How accurate is the MPG indicator.??

[John Ratsey]

Why is there a consistent variation?

It must be designed that way.  I think the measurement of fuel consumed is based on regular sampling and it’s clearly in the maker’s interests to err on the side of flattering the consumption, making us all feel better.  I don’t suppose there are any regulations specifying accuracy as it is not a safety issue.

The error on the fibometer seems to be quite closely related to the speedometer over read. Both over reading about 4/5%.

Is the false calculation of petrol consumption perhaps based on a false reading of speed?

Remind me, why does the speedometer over read?

If the mpg reported by the fibometer was an unbiased best estimate based on measurements of fuel into engine and distance travelled then some people would see high mpgs, others low mpgs and many close to correct. However, no one has ever reported the fibometer reporting an mpg lower than the fuel into tank calculation so there must be built-in bias.

I noted when comparing the Mk. 4 speed (a digital display) with the sat-nav speed that the Mk. 4 speedo displays true speed + 1 or 2 mph rather than add a percentage which means that the difference increases with speed. There's no reason for the vehicle to not know the actual speed subject to the limitations caused by tyre pressure and tyre wear (low pressure or worn tyres will increase the apparent speed) as the system counts the wheel revolutions (sensibly the average of all four wheels).

[Jocko]

However, no one has ever reported the fibometer reporting an mpg lower than the fuel into tank calculation so there must be built-in bias.

With my Mk1 I have experienced some regular incidences where the calculated mpg is higher than the "Fibometer". About 10-12% of the time, but always with exceptionally good mpg figures (60 mpg+).

[JamtartJazzman]

Did a 275 mile trip in the last couple of days. By the end of the trip, the fibber was registering 65 mpg. I re-filled and calculated the consumption at 60.01 mpg.  The last bike I had that was as good was a 250 MZ!

[Jocko]

I had a top-up yesterday and for the first time, the fibometer and the calculation matched exactly. 61.6 mpg.

[MRCLICKCLICK]

Could I point out that mpg is dependant on many variables. 1 - how many passengers, 2 - weight of each, 3 - luggage, 4 - weather conditions, 5 - how you drive - heavy or light foot, 6 - fuel manufacture ( I know they should all meet the same specification - but they differ from supermarket to supermarket and petrol station brand to another). 7- others. So - please when giving your mpg please state the variables that you are using. And don't forget - manufactures mpg are lies - they aren't done in the real world.

[ColinB]

And don't forget - manufactures mpg are lies - they aren't done in the real world.

That’s a bit harsh. Manufacturer’s figures are measured under precisely specified and controlled conditions in an attempt to ensure the figures are comparable across marques. The whole point of the mandated testing is to eliminate the kinds of factors you mention as much as possible to give consumers an objective comparison across brands and models. Of course they don’t represent real world consumption because of all those factors (to which one could add things like ambient temperature, hills, wind strength and direction etc, etc), but that doesn’t make them “lies”.

[MRCLICKCLICK]

OK ColinB- maybe a bit harsh - but don't forget how there are several manufacturers that are being taken to court etc for "falsifying" test results to "improve" result - that his lying in my books. However to be fair - Honda, as far as I am aware isn't in this situation. But I have always taken mpg figures with a pinch of salt. My main point is that the only way you are likely to get the quoted MPG is to drive the car in the same way as it was tested - in a laboratory situation - which we cant do - but the thrust of the point is that so many variables do affect mpg - so the only way to give the best comparisons is the state the conditions. For myself I run 2 up - slightly over average weight, no luggage, the same petrol station fuel, no motorway, light footed, eco mode on.

[culzean]

And don't forget - manufactures mpg are lies - they aren't done in the real world.

That’s a bit harsh. Manufacturer’s figures are measured under precisely specified and controlled conditions in an attempt to ensure the figures are comparable across marques. The whole point of the mandated testing is to eliminate the kinds of factors you mention as much as possible to give consumers an objective comparison across brands and models. Of course they don’t represent real world consumption because of all those factors (to which one could add things like ambient temperature, hills, wind strength and direction etc, etc), but that doesn’t make them “lies”.

The disparity between EV makers claims and ICE car claims seems to be even wider,  take an EV on motorway and expect 60% of claimed range, on a wet motorway even worse, on a cold, wet motorway even worse.  One bloke took Nissan to court  ( or threatened to - I may have posted a link in electric car thread a while ago ) because he waited for new Leaf with claimed 235 mile range and fast charging, only to find on the longer trips he was getting about 150 miles,  and could only use fast charge once a day,  so he was stuck on a motorway services for hours waiting for car to slow charge and the computer said NO ! ( it is the car computer that controls charging to 'save the battery' ).  Take an EV above 50mph and all the miles per charge claims are out the window, but around town are better than ICE.

