Z4 MPG computer gee whiz question

[Aebous]

Hey I was just wondering, how often/what parameters does the MPG computer use?

Basically does it calculate based off of amount of gas going into the engine? Or is it based on a certain distance travelled?

Also is there a point during the trip that it stops calculating? It sure seems like after 5-10 minutes my MPG stops feeding me up to date info.

 

[mmm-five]

Average of mileage travelled divided by the amount of gas going into the engine - so it will be erratic from it's initial calcs and then gradually smooth out as the data points increase.

I only reset my MPG2 after servicing, MPG1 is reset every couple of thousand miles.

 

[ab7]

I'm sure someone can give a more detailed response - but it would have to be based off both the fuel input to the engine and distance as you need both variables for the calculation.

In terms of it stopping calculating, I'd imagine that's just a reflection on the numbers averaging out, the longer you drive after resetting it - the more it's an average for the 'tank' or whatever rather than instant read out.

My old car had two mpg read outs - one instant and one average which I found quite interesting.

 

[Aebous]

Yeah I guess that does make sense that it would get more accurate (less movement) with more data points.
One other odd thing I noticed, is the engine not as efficient at low speeds? The other day I was in traffic doing all of 20 mph and my MPG was around 16... I think I even switched gears to get a reading at multiple RPM's. I didn't think about resetting at the time.

The main reason I was wondering is I just swapped out spark plugs so I've been paying more attention than normal.

 

[mmm-five]

One other odd thing I noticed, is the engine not as efficient at low speeds?

If you think about it, it's obvious.

A car doing 20mph at 3000rpm in 2nd gear will not be as efficient as a car doing 20mph at 2000rpm in 4th gear as the first car has to use 50% more fuel as the 2nd car because it's got to 'fill' those cylinders 50% more frequently.

 

[ab7]

^ Spot on.

I'm still suprised by the number I get now and again...

This was a long run over to London and back on Monday/ Tuesday this week. I (obviously) reset the mileage and mpg trip at the start of the tank...

I used the motorway on the way back which actually dropped the mpg slightly (A & B roads on the way there) although there was plenty of traffic and roadworks keeping me at more economical speeds.

 

[KarlosG]

As said above the MPG read out shows 'Distance Travelled / Fuel Used' since you last reset it, so yes the longer you travel the more accurate an average it will read.