[Closed] My car's a liar - mpg figures.....

It's normal for some* cars but not others. Mine are accurate.

* Ford are terrible for this, but many others seem to be too.

Most cars will have a variation between actual and indicated mpg.

There is no legal requirement to report accurate mpg.

I thought I was only getting 21mpg out of my landrover. Turns out I was using the US gallon conversion from Litres 😳

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Mine's the opposite - reports 40ish but actually gets 48ish.

Yup - it's a ford.......

Ford Focus Econetic here. Compared to an excel spreadsheet of what I've actually used, the trip computer seems to think things are a few mpg better than they really are.

They're not going to tell you it's worse, are they!

My wife's Laguna is a little pessimistic reports about 43 and actually does about 44.5

My Friends Mondeo on the other hand always manages about 52 and I'm very very doubtful if it actually manages much better than the Laguna.

They all seem to be inaccurate.
I have a BMW 320 ED.
Overall the computer claims 61.9 and my current journey suggests 63. The reality when calculated by miles travelled on a tank and litres put in at the pump suggest an average of between 57 and 58 over the 6 months I've had it.

Mine is pretty accurate to the odometer, but the odo overreads so the mpg ends up optimistic.

My renault is supposed to get 64.5 mpg on the highway. I could only manage 72mpg yesterday 😛

'14 VW Passat here - the MFD in the car suggests i'm averaging 2-3mpg more than I actually get.

Usually a 0.5-1 mpg max discrepancy (positive or negative)on mine compared to calculated (Audi). Also, the speedo is also much more accurate compared to other cars I've owned (1mph out out 70 mpg (GPS reading), compared to 4-5 mph out on previous cars).

I had a Fiat Punto HGT that was outrageously optimistic - computer would say 55 to 58mpg, and I never ever, driving as carefully as I could, measured anything better than 32. Not checked it recently but Mrs Pondo's Panda was pretty accurate, as far as I could make out.

My BMW 320d measures about 10% over so 55 mpg is really about 49.5. Still better than my old Subaru used to get. And faster too!

charliemort - Member

Last tank it said it had done 48mpg, actual was 40 mpg

Is this normal / legal?

Depends on how the average is being measured - over what time frame. I tend to hire a lot of cars and use them only for long, steady motorway hauls - glasgow to kent and back, that sort of thing. For the whole journey I should be getting pretty much the optimum MPG but it can take 500 miles or so before the MPG figure being displayed starts to shift much on some cars so the average, being displayed is mostly reflecting the driving style of the previous hirer - then the figure keeps improving for the next 500 miles. So in a lot of cases the 'average' you seem to be getting from one tank of fuel will be influenced by the driving patterns from the previous tanks-worth of milage at least.

Calculated into any average is any time you spend stationary - waiting at the light you are doing ? gallons/mile

So in a lot of cases the 'average' you seem to be getting from one tank of fuel will be influenced by the driving patterns from the previous tanks-worth of milage at least.

I think most of us are bright enough to reset the computer 🙂

My Civic get's it wrong from -3% to 19%. It gets more accurate over longer (~150mile) runs hence the -ve values.

think most of us are bright enough to reset the computer

only if you're choosing to - if I owned a car that calculated fuel consumption I'd be more interested in one big average than a lot of small ones.

One variable of course is.... as accurately as you try and measure the fuel a car burns- and the car might be able to measure that very precisely, you don't have all that accurate a measure of how many miles you're travelling - how well calibrated is the milometer? - certainly speedos are set to over-report your speed, so is the milometer over-reporting distance? wheel and tyre profiles vary for different models of the same car, tyre wear is a factor - that means you'll be travelling further per wheel revolution with one wheel rather than another or on a new tyre than an old one. So you can take as accurate measure as you like of the gallons, you only have an approximate measure of the miles

My Navara reads 10% over - as does the speedo.

This makes me think, does the odometer read 10% over too? Annoying if so, as my 81,000 miles should only be 73,600 odd - thereby reducing it's resale value!

Compared to an excel spreadsheet of what I've actually used

Oh

My

Lord

Do you actually have a life? 😉

Compared to an excel spreadsheet of what I've actually used

Oh

My

Lord

Do you actually have a life?

[Quietly hides his spreadsheet and fuelly browser tab]

How sad are graphs then? 😀

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They all seem fairly optimistic, in every respect. The worst is the odo on the KTM, which reads about 10% further than it's actually traveled. I've also noticed that VW group air temp gauges read about 1-1.5C above reality. FIAT clocks are notorious for suffering from slow time, losing about a second a day. How that's possible with a quartz crystal I'll never know.

pondo, have you got the fiat ecodrive app installed on the panda? I ran this for a while on the abarth and having hacked open the SQLite database it kept, was astonished at the detail in the info. It knew everything: throttle position; time on the brakes; steering input angle; ambient temperature, gear position, speed. Just like a full-on race datalogger.

if I owned a car that calculated fuel consumption I'd be more interested in one big average than a lot of small ones

Exactly. If you're comparing brim-to-brim with the computer average then you need to reset the computer at each fill - you'd have to be pretty dim not to realise this, no?

