaves in exactly the same way as you describe. (except that the figures are a lot lower :-(
varying fuel economy.. - Ben79
I have read before that Beemers often overread on the fuel computers.

Just for the record, my C5 always seems to underread average consumption by 2mpg, i.e. i always get 2mpg more when calculating manually.

On my 3rd Citroen. Saxo, Xsara, C5.
varying fuel economy.. - JohnM{P}
Depends where you started and ended your trip... the M4 is not flat the whole way! Chippenham to Newbury is obviously uphill, but there are other more subtle up and down gradients also.
varying fuel economy.. - CM
start J1 - end J13 Newbury

speed and mpg readour reset and CC set immediately.
varying fuel economy.. - Rojer
A small slope will make a huge difference.
Warm tyres too ..
Astra, Renault 18, Renault 25 TXi, Astra Est, Passat Est, Mercedes 190E, Mercedes


Value my car