My OBC says i'm getting 44 mpg and my calculations say 32mpg. I'm sure my next question has been settled but for the purposes of this thread I will ask it. Is our tank 13.2 or 15.3 gallons? Using the wrong one could throw off the calculations.
I'm wondering if this is common to you guys as well or if my OBC is screwed up a little bit.

 

certainly not 15.3 gallons. Only way to get accurate measurement is to fill tank, reset rip odometer, and then refill and divide miles covered by gas needed to refill the tank.

I find the OBC mileage to be very close.

 

The size of the tank isn't as important as reserve, fuel line capacity, and the like. I believe the official documentation says the tank is 13.2. I've put that much gas in my car before though, so obviously I wasn't driving around on air.

Hand calculations shouldn't have anything to do with the capacity of the tank. Fill the tank to the top, drive to a certain point, fill up, divide miles driven by the amount of fuel you put in. .

 

I'm confused. Why would the capacity of the fuel tank make any difference in the manual calculations? You fill up. Drive. Record miles driven. Fill up again. Divide miles driven by gallons to full. It shouldn't matter what the fuel tank's capacity is.

Am I missing something?

 

Oh, and for the record, my OBC Average MPG is within .5 to 1 MPG of calculated... consistently.

 

sorry, I suppose it doesn't. lol. But I'm still wondering about my OBC.

 

I think i'm just going to call the service manager and see what he says.

 

This is probably a stupid question... but are you resetting your average MPG after refilling?

Also, I'm assuming you're not confusing instantaneous MPG with Average?

A 44 MPG average would be unlikely on any Mini. Unless you've figured out how drive downhill 75% of the time... OR... your OBC was FUBAR.

 

No I am not. but I do think it is a legitimate question. I have reset it several times and have gone at least 3 or 4 tanks before resetting. The reason I reset it is because I was getting the weird avg and wanted to see if it would fix it self, but it hasnt.

I'm pretty sure I cant even find the instant mpg reading anywhere. I think it's below it but somewhere around the 44 it says avg. mpg

I think it's the OBC.

 

My OBC is off by 3-4 MPG.

 

Make sure the language is NOT set to UK English on your on-board computer. There is a UK English and US English option for the language. If it is set to UK English, the computer will use the British gallon which is larger than the US gallon to compute your mpg. That might explain the discrepancy...

 

oh! That never even crossed my mind but come to think, and I'll check it tomorrow morning, I'm pretty sure thats the issue. Thank you uzun. :-)

 

If you are not reseting the OBC every time you fill up, you are actually averaging all tanks together since your last reset. Meaning, on this tank you got 40mpg and on the last tank you got 48mpg so you would average 44mpg between the two. (These numbers were just used as an example).

As far as the fill up total gallons being greater than 13.2 could just be the pump not being 100% accurate. My father in-law owns a Texaco....the pumps are legally allowed a certain percentage margin of error. Exactly what that margin is I don't know, and I'm sure it verys according to local law. If they are allowed a 5% error, as example, then you would be filling up to 13.86gal....7% would be 14.124.

I also wanted to point out that according to BMW USA a 10% margin of error on our speedo/odo is acceptable. In my personal car my speedo/odo is 6.667% slow. So, if my speedo says 66mph, i'm actually going 61.6mph (verified by two GPS systems). This 6.667% also effects my OBC in the exact same way. I'm very OCD (according to my wife) about checking my fuel mileage on every fill up by hand and then comparing it to the OBC.

 

My OBC is getting less reliable as time goes by....shortly after I bought my R56, it was only 1-2 mpg off, but now (w/ 12k miles) it's closer to 4-5 mpg off. Annoying.

 

MY OBC is off about 2-3 MPG. But, as stated above, the calculation starts with the odo/speedometer. When going through those cute little radar warning signs as you enter construction zones, I've noticed my speedo is always off my about 5 MPH. I'm not saying those things are something to set your watch by, but the variation seems rather consistant. Then, I've noticed on a recent road trip, using the nifty new .1 mile markers on the interstate, that the odometer is short by a few yards/meters on every .1 mile.
My OBC tells me I'm getting about 42 MPG and I'm actually getting about 40 MPG. The variation with the speedo/odometer readings seem to match up with the OBC discrepency as well. A while ago I noticed as similar descrepency in my Jaguar as well. The mechanic told me the Fed allows speedometers to be off by a bit AS LONG AS ITS HIGH! In other words, they'll allow a manufacturer to tell you you are going faster that you really are, but not slower. Keeps us safer they think. However, (here comes my conspiracy theory) I think the manufacturers are using this "loophole" to shorten our warranties. Think about it. If the speedometer reads a few MPH high, the odometer turns a bit quicker, racking up more miles than reality, hence, shortening your warranty if you are a high mileage driver. And from what I've read on lots of posts here, MINI drivers DRIVE, alot!

 

Keep in mind: our various tire/wheel fitments produce varying diameters, and that means variations in shaft revolutions for a given distance. Speedometer calculations are typically based on transmission output shaft revolution.

 

I'm fine with it being off a little bit but I know my speedo is only off by about 1-2 mph because I got pulled over once for speeding.

This is all good info to keep in mind. Thanks for the help ya'll.


I'm going to go out and check my language setting and see if that wont fix this problem. I'm fine with it being off by maybe 2-3mpg's but you all know that being off by 12 is just wrong. haha. I'll report back this afternoon.

 

Why was my 1982 Mustang and my current '94 F150 (and countless other cars) pretty much right on the dot! Seems that with all this "drive by wire" we should be MORE accurate.

 

I got 43.3 last tank on premium.
The OBC read something like 45.7.
I tried regular this tank.
So far (~400 miles), OBC says 44.

 

The OBC is for entertainment purposes only!
So is the speedo. And the odo.
If you check it all by GPS you'll notice that none of the data is accurate....

Now that we agree on that, go put on your aftermarket wheels and rubber and check it again against your GPS.
My advice is to not sweat it.

 

Turns out the language was the issue, now it's almost right on. GPS is more accurate by the way in terms of speed and what not. Thanks for the help.

 

+1 Running 16" wheels and the runflats that came on it.

 

Mine too. So close i stopped doing the math when i fill up.

 

Ditto on that. Mine's pretty accurate.

 

Hmm, you guys are getting accurate measure on your 16" & 17" wheels, I wonder if there is a significant diff in OD with the factory 15" tire wheel combo and they didn't calibrate for it.

It wouldn't really bother me too much, as someone above said the OBC is for entertainment, if it didn't reflect the actual miles on the odometer and then the subsequent warranty issue. Seriously a 5 mph error on 10,000 miles, say @ 60 MPH is 833.3 miles. (that's less than the 10% someone said was allowed)so on a 100,000 mile warranty one could lose 8333 miles of warranty if the speedo/odo was of by 5mph.