Onboard computer MPG (miles or meters per gallon?)

[Tim T]

Bi-turbo, on winter fuel.

I'm having some trouble believing it as well. Although it appears to me that there has been a firmware upgrade that has reduced the damping on the average mpg it still seems that the compression damping is somewhat less than the rebound. The figure moves up quite readily but is reluctant to come back down with quite the same rigorous adherence to accuracy. It's quite easy to keep it above 40mpg and yet the full to full tank refill over the "trip" reveals a pretty constant 36-38mpg over the year.

Anybody else finding the onboard about as accurate as a DFS Sale closing date?

 

[Ozzy Pete]

The onboard computer is really only a guide. The only accurate way to get a true reading is filling the tank full to full a few times then taking an average.

 

[Tim T]

The onboard computer is really only a guide. The only accurate way to get a true reading is filling the tank full to full a few times then taking an average.

I understand what you say, but a guide on my model that has a consistent 10+% over-read is poorly calibrated. If it's over the entire fleet it becomes more of a marketing tool. The reading on mine seems to be more about making me feel good than providing information... ;-)

 

[scottk]

I’m sure there is a way to adjust the calculation with VCDS. My current car is pretty spot on but the previous ones have all been over by about 10%

 

[WelshGas]

View attachment 88756
Bi-turbo, on winter fuel.

I'm having some trouble believing it as well. Although it appears to me that there has been a firmware upgrade that has reduced the damping on the average mpg it still seems that the compression damping is somewhat less than the rebound. The figure moves up quite readily but is reluctant to come back down with quite the same rigorous adherence to accuracy. It's quite easy to keep it above 40mpg and yet the full to full tank refill over the "trip" reveals a pretty constant 36-38mpg over the year.

Anybody else finding the onboard about as accurate as a DFS Sale closing date?

Try changing to Memory 2.
This gives the Average MPG over 20,000 miles. Much more accurate and Real World.


Press the Steering Wheel OK button wWhen Ignition is switched On.

 

[Ozzy Pete]

Mine is the same. I constantly get 40-42 mpg on the computer on a run but full to full it’s nearer to 37 mpg. Nothing’s changed over the 7 years we have owned it. You can’t expect a van that weighs as much as the California to give what vw says you should get. I recon 37 is probably as good as you can expect.

 

[Wildcamper]

My MDF mpg calculation on trip to trip v tank to tank is fairly consistent. On a reasonable length trip I will get between 37-40mpg while a short trip may be 28mpg. On MDF screen two the overall average mpg is 35 mpg and this tallies well with my fill to fill record of all fill ups over 28000 miles.

 

[Tim T]

Try changing to Memory 2.
This gives the Average MPG over 20,000 miles. Much more accurate and Real World.


Press the Steering Wheel OK button wWhen Ignition is switched On.

Thanks, this does sound good. But... I normally use the MPG, or economy, to give a rough idea on the health of the engine/van where it's useful to know tank to tank. So if the long range average is accurate and easier to program because of an inherently smaller error, then the short range average must be programmed to over-read (it must be if it's consistently higher). Perhaps VW worry about how false low readings change user's perception of economy... Still means if it's not accurate it's just a "feel-good button".

@Ozzy Pete -Yes, I agree. 37MPG is pretty good for the weight of the van and a Bi-Turbo engine. I'll still be doing the ful to full over the trip calculation on the iphone calculator, it's quite easy. But I long for a reset/trip on MPG from tank to tank, it can't be that hard. ;-)

 

[WelshGas]

Thanks, this does sound good. But... I normally use the MPG, or economy, to give a rough idea on the health of the engine/van where it's useful to know tank to tank. So if the long range average is accurate and easier to program because of an inherently smaller error, then the short range average must be programmed to over-read (it must be if it's consistently higher). Perhaps VW worry about how false low readings change user's perception of economy... Still means if it's not accurate it's just a "feel-good button".

@Ozzy Pete -Yes, I agree. 37MPG is pretty good for the weight of the van and a Bi-Turbo engine. I'll still be doing the ful to full over the trip calculation on the iphone calculator, it's quite easy. But I long for a reset/trip on MPG from tank to tank, it can't be that hard. ;-)

You can reset Memory 2 tank to tank if you wish.

The MFD display does not take account of Parking Heater usage, just fuel usage related to miles covered, so will vary due to tyre wear, distance covered/revolution greater with new compared with worn tyres, tyre pressure in relation to varying load also vary the tyre circumference and hence distance covered/revolution also using the Coast function or engine braking downhill where you get less fuel consumption for distance covered all produce subtle errors that add up making it impossible to get an accurate reading in the real world, compared to the laboratory type conditions used to give the manufacturer published fuel economy figures.

 

[2into1]

The dash figure was very accurate on the T5 and got progressively worse (optimistic) on the newer models. T6s seem 10% out.

 

[andyinluton]

On a T6.1 the Reconnect App records fuel usage & you can display it under 3 different tabs: Since last start, Long term and since refuelled.
On the since refuelling tab it records the date you filled up, distance covered, ave speed, driving time & Ave consumption for that tankful.

 

[Ozzy Pete]

The most accurate vehicle mpg figures we have had has been on a Mini Cooper D we had a while ago. On a run it calculated 72 mpg and full to full was 71 mpg but every other vehicle has been about 10% out. Like someone said on here it’s just a feel good thing thinking your van is much more economical than it really is and makes it look like the VW consumption figures maybe almost correