M.P.G. readout...... how accurate?

m2n · 18th February 2019, 18:33

Reset my MPG readout and decided to drive like a vicar on/around local roads(as usual),when I checked the readout it gave me 31.4 mpg...... Now this is a 2.5 Auto petrol monster machine........ normal readouts been anywhere between 20-25 on said same roads....

another_clean_sheet · 18th February 2019, 19:38

Quote:

Originally Posted by m2n

Reset my MPG readout and decided to drive like a vicar on/around local roads(as usual),when I checked the readout it gave me 31.4 mpg...... Now this is a 2.5 Auto petrol monster machine........ normal readouts been anywhere between 20-25 on said same roads....

The only way to check it is to do brim to brim preferably over say 1000 miles and then apply a correction factor to the OBD. Then you will be able to believe the mpg read out in the future.

stevestrat · 18th February 2019, 20:05

If I remember correctly the mpg figure is an average over the last 100 miles or something like that. If you've cleared it will more or less give the current mpg. Its not that accurate anyway, highly optimistic.

another_clean_sheet · 18th February 2019, 20:29

Quote:

Originally Posted by stevestrat

If I remember correctly the mpg figure is an average over the last 100 miles or something like that. If you've cleared it will more or less give the current mpg. Its not that accurate anyway, highly optimistic.

No. It is the average since you last reset it.

T-Cut · 18th February 2019, 21:22

@m2n

The system is not as smart as many owners might imagine. You can coax virtually any average MPG readout you want, high, low, good, bad or indifferent. It all depends on how the car's been driven and how far, after the last reset. I made this one when I was trying to understand its logic. I have a 1.8T by the way not a seven litre Bentley. You did the opposite and got a surptrisingly good result.

The mpg display will be reasonably accurate if you drive the car very consistantly over a lot of miles to allow it to aggregate fuel consumed and distance travelled. The computer algorithm should first be calibrated using real world data (brim-to-brim calculations) over a 500-1000 miles period driving in your consistant fashion. You calibrate the computer by following the notes given on the Trip Screen Diagnostic Mode here: https://www.the75andztclub.co.uk/for...ad.php?t=55585

You need Sections 5.0 and 20.0 plus the associated notes.

If you want to see a consistant and realistic figure, never reset the OBD computer. If you do you'll have to wait 500-1000 miles again till the aggregation settles down. Doing this stops any unusual dragster type dash now and then from affecting the result. It gets 'diluted' with the large quantity of standard data. A change in driving style (say a different driver) would require a recallibration using their real world data.

TC

18th February 2019, 18:33	#1
m2n Gets stuck in Rover 75 2004 Contemporary SE 2.5 Auto Join Date: Feb 2018 Location: Preston Posts: 513 Thanks: 67 Thanked 56 Times in 46 Posts	M.P.G. readout...... how accurate? Reset my MPG readout and decided to drive like a vicar on/around local roads(as usual),when I checked the readout it gave me 31.4 mpg...... Now this is a 2.5 Auto petrol monster machine........ normal readouts been anywhere between 20-25 on said same roads.... __________________ To have and not need, than need and not have. Eve was born on: Tuesday 14th September 2004 @ 07.54.23 Date of first Registration: Friday 24th September 2004 She was the: 103,715th 75 to run off production line, out of 112,381 She was the: 382nd 75 2.5 V6 Contemporary SE to be made out of 686 She was the: 1,415th 75 in Firefrost Red (code CEV) to be made out of 2,089 Firefrost Red 75's

18th February 2019, 20:05	#3
stevestrat This is my second home ZT260 #243 (resting) Join Date: May 2010 Location: Prestonpans Posts: 22,463 Thanks: 650 Thanked 4,624 Times in 3,358 Posts	If I remember correctly the mpg figure is an average over the last 100 miles or something like that. If you've cleared it will more or less give the current mpg. Its not that accurate anyway, highly optimistic. __________________ Steve T4 Location Map

18th February 2019, 21:22	#5
T-Cut This is my second home Rover75 and Mreg Corsa. Join Date: Nov 2006 Location: Sumweer onat mote o'dust (Sagin) Posts: 21,752 Thanks: 341 Thanked 3,660 Times in 2,924 Posts	@m2n The system is not as smart as many owners might imagine. You can coax virtually any average MPG readout you want, high, low, good, bad or indifferent. It all depends on how the car's been driven and how far, after the last reset. I made this one when I was trying to understand its logic. I have a 1.8T by the way not a seven litre Bentley. You did the opposite and got a surptrisingly good result. The mpg display will be reasonably accurate if you drive the car very consistantly over a lot of miles to allow it to aggregate fuel consumed and distance travelled. The computer algorithm should first be calibrated using real world data (brim-to-brim calculations) over a 500-1000 miles period driving in your consistant fashion. You calibrate the computer by following the notes given on the Trip Screen Diagnostic Mode here: https://www.the75andztclub.co.uk/for...ad.php?t=55585 You need Sections 5.0 and 20.0 plus the associated notes. If you want to see a consistant and realistic figure, never reset the OBD computer. If you do you'll have to wait 500-1000 miles again till the aggregation settles down. Doing this stops any unusual dragster type dash now and then from affecting the result. It gets 'diluted' with the large quantity of standard data. A change in driving style (say a different driver) would require a recallibration using their real world data. TC Last edited by T-Cut; 18th February 2019 at 21:25..