How accurate is the MPG display?

G

GaryH

Member
My first weekend with my car and im loving it. Even the sun has come out since I picked it up on Friday. One question though, how accurate is the onboard computer MPG readout, and the miles remaining indicator?

Just that I have a 2.0 and doing a drive up the motorway on the weekend at a steady 70mph it was showing 46mpg for the entire journey!!! I was hoping for maybe close to 40 but not this much! Also, is the miles remaining display accurate or should we just rely only on the actual fuel guage?
 
To answer question 1. I find its more of a ballpark figure. Nothing beats doing a brim to brim fill up and using the trip computer to manually calculate.

To answer question 2. I find its more of a ballpark figure. You shouldnt let it go below 50miles anyway as advised in the instruction manual as it can cause damage.
 
I suspect its as accurate as the speedo, since one assumes it takes its distance/speed reading from the same place. So if your speedo is 5% out against satnav (which is 100% accurate on the flat) then your computer figures will be too.

Even a "brim-to-brim versus speedo" measurement isn't accurate for that reason.

In other words, if a speedo is over-reading by 10% (as my Nissan 4x4 does) it will show me as having done 110 miles when I've actually only done 100. So if I use 4 gallons of gas (which I do in that thing) I think I've done 110/4 = 27.5mpg, when in fact I've actually only done 100/4 = 25mpg.

Sad isn't it.

Easiest way to check this hypothosis is to do a couple of long journeys against satnav, noting the mileage it says you will do at the outset (since its measuring accurately), then see what the actual was on your speedo.
 
I find the odometer is more accurate than the speedometer.

I can do 60 mile motorway journey (according to the sat-nav), and the odometer will count upwards as the sat-nav counts down, and they'll both deliver the same mileage - so there must be another sensor which reads the distance. Also, the speedo may say I'm sat at 70mph on cruise for 1 hour, but the odometer will say I've covered 65 miles in that period - which is closer to the actual road speed the sat-nav reports.
 
mmm-five said:
I find the odometer is more accurate than the speedometer.

I can do 60 mile motorway journey (according to the sat-nav), and the odometer will count upwards as the sat-nav counts down, and they'll both deliver the same mileage - so there must be another sensor which reads the distance. Also, the speedo may say I'm sat at 70mph on cruise for 1 hour, but the odometer will say I've covered 65 miles in that period - which is closer to the actual road speed the sat-nav reports.
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Interesting and not what I'd have expected but since when did logic come into it LOL. I've got to do a 150-mile round trip to Welshpool this afternoon to collect a stub axle for my 4x4 so I'll follow my own advice with the satnav comparison. :thumbsup:

I can't help wondering why BMW would take two different readings of effectively the same nature and have one accurate and one not :headbang: Still, I'm sure theres a good engineering explanation.....
 
IIRC there is no other sensor. The signal going to the speedo has a correction factor applied (5~10% or +2mph etc) to ensure it complies with UK law (A speedo must not under-read).
 
I find that my MPG computer reading is consistently 2-3 mpg above actual (I calculate by doing a brim to brim).

Gives me false hope when seeing nearly 30mpg.... only to get 27mpg on the real calculation :(

Ho hum....
 
Adamski said:
I find that my MPG computer reading is consistently 2-3 mpg above actual (I calculate by doing a brim to brim).

Gives me false hope when seeing nearly 30mpg.... only to get 27mpg on the real calculation :(

Ho hum....
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That agrees with what I find - not sure you do the same but I always reset my trip computer at each refill. I started manually logging fuel consumption when I owned a diesel (trying and failing to justify it over a petrol...) and continued when I got the Z4. Mine's all in an excel spreadsheet but the basic calculation (for UK-sized gallons) is:

MPG = Miles for tank / (litres used x 0.219969157)
 
Yeah, a11y - that's the MPG calc I use (ish).

Funnily enough, I find my speedo is roughly 2-3mph out too (vs satnav reading)......

2-3mpg difference, 2-3mph difference... :?
 
I find my mpg within 98% accurate. At 70mph my speedo is actually 100% accurate (vs my road angle on the flat) I presume that may be down to running slight larger profile tires.

as for the miles left.... i once very scarily covered 50 miles after the obc said 0 miles remaining... was in the middle of nowhere with no petrol station nearby... and when i filled the car still only took 53.7 lites... so 1.3 lites left in the tank :roll:
 
I find mmm-five guilty of being right. :thumbsup:

Did a 160 mile round trip to Welshpool yesterday....at 60mph satnav shows actual to be 57mph so approx a 5% over-read on speedo, yet odometer was accurate to within a mile against satnav, so R26Andys comment sounds correct.

The only issue I'd mention if I'm being anal :? is wondering if satnav distances are 'straight and level' or whether they take account of the road undulations (which in Wales would easily add 5-10% to any journey). For example, if the satnav was reading distance as you might off a map (i.e. flat), the actual could well be longer in hilly country but you'd get an identical reading against the odometer if the latter was indeed 5% out....?

Think I might try it on a long flattish journey sometime just to see. :yawn:

I'm sure there are people bored witless reading this crap, but I like to know what makes things tick LOL. :oops:
 
lacroupade said:
...... The only issue I'd mention if I'm being anal :? is wondering if satnav distances are 'straight and level' or whether they take account of the road undulations .........
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Yes, GPS will take account of the undulations and calculate altitude accurately.
 
I have noticed that the last 100 miles on the computer goes faster than the first 100 miles. I think the computer tries to catch up when it gets closer to empty. I agree with everybody as far as the 5-10% accuracy is concerned.

As far as when to fill up..ie..1/4 tank so you don't suck up the gas gunk. Well, I have a theory about that. If you always wait till the computer says 0 miles to go or so, then you will never have gas gunk cause you are constantly sucking it up.
With that said, though. When I do go get gas, I rarely get over 13 gallons, so there is at least a 1 gallon reserve which may hold the gas gunk.
My first Z4 had 236K on it and I did this regularly and never a problem.
 
I've just got my new car yesterday a 2.0. I was interested to see mpg so filled up, driven 400 miles and filled up again. As per the display and my calculations I got 40 mpg. That's almost annoying as I've been driving around in a peugeot 406 diesel estate and only been getting 40mpg out of that.... If I'd have realised the mpg would be so good I would have bought one a long time ago! :D
 
Just did a highway trip this weekend from Calgary to Lethbridge (Alberta, Canada) and return. Total distance was around 400+ kilometers, 200 km (120 miles) each way, and I reset the OBC mileage when departing Calgary.

I took back roads on the first leg, speed mostly at 110 km/hour (68 Mph), with some slower sections for road construction. The computer showed fuel consumption at no more than 6.7 l/100 km for the first 100 km or so, then gradually increased as I picked up a bit of a headwind. On arriving at Lethbridge after 200 km and about 2 hours driving, the OBC had just changed from 6.9 to 7.0 l/100 km. That works out to 40.3 miles per imperial gallon.

I left the OBC active while doing some city driving in Lethbridge, and did not reset it before driving back to Calgary that night. This time I used mostly the main divided highway at 120 km/hr (75 Mph), with occasional slowdowns for a couple of small towns, and the computer was showing 7.2 l/100 km when I arrived back at Calgary. Conversion is 39.2 mpg.

The whole trip was done with the top down, occasional slow downs as noted, and occasional 'spirited' passing on two lane sections of road. No complaints on fuel mileage here - very happy with both the car's mileage and performance. I haven't double checked speed with my GPS as yet, so can't comment as to accuracy of the speedometer calibration.
 
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