In the past I have always reset the OBC after each fill up and compared the mpg to my manual computation. I also might add that I have filled up most of the time at the same station and usually fill until the pump clicks of then add more slowly until it clicks of again. The OBC has been within 1 mpg each time and usually the OBC is reading less than I compute.

This weekend I thought I would experiment while doing a long trip. I DIDN'T reset the OBC after each fill up. I filled the same way during this trip but filled twice from different pumps and the final fill I did until I reached overflow.
The Results:
1st fill at 178 miles OBC 36.7mpg calculated 35.3mpg
2nd fill at 546 miles OBC 38.5mpg calculated 40.1mpg
Last fill at 989 miles OBC 38.5mpg calculated 38.8mpg

I am thinking that it is the pump "shut off" that is causing various levels of "full" to then result in conflicting OBC vs calculated values. Any thoughts or comments?

I also noticed something else that I had never watch for before. After the 1st filling I left the MPG displayed and glanced at it often, I saw it go from 36.7 to 37.9, 38.5 and 39.2 then back to 38.5. After the second fill it went from 38.5 to 37.9 then back to 38.5 where it ended. NO values other than these. Anyone else see anything like this? Why no changes of .1 at a time?

 

I think it has to do with the intervals that it takes the readings at. It doesn't constantly calculate it, but rather it calculates in an interval. The length is probably the same rate that you see it change. So unless the mpg changed by .1, in that interval, you're not going to see the change.

I think it would be nice to have a constant calculation but I'm not too picky. This is my first car with any sort of on board computer so I think it's neat but from what I've seen on other cars it could use some improvement.
-Chris

 

I notice my MPG figures change in .4 intervals up or down.

retroom

 

>>I think it has to do with the intervals that it takes the readings at. It doesn't constantly calculate it, but rather it calculates in an interval. The length is probably the same rate that you see it change. So unless the mpg changed by .1, in that interval, you're not going to see the change.
>>-Chris

Chris,
Thanks for this! I have been looking for an interval explanation but can't seem to hit upon it.
Can you give me an example of what you have said?
Is the inverval a variable length? If so, how is it arrived at?
What unit of measure is the interval - miles, gallons, time?

 

How do you reset the OBC? I seem to have missed this in the manual.

 

I just fill it up when I get to a quarter tank and drive....All that figuring stuff to me is a pain. Besides I got the car for fun and if it needs more gas to have more fun then baby I'm looking for a gas station!!!

 

To reset it, just hold down the button that changes the different modes on the left stalk.

As for the interval, I think it is a matter of time. A few seconds or so and then it recalculates it. It's not as noticable as you are cruising at a steady speed because it stays the same. However, once you reset it, it changes a lot so you can see how often it changes. Once it settles down the change happens less often so it's less noticable.
-Chris

 

I believe I have solved my MPG readings issue. The ECU takes a momentary fuel consumption reading every (?) time period and these readings are sent to the OBC for it to add and average. As in any averaging as the total gets larger each new reading has less and less individual impact on the average so there are many changes at the begining and then less as time passes. It was the size of the change that was confusing me. I was seeing changes of .7mpg each time and retroom says he sees .4mpg changes.

That is because the ECU is taking its readings in Liters/100Km the OBC averages them and then converts to MPG. The readings are only to one decimal place (ie 6.3/100KM) and I believe the average also uses only one decimal. Therefor using the 6.3/100Km example it would convert to 37.9MPG. If you move to 6.2/100Km the MPG becomes 38.5. These were reading that I saw.

The situation that retroom describes would fall in the range of 8.2/100Lm to 7.4/100Km or MPGs of 28.8, 29.2, 29.6, 30.0, 30.4, 30.8, 31.2, 31.6, and 32.0.

Probably of no importance but I'm glad that I have explained it to my own satisfaction. :smile:

 

edlfrey, nice explanation! I had always wondered about this, too. That darn rest of the world and its metric system!

 

Mine computer reads high by anywhere from 2.5 MPG to 4.0 MPG too high. And it is notchy in it's digital readout as well.

 

>>Mine computer reads high by anywhere from 2.5 MPG to 4.0 MPG too high. And it is notchy in it's digital readout as well.

If I understand your term "notchy" to be the same as what I have attempted to describe then ALL OBCs will readout in that fashion.

Your computer reading high is still an unknown. However, I would say that you have an 'S'. This seems to be a common issue with people having the OBC and an "s" vs the Cooper. I asked my service tech today about this subject and he confirmed what I have said about the readout but knew nothing about 'S' OBCs reading to high.

Take it to your dealer and complain, then tell us why it was wrong and what they did to fix it.

 

The OBC is way off, Do not trust this as a way to detrmine when to get gas. The Outside temp. gauge is accurate, so I leave mine set on it.

