Calibration of OBC

[Mint]

Does anybody know how to re-calibrate the MPG aspect of the OBC as mine is wildly inaccurate. On this weekends Lake Cruise the OBC registered 43.0 MPG including travelling there and back. Now I know I drive like Miss Daisy, but not all of the time so that's ridiculous. Calculating from fill up to fill up I actually achieved 32.23, which I can believe.
I know how to reset the OBC but the MPG side is always over what I actually get.
Anybody help?

 

[Brian H]

This should help.

https://zroadster.org/threads/bmw-z3-obc-calibration.8677/

Brian

 

[Mint]

Excellent, cheers Brian

 

[g8jka]

I've been trying to calibrate mine over the last few fill ups but I keep getting it miles too high or too low. Was originally a couple of MPG high but I think I'm somewhere close now. Will fill up this week and see how far out it is. The higher the number you put into the OBC (upto 1250) the lower the MPG will read if I remember. I couldn't seem to get the calculations correct so I've been guessing

 

[Mint]

I'll have a go following the instructions in Brian's link. I'll let you know

 

[g8jka]

Its easy to change the number to alter the figure I'm just having a few attempts at getting it spot on! Good thing is at least it can be done and you can get it somewhere near.

 

[Mint]

Had a go at changing the correction factor. It was set at 1000 and using the formula in the other thread,

New correction factor (CF) = Old CF x (actual MPG/OBC MPG)
1000 x (32.23 / 43)

Brings the new correction factor to 749.5, so I've set it to 750.

The higher the number you put into the OBC (upto 1250) the lower the MPG will read if I remember.

This seems to be at odds with what you said Dave, I've lowered the number so the MPG may go up. Only time will tell. Will let you know...

 

[g8jka]

Had a go at changing the correction factor. It was set at 1000 and using the formula in the other thread,

New correction factor (CF) = Old CF x (actual MPG/OBC MPG)
1000 x (32.23 / 43)

Brings the new correction factor to 749.5, so I've set it to 750.



This seems to be at odds with what you said Dave, I've lowered the number so the MPG may go up. Only time will tell. Will let you know...

I could be wrong, I had trouble with the new correction factor not providing a figure that responds to the MPG so I've just altered it everytime I've filled up until it's somewhere near. If you reset the MPG when you've filled up and drive around for 50 miles or so you will be able to tell if it's far out as you have an idea already of what it normally does. When I set the CF to 1250 the MPG came right down to 20. Hopefully you'll get it spot on first time and won't have to keep guessing like I am

 

[Mint]

Just done 30 miles or so today and the MPG rose to 49MPG. So I think you're right Dave, @g8jka, the smaller the CF the higher the MPG. So I've reversed the calculation and the new CF is 1334, so I'll set it to 1250 and see how that goes. Standby...

 

[Redline]

Some advice - it’s best to do the calculation based on a near empty to full tank of fuel (I would suggest brim to brim filling preferably from the same pump.
Two things at play. Firstly small differences where the pump stops pumping will make less difference to your mpg calculation.
The second is the temperature and hence the density of the fuel in the storage tank. The lower the temperature the greater the density so you get more energy in the tank.
Keep things as close as possible.

The larger number of miles you do the calculations over the more accurate the new correction factor. Over a few tanks you should be zeroing in on the right cf.

If you try and do this on a small number of miles you will end up with possibly wild fluctuations in the cf. it’s just the way the maths works.

 

[Mint]

Yep, appreciate what you are saying Ian. I realised as soon as I checked it today that the CF was incorrect. There was no way I was doing 49MPG even over a small distance. I have logged all of my MPG figures, so I know roughly what the OBC should be showing. I do not want/need it to be exact but just a bit more realistic

 

[g8jka]

I use an app that gives me all the MPG figures so I know what I'm doing but it annoys me not having the OBC correct so I'm trying to get it as close as possible. You may need to keep trying Dave as once I started like you I didn't know whether to use the original CF or the new one etc and now I've just been altering it a little once I fill up. I think I'm almost there, if I get it right I will post what MPG I'm doing and what the CF is.

 

[Jack Ratt]

Does anyone know how the OBC calculates the mpg figure? Does it reset every time the engine stops or is it a rolling average calculation?
I have recorded every single fill up since their purchase for both my cars and by calculation my auto is doing an average 28mpg and the manual 30mpg between fill ups.
Granted, I don't teararse around the streets anymore as years of motor cycling have satisfied that urge but neither do I get people coming up behind me and flashing me to get out of the way.

 

[Mint]

...Does it reset every time the engine stops or is it a rolling average calculation?...

Unless you reset it by pushing the stalk in for about two seconds, it's calculating from the last reset.

 

[g8jka]

I have recorded every single fill up since their purchase for both my cars and by calculation my auto is doing an average 28mpg and the manual 30mpg between fill ups.

I wish I could get near rhem figures, average of 23.9 for almost 17500 miles and I don't thrash it round either probably only 10% if that. Most of my journeys are short, 3 - 5 miles each time with a couple of 15 milers each week but my MPG is still terrible.

 

[Redline]

I wish I could get near rhem figures, average of 23.9 for almost 17500 miles and I don't thrash it round either probably only 10% if that. Most of my journeys are short, 3 - 5 miles each time with a couple of 15 milers each week but my MPG is still terrible.

It’s the 3-5 milers that’s hammering you, especially if it’s stop-start. Need to take a longer more scenic route