If you look on various sites Honda seem to be in the top of the league when claimed MPG vs real mpg is concerned.

https://www.bbc.co.uk/news/business-44575399

[Expatman]

Based on this and a bit of online research it now seems that EV's are totally unsuitable for longer journeys. If you can only fast charge once on a journey then the useable range is dramatically reduced.
Has this been hidden by EV makers or is this information only just being understood by the industry?

[Kremmen]

A friend bought a Jag iPace some years ago. The brochure at the time very cleverly said 'up to 250 miles'.

After a few years use through all seasons and various equipment usage his average range is ~175 at best. His main problem is that there are various networks of chargers and the best he could find was 2 miles from his office near Croydon. Using chargers without a contract is very expensive.

Twice a week he has to park up and get a bus as he has no way of safely charging overnight near where he lives. 

[ColinB]

OK ColinB- maybe a bit harsh - but don't forget how there are several manufacturers that are being taken to court etc for "falsifying" test results to "improve" result - that his lying in my books.

Again, a bit harsh and possibly incorrect. There has been plenty of publicity about falsifying emissions testing results (for example: https://en.wikipedia.org/wiki/Volkswagen_emissions_scandal), but I don't recall hearing anything  about anyone falsifying the economy figures.
If you know of any sources about falsified economy and/or court cases for ICE cars, please provide a link.

Culzean's point about EVs is interesting, but I think EV charging and range claims are a whole different problem. Was there deliberate lying, or perhaps Nissan didn't make the practicalities as obvious as they should, or did the customers misunderstand? It's noticeable that the BBC link does point out that some of the issues are described in the car handbook, and that the changed test protocol - 3 years ago - brings the claimed range down much closer to what drivers actually experience. And is affected by all the real-world factors identified earlier, plus the added uncertainty of the charging procedures. I suspect the EV industry is only gradually understanding what information needs to be "front and centre" to the consumer.

[MRCLICKCLICK]

ColinB - point taken :-). still doesn't alter my views re mpg form Manufacturers - take with a pinch of salt. Been like this since 1965 when I started driving - never been any different.

[MRCLICKCLICK]

Forgot to say - had Crosstar since Oct last year - done 2600 miles. 2 up, no luggage, no motorway, 50% town / under 40 limit, Eco on, light foot on throttle - ave now 65 mpg. Was lower in winter - the ICE came on more and longer. Have EV regularly running at 50 - 60 MPH - even seen on a short run at 70 MPH. See what happens over the coming Autumn and winter - I think will drop back to 50MPG or so - depending.

[embee]

As said, vehicle manufacturers are bound to do things in a very closely specified way. I've been involved in vehicle engineering with various manufacturers and for example emission tests are done in accordance with very closely specified procedures, for example vehicle pre-conditioning and soak times/temps etc before the test is carried out.
Like any business, manufacturers will try to present their product as best they can, and in terms of emissions that means being able to sell cars at all (no compliance, no certificate, no sales) and for fuel economy (CO2) that puts the products into various tax categories etc which can have significant influence on market sales.
For economy, what you can and can't do to a vehicle to prepare it for test is laid down, things like tyre wear etc (rolling resistance). Stuff used to be done which was stretching the regs to the limit (alternator pulley diameters anyone?), but largely stopped now.
Manufacturers have had to do economy tests in accordance with the regs, the fact that it hasn't historically represented real world conditions is not down to the manufacturers. That has changed largely now with the "real world" test cycles, but that is strictly specified too.
Whether an on-board trrip computer is realistic or not is nothing to do with regs however, that's up to the manufacturer. There's little excuse for being wildly wrong on those, anything more than 10% optimistic is not really forgivable.

If you don't like the regulations, blame the regulators not the manufacturers.

Using any means to deliberately make a vehicle run differently during conditions not represented by emission certification testing has always been illegal, the so called "defeat device" ("device" is a legalese term meaning any means by which it is done, it doesn't mean a specific physical object). One of the first things drummed into us when I started in the industry was that you DO NOT do anything which could be interpreted as a defeat device. Any manufacturer who has deliberately done this deserves all the penalties they get, they know what they are doing and they know defeat devices are illegal.

One major factor which influences real world economy is the wind, which is often not appreciated. Drive with a 20mph tail wind and the economy will be very much better than driving into a 20mph head wind.
Heavy rain too, look at the spray coming off wheels and think what is powering that, you need a decent pump to shift that much water and that pump takes power.

[peteo48]

I do find the WLTP figures a lot more real world than the old NEDC figures. I think the combined figure for my car is 46.3 on WLTP and I get around 42 mpg but, to be fair, my usage isn't really balanced with the majority of my journeys being 3 to 5 miles.

EVs do have a problem with over promising and under delivering on a truly epic scale. Here WLTP seems some way from reality. The Honda E is a sub 100 miles car and yet, I think, the WLTP figure is about 135 - it's hard to imagine how perfect the conditions would have to be for a Honda E to attain that.