 

my only concern is this.. is teh mpg falling short of actual mpg or is it overcalulating sayignyour gettign better than average if its below averge I can live with it.. if its showing overactual then I'm a littled miffed.

 

Everyone, needs to do a little experiment before they claim the OBC is wrong.

1 Fill up the car, fill it to the max....
2 Reset the OBC to zero and reset the mileage on the car without the car started prior to driving it away from the station.
3 Drive the car for something less than a tank of gas, hopefully some number approximating a quarter of a tank.
4 Return to the same pump at the same gas station and make sure the car is parked right where it was last time. (Ie you want the car to be in the same position so that gravity will be consistent regarding the fuel tank and it will hold the same amount of gas(you wouldn't want the tank to be sloped one way one time and another the next... this might cause the tank to hold more or less gas ... thus screwing up your manual calculation))
5 Fill up the car, fill it to the max.
6 Look at the number the OBC provides ... now look at the mileage and divide this number by the amount of gas you just pumped into the car. How close are the numbers?

I would expect the small deviations to be due to the slight differences between one full tank and the following full tank.

I have done exactly this and when I did I find the OBC and my manual calculation have been very close, but not exact. Close enough for government work, though.

I have a 2003 S that was built in January....

jc

 

>>Mine computer reads high by anywhere from 2.5 MPG to 4.0 MPG too high.

Well after many months of talking with the dealership and MINIUSA and providing them with spreadsheet data on over 21,000 miles of MPG comparisons and allowing them to change components on my MCS to see if it could help, MINI AG has finally excallated this issue to an "Offical problem" to be resolved by their engineers.

 

Oh, here's the latest data...

* the Sept 12 and Oct 20th datapoints for ECU MPG are bogus. This is because the dealership had reset the setting for their testing...

