BILLYBOB

Has anyone else noticed that the trip computer estimate of fuel economy (and therefore the mile to empty as well) is wildly optimistic? I make several 300 mile interstate highway trips a month, and usually fill up at the beginning of the trip and again as soon as I arrive (so as to not have to gas-up the next morning when I'm in a hurry to get on the road). As a result, my actual fuel economy truly is a reflection of "highway only" driving conditions. And, I have yet to do bettter than 30 miles per gallon, yet the trip computer (yes, I reset it before I start the trip) consistently calculates fuel economy at about 37 miles per gallon. I have owned several cars with trip computers, and I always marveled at how accurate their on-board trip computers were. I would expect a BMW to do better than a 20%+ margin of error.

Has anyone else had a similar experience?

Does anyone know if the trip computer can be adjusted by a service tech?

Thanks, Billybob

JDN

Below are my actual MPG readings for 15 consecutive fill ups through 4500 miles. The WEIGHTED average is at the bottom. The OBC has overstated mileage about 4% overall.

ACT -- OBC
18.6 -- 18.8
17.1 -- 17.2
24.2 -- 25.0
24.1 -- 23.7
18.4 -- 19.1
17.5 -- 18.2
15.7 -- 15.2
25.4 -- 26.7
24.9 -- 25.5
24.0 -- 25.0
24.1 -- 25.0
22.2 -- 22.6
21.2 -- 21.7
23.5 -- 24.0
24.4 -- 25.0

22.16 - 23.05 Weighted averages for 4500 miles. Will probably continue this record into the future.

ULtraPLanet

Nice post JDN. What happened on that 15 mpg tank? End of the break-in period

JDN

Ah, that was a top up with only 77 miles driven before some extended highway driving. The next eight tanks were predominantly open road cruising. The first two were old maid style yo-yoing for four 130 mile spins. The price for a more exciting cruise style is about four MPG -- not bad.

The V-8 is really thirsty in stop and go traffic -- like 10-12 MPG.

535D Sport

JDN,

......how did you reach your actual figures.i.e.did you fill up at the same time every day at the same temperature,at the same pump.In the mornings when it is cool you will get more fuel in your tank than you would at midday temperatures.The 4% difference could be down to human error ?

JohanDC

I think that the speedometer is causing the difference. As reported by a lot of people the speedometer seems to be a little bit to optimistic, so this could explain the difference in real mpg's and the ones that are calculated by the OBC.

535D Sport

I'll have to check my speedo against what the sat nav said and see what the difference is ?

545i2b

Excellent discussion - will use these numbers as a baseline when I FINALLY get mine

JDN

I always filled up in an air conditioned, controlled humidity environment like everybody else.

On a more serious side, the conditions do vary and therefore the tank by tank comparisons jump around a bit. Most of my local refuelings are at the same location, but not always the same time of day. Temperature in Dallas has varied from 0 F to 122 F since I have lived here. However, during my 2+ months of ownership, it has been almost like an isothermal belt. No really hot and no really cold days. The OBC also may change in increments of .3 mpg which is in itself over 1%. So far, the 545 parallels my experience with the 540 as far as overstating MPG, but to a lesser degree. Four percent is not all that bad, is it?

Besides recording my fuel receipts and the OBC MPG, all I have to remember is to reset the odometer and the MPG while still at the pump. I ususually top up with a last liter or so after the automatic pump cut off. Over the longhaul, I believe what I am doing will establish the difference between the two measurements -- certainly to my satisfaction, anyway.

SkiScubaSailDude

Now granted I don't have the V8... but check out these fuel economy numbers from both 170 mile drives I made yesterday, and the total, showing the influence of around town driving to the averages.

Going...


Comming back...


Total


35+ MPG?!? Granted thats mostly flat highway driving.... 70+ MPH too!!! Hell, if the estimates are 5% inflated... I'm still making out like a bandit!

Just another reason for me to laugh at the Surburban/Tahoe boys....

Sorry the pics are so fuzzy, my phone doesn't take great pictures...