stupid inaccurate mpg computer

[Trustmeimadoctor]

finally after 4 years of having this crappy micra i decided to work out what mpg i was actually getting as i'd always thought there was a bit of a discrepancy between what it said i was getting and how often i was having to fill up and how much i was having to put in.

anyway computer said 51.9 mpg average over 381.7 miles what it worked out at was about 46mpg ! why the hell is it so different ? its on standard sized tyres its fully serviced on the dot every 9k but it has done 72k

anyone any ideas

[ChiChoAndy]

It's telling you what it could be if you weren't so fat.

(I kid! I kid! )

[DannyVTS]

My Clio is currently showing 41.4mpg... I hope it's accurate but I'm kinda scared to try!

[soda]

All onboard computers are highly optimistic in my experience. Some by 5%, others by 20%. The cynic in me reckons this is a deliberate manufacturers ploy to make you think you have a more economical motor.

[Trustmeimadoctor]

The Crack Fox said:

And just look at all the more interesting motors you could be driving with 40ish MPG...

lol i agree funnily enough its only a 1.2 as well tbh its not a bad car it just lies about its drinking habits and its now developed a nasty sounding metal on metal squeak

as for clios my 172 was also very optimistic claiming over 40mpg at times when really it was mid 30's at best

[Crook]

E36 M3 pretends to get 29ish. It even boasted of 40+ when reset on the motorway. Reality is 25ish O/A.

[300bhp/ton]

Trustmeimadoctor said:

finally after 4 years of having this crappy micra i decided to work out what mpg i was actually getting as i'd always thought there was a bit of a discrepancy between what it said i was getting and how often i was having to fill up and how much i was having to put in.

anyway computer said 51.9 mpg average over 381.7 miles what it worked out at was about 46mpg ! why the hell is it so different ? its on standard sized tyres its fully serviced on the dot every 9k but it has done 72k

anyone any ideas

How exactly did you work out your mpg? And how certain are you that the mileage you are reading is correct and tallies with the figure the computer is using?

[Trustmeimadoctor]

soda said:

All onboard computers are highly optimistic in my experience. Some by 5%, others by 20%. The cynic in me reckons this is a deliberate manufacturers ploy to make you think you have a more economical motor.

i think i agree the mrs is having i different one with her new giulietta that seems to never shift from 51mpg even though its supposed to be doing well over that 64.2 combined 76.4 extra urban thats done almost 12k so far since start of december

[Galsia]

My 2008 Fiat Grande Punto's trip computer says that I get an average of 37mpg but figuring it out manually gives me a figure of 31mpg.

[Trustmeimadoctor]

300bhp/ton said:

How exactly did you work out your mpg? And how certain are you that the mileage you are reading is correct and tallies with the figure the computer is using?

the standard way
rest both trips when filling one on the odo the other the obc fill to brim drive till i wanted to fill it up again then brim it again

then work it out from there

i mean yes maybe the car is really under-reporting the distance travelled but i doubt it

[killsta]

My Z shows an average of 23.8mpg and the RoadTrip iPhone app shows 24.05mpg, so it's not too far out

Edited by killsta on Thursday 19th May 11:16

[300bhp/ton]

Trustmeimadoctor said:

the standard way
rest both trips when filling one on the odo the other the obc fill to brim drive till i wanted to fill it up again then brim it again

then work it out from there

i mean yes maybe the car is really under-reporting the distance travelled but i doubt it

Sorry, wasn't really questioning you so much. But there have been times on here where people have used the wrong calculations to get get litres to imperial gallons. Also some converters (online or iPhone apps) only convert to US gallons which are quite a bit smaller, so mpg would be less if you used them.

[mike325112]

soda said:

All onboard computers are highly optimistic in my experience. Some by 5%, others by 20%. The cynic in me reckons this is a deliberate manufacturers ploy to make you think you have a more economical motor.

Mine under estimates by about 3-4mpg - always has. I was quite pleased when I first did it properly, then realised that actually 41 mpg is crap for a 2.0 litre diesel shopping trolley...

[ewenm]

Does anyone use the figures for anything? I find the mpg readout on the Volvo to be interesting but mostly useless. If it suddenly dropped then it could be an indication of a problem, but the indicator would be the sudden change, not the actual figures themselves.

[Matt UK]

soda said:

All onboard computers are highly optimistic in my experience. Some by 5%, others by 20%. The cynic in me reckons this is a deliberate manufacturers ploy to make you think you have a more economical motor.

I agree. Some are worse than others though.

My BMW would tell the odd white lie.
The wifes Lexus claims an ave of 18.4mpg which is spot on to the decimal point based on my records.
Yours sounds like a right little fibber though.

[V88Dicky]

I suspect manufacturers will put in an error reading so that they are more optimistic, especially diesels and ECO claptrap rubbish.

[kambites]

Trustmeimadoctor said:

i mean yes maybe the car is really under-reporting the distance travelled but i doubt it

I'm certain that the car is over-reporting the distance travelled. In my experience the speedometer and odometer tend to read the same and the speedometer will often be off by around 5-10% depending on the car. It's quite possible that the trip computer is correct and the discrepancy is simply due to the inaccuracy of the speedometer.

[Trustmeimadoctor]

300bhp/ton said:

Sorry, wasn't really questioning you so much. But there have been times on here where people have used the wrong calculations to get get litres to imperial gallons. Also some converters (online or iPhone apps) only convert to US gallons which are quite a bit smaller, so mpg would be less if you used them.

i was doing cost per litre / cost to fill = litres used x 0.2199692483 = gallons used / miles done

[Trustmeimadoctor]

kambites said:

I'm certain that the car is over-reporting the distance travelled. In my experience the speedometer and odometer tend to read the same and the speedometer will often be off by around 5-10% depending on the car. It's quite possible that the trip computer is correct and the discrepancy is simply due to the inaccuracy of the speedometer.

actually the speedo is out will have to trace the distance with satnav as well see if reported trip distance is out

[Crook]

kambites said:

I'm certain that the car is over-reporting the distance travelled. In my experience the speedometer and odometer tend to read the same and the speedometer will often be off by around 5-10% depending on the car. It's quite possible that the trip computer is correct and the discrepancy is simply due to the inaccuracy of the speedometer.

Where would the trip computer get its distance reading from?