[code:1:61b2ffe95b]
Date Mileage miles Gallons My-MPG ECU-MPG MPG Diff % Diff Gal Used ECU Gal
27-Feb-03 0
01-Mar-03 219 219 9.549 22.9 26.4 3.5 15.1% 9.549 8.295
01-Mar-03 519 300 10.429 28.8 32.2 3.4 11.9% 19.978 17.612
05-Mar-03 791 272 11.726 23.2 25.8 2.6 11.2% 31.704 28.155
08-Mar-03 1,075 284 11.399 24.9 27.6 2.7 10.8% 43.103 38.445
13-Mar-03 1,386 311 11.918 26.1 29.7 3.6 13.8% 55.021 48.916
18-Mar-03 1,720 334 12.230 27.3 29.7 2.4 8.8% 67.251 60.162
21-Mar-03 1,985 265 10.075 26.3 28.6 2.3 8.7% 77.326 69.428
26-Mar-03 2,283 298 12.014 24.8 28.3 3.5 14.1% 89.340 79.958
29-Mar-03 2,590 307 11.681 26.3 29.0 2.7 10.3% 101.021 90.544
06-Apr-03 2,887 297 11.315 26.2 29.0 2.8 10.5% 112.336 100.785
10-Apr-03 3,201 314 11.853 26.5 30.1 3.6 13.6% 124.189 111.217
15-Apr-03 3,555 354 12.001 29.5 33.6 4.1 13.9% 136.190 121.753
19-Apr-03 3,785 230 9.300 24.7 28.0 3.3 13.2% 145.490 129.967
23-Apr-03 4,120 335 12.001 27.9 31.3 3.4 12.1% 157.491 140.670
29-Apr-03 4,453 333 12.304 27.1 30.9 3.8 14.2% 169.795 151.447
04-May-03 4,771 318 11.879 26.8 30.1 3.3 12.4% 181.674 162.011
08-May-03 5,085 314 12.000 26.2 29.4 3.2 12.4% 193.674 172.692
12-May-03 5,232 147 5.500 26.7 30.1 3.4 12.6% 199.174 177.575
15-May-03 5,425 193 6.480 29.8 33.6 3.8 12.8% 205.654 183.320
15-May-03 5,762 337 10.577 31.9 35.6 3.7 11.7% 216.231 192.786
16-May-03 6,047 285 9.822 29.0 32.2 3.2 11.0% 226.053 201.637
16-May-03 6,249 202 7.247 27.9 30.5 2.6 9.4% 233.300 208.260
19-May-03 6,507 258 12.265 21.0 23.5 2.5 11.7% 245.565 219.238
19-May-03 6,882 375 12.179 30.8 34.0 3.2 10.4% 257.744 230.268
20-May-03 7,192 310 10.411 29.8 33.6 3.8 12.8% 268.155 239.494
21-May-03 7,480 288 10.343 27.8 31.3 3.5 12.4% 278.498 248.695
22-May-03 7,793 313 11.224 27.9 32.2 4.3 15.5% 289.722 258.416
29-May-03 8,088 295 11.227 26.3 30.1 3.8 14.6% 300.949 268.216
04-Jun-03 8,409 321 12.583 25.5 29.4 3.9 15.2% 313.532 279.135
07-Jun-03 8,709 300 10.501 28.6 31.7 3.1 11.0% 324.033 288.599
12-Jun-03 9,014 305 12.181 25.0 30.1 5.1 20.2% 336.214 298.731
19-Jun-03 9,346 332 12.451 26.7 30.5 3.8 14.4% 348.665 309.617
25-Jun-03 9,668 322 12.484 25.8 29.4 3.6 14.0% 361.149 320.569
28-Jun-03 9,816 148 5.596 26.4 30.1 3.7 13.8% 366.745 325.486
01-Jul-03 10,152 336 12.075 27.8 32.6 4.8 17.2% 378.820 335.793
09-Jul-03 10,400 248 9.482 26.2 30.1 3.9 15.1% 388.302 344.032
12-Jul-03 10,592 192 7.549 25.4 29.4 4.0 15.6% 395.851 350.563
12-Jul-03 10,848 256 10.352 24.7 29.7 5.0 20.1% 406.203 359.182
14-Jul-03 11,124 276 9.018 30.6 34.5 3.9 12.7% 415.221 367.182
19-Jul-03 11,433 309 11.884 26.0 30.1 4.1 15.8% 427.105 377.448
25-Jul-03 11,762 329 11.692 28.1 32.2 4.1 14.4% 438.797 387.665
31-Jul-03 12,080 318 11.325 28.1 32.6 4.5 16.1% 450.122 397.420
02-Aug-03 12,246 166 5.737 28.9 32.6 3.7 12.7% 455.859 402.512
02-Aug-03 12,438 192 8.109 23.7 27.3 3.6 15.3% 463.968 409.545
03-Aug-03 12,682 244 8.766 27.8 30.9 3.1 11.0% 472.734 417.441
04-Aug-03 13,026 344 12.180 28.2 33.1 4.9 17.2% 484.914 427.834
05-Aug-03 13,362 336 12.020 28.0 31.7 3.7 13.4% 496.934 438.433
07-Aug-03 13,569 207 7.817 26.5 31.7 5.2 19.7% 504.751 444.963
11-Aug-03 13,899 330 11.557 28.6 33.1 4.5 15.9% 516.308 454.933
15-Aug-03 14,178 279 10.400 26.8 31.3 4.5 16.7% 526.708 463.847
19-Aug-03 14,403 225 8.005 28.1 31.7 3.6 12.8% 534.713 470.945
22-Aug-03 14,733 330 12.311 26.8 31.3 4.5 16.8% 547.024 481.488
23-Aug-03 14,874 141 5.392 26.1 29.4 3.3 12.4% 552.416 486.284
26-Aug-03 15,165 291 9.732 29.9 34.0 4.1 13.7% 562.148 494.843
03-Sep-03 15,479 314 10.848 28.9 32.6 3.7 12.6% 572.996 504.475
07-Sep-03 15,756 277 10.133 27.3 31.3 4.0 14.5% 583.129 513.324
08-Sep-03 16,051 295 11.604 25.4 30.1 4.7 18.4% 594.733 523.125
12-Sep-03 16,336 285 9.836 29.0 29.0* 0.0 0.1% 604.569 532.953
14-Sep-03 16,538 202 6.845 29.5 33.6 4.1 13.9% 611.414 538.964
16-Sep-03 16,874 336 12.409 27.1 30.9 3.8 14.1% 623.823 549.838
21-Sep-03 17,211 337 12.512 26.9 31.7 4.8 17.7% 636.335 560.469
21-Sep-03 17,373 162 6.389 25.4 28.0 2.6 10.4% 642.724 566.255
24-Sep-03 17,711 338 12.120 27.9 32.2 4.3 15.5% 654.844 576.752
28-Sep-03 17,961 250 8.867 28.2 32.2 4.0 14.2% 663.711 584.516
04-Oct-03 18,219 258 9.478 27.2 31.3 4.1 15.0% 673.189 592.759
05-Oct-03 18,488 269 10.332 26.0 30.5 4.5 17.1% 683.521 601.578
05-Oct-03 18,699 211 4.752 44.4 47.0 2.6 5.9% 688.273 606.068
11-Oct-03 19,043 344 12.529 27.5 31.3 3.8 14.0% 700.802 617.058
12-Oct-03 19,227 184 6.695 27.5 30.5 3.0 11.0% 707.497 623.091
15-Oct-03 19,537 310 12.291 25.2 29.4 4.2 16.6% 719.788 633.635
20-Oct-03 19,629 92 5.540 16.6 16.6* 0.0 0.0% 725.328 639.177
25-Oct-03 19,951 322 11.215 28.7 32.6 3.9 13.5% 736.543 649.055
01-Nov-03 20,176 225 8.623 26.1 29.0 2.9 11.1% 745.166 656.813
08-Nov-03 20,513 337 12.411 27.2 30.5 3.3 12.3% 757.577 667.862
09-Nov-03 20,791 278 10.088 27.6 30.1 2.5 9.2% 767.665 677.098
09-Nov-03 21,008 217 10.827 20.0 22.4 2.4 11.8% 778.492 686.786
[/code:1:61b2ffe95b]