My car has two averages anyway, one per trip and one you can reset when you want.

This makes me think, does the odometer read 10% over too?

All UK cars seem to be configured to show 10% over true - a legal thing I think. However the odometer should be accurate. In my Passat at least, I know it is, because if you drive at an uninterrupted 70mph for a long time the average speed reads 63mph.

having hacked open the SQLite database it kept, was astonished at the detail in the info.

I think most cars know all this stuff. Modern cars are made of a dozen or more computers linked with their own network on which they broadcast all the data they use to make decisions.

My Honda's about 10% optimistic!

pondo, have you got the fiat ecodrive app installed on the panda? I ran this for a while on the abarth and having hacked open the SQLite database it kept, was astonished at the detail in the info. It knew everything: throttle position; time on the brakes; steering input angle; ambient temperature, gear position, speed. Just like a full-on race datalogger.

Compared to an excel spreadsheet of what I've actually used

Oh

My

Lord

Do you actually have a life?

Fuel spreadsheet here as well.
However it is a company car, which is leased from a bunch of complete ****iwits, who also manage my fuelcard spend, and regularly cock-up my salary deductions for private miles.

The fuel spreadsheet is vital for checking my records against theirs.

my BMW 116 'efficient dynamics" which is supposed to get an average of 73MPG has only ever got a maximum of 55mpg, and thats even by driving on dual carriageways at 59MPH non stop for 400 miles!!!

I even went on a driving course to learn to drive more efficiently as i thought it was me, but nope... just standard BMW rubbish!

Sounds like an issue there millzy, it should be better than that. And I'm no particular BMW fanboi.

I keep a spreadsheet to see how the mpg is doing.

Seems to be a bit less in real life than the car tells me... A little under 28mpg in real life - I usually get around 29 - 30 on the readout, but there are a few short trips around the 25 mpg mark (into town and back), so I suppose it's probably reasonably accurate 😯

Mines amazingly accurate. When it's empty a little light comes on and a low fuel alarm sounds. Then I fill it back up to the top.

Spreadsheets?? You get apps for that now 😉

Mine is pretty accurate but I try not to focus on it to much, you use fuel you fill it up. I don't tend to drive for the best MPG.

I have actually asked Audi if they could disable my mpg reading as it was upsetting Mrs Smartboy at first. They can't apparently!

If I used my car to commute to work, then I would buy a modern diesel like a normal, sensible person, but owning an RS 6 and worrying about mpg, is like owning a MTB and worrying about getting it muddy!

It does actually think that over the 5 years of ownership, it has averaged 19.6mpg. That is over about 25k miles. Pretty reasonable?

Hey Smartboy, thanks! I feel better about my fuel consumption now. I reckon you've had reasonably good fuel consumption out of it given the size of the engine and the car.

Bet you've had an unreasonably good time driving it though! Mines the 3.0 A4 Cabrio and that's plenty of fun - I bet the RS6 is awesome.

Wonders what the RS4 Cabrio is like (and instantly decides he can't afford it! - or even the S4...)

As noted, the Fords are somewhat innacurate - It seemed to me when I had one that it would conveniently ignore the first few minutes of any journey when the engine was cold etc...

Skoda seems pretty accurate (from looking at the since last fill average compared with the receipt/trip meter - I feel so horrendously disorganised for not having a spreadsheet!)

Mine says it's getting 13 MPG and it's bloody right!

The mpg read out on my 9 year old Clio is surprisingly accurate

I haven't read all that lot prior but I can safely say I trust my new transits computer about as much as myself with Tina from corrie!
On leaving work today it said I had 8 miles remaining, bollox I thought, the journey home to my local Morries is 15 miles. Ill risk it I thought. Whilst driving along the miles remaining counted up to 12 them slowly back down to 9, so on reaching morrisons I had done minus 1 mile. If only all journeys were this economical 🙄

The 'miles remaining' is based on a rolling average of the last 10 or whatever miles - obviously it can't actually predict how far you have left to drive since it can't tell how you're going to be driving in the near future. So it's never going to be particularly accurate.

A Toyota I hired was a complete fraud. Told fibs making out it had gone further then it really had, and therefore an erroneous MPG figure. Also by it's own calculation it had used less fuel than passed out of the tank, doubling the b***s**t it spouted forth.