[code:1:61b2ffe95b]
_______________
/ ||
/_______||______)______
/ / ~~ | ~ ()
/ ---- |HAPPY | ----
|/ () | MOTORING|/ () _|
`.__.'=============`.__.'
[/code:1:61b2ffe95b]

 

two major points to make:
1) a few tenths will make a substantial diffnce in computed figures. I ALWAYS fill to the absolute BRIM with car on a slight down and right slope if attempting to compute accurate mpg's (as little air as possible trapped in tank).

2) your OBC readings only SEEM to be jumping around....Please understand that BMWs "think" in metric units i.e. ___liters per 100km___and then they CONVERT that value to the English system. Liters/100km is a coarser interval than MPG and a .1 liter increment may translate into several tenths of a mile per gallon. It took me several years before I figured out what was going on on my 3-series.

I hope this helped... :smile:

 

>>a few tenths will make a substantial diffnce in computed figures.

For a single tank one time testing is sure will. But with more than one fill-up, those tenths go away quickly and the law of averages quickly plays into the equation (pun intended LOL ). For example, at 21000 miles, my records indicate 778.492 gallons of gas being used by my car over a span of 77 trips to the gas pumps. So for the first 21000 miles, I averaged 26.975 MPG. The OBC showed a consumption of 686.786 gallons over the same 21000 miles. The OBC thought I was getting 30.577 MPG. The % difference is 13.3% over those 21000 miles is significant.


>>2) your OBC readings only SEEM to be jumping around....
>>Please understand that BMWs "think" in metric units i.e. ___liters
>>per 100km___and then they CONVERT that value to the English
>>system. Liters/100km is a coarser interval than MPG and a .1 liter
>>increment may translate into several tenths of a mile per gallon.
>>It took me several years before I figured out what was going on
>>on my 3-series.

Actually, I ask you to plot out those % difference numbers (which is a relationship between my hand calculated MPGs and the OBC MPGs) on a graph, they are trend very consitantly around 13%...allowing for acceptable errors in fuel fill-ups, pump differences, and rounding errors. Another interesting point you should note when you plot these out, the % diff gets higher during the hotter months and trends to be smaller during the cooler months. If this was a conversion and a problem with me topping off the fuel, then ambient temperature should not have anything to do with it. but in this case, it does.

The dealership's head technician and now MINI AG both agree there is a signifanct enough of a problem to warrant engineering time to be spent troubleshooting this issue. Since there's autos out there that have a very accurate reading (within a few tenths of a gallon) and others like mine that read high and others that chronically read low. This is clearly a calibration issue rather than a metric-standard conversion issue.

 

Absolute accuracy in the OBCs mpg computation is not a concern for me as I keep my eye on the "relative change" (+ or -) in mpg. This allows me to understand the effects on changes in mods, driving conditions and/or driving styles. Enough said.

 

I have ~10,000 miles on a Aug 03 US MCS and I found that the OBC is about 10% optimistic. The very first tank from the dealer was accurate. Every tank after that the OBC was high. I'm due for an oil change in a couple weeks and I'll bring this up with my dealer.

Sean

 

My 03 MC consistantly gives me pessamistic readings by about 5%. I've checked this using odometer mileage vs. actual gallons of fill-up on seven conscutive fills. While I'm pleasantly surprised by this it makes me wonder if the odometer milage is reading high???

>>I have ~10,000 miles on a Aug 03 US MCS and I found that the OBC is about 10% optimistic. The very first tank from the dealer was accurate. Every tank after that the OBC was high. I'm due for an oil change in a couple weeks and I'll bring this up with my dealer.
>>
>>Sean

 

xpcdoojk suggested


Like dominicminicoopers, I have followed this procedure For some 14,000 miles. I haven't (yet) built a spreadsheet, but my experience is that the OBC is always high by about 2 MPG. My MCS is a mid-2002 build.

Only once, during a long, lazy trip using the cruise control at 70-75 MPH have I ever done better than 28 MPG (actual), and the OBC showed about 31.

HAPPY NEW YEAR -- Tom.

 

It's now an official MINI AG problem. If you are concerned about it, let your dealership know and have them write it up. The more cars with this problem that MINI AG can be made aware of, the better for us who are waiting for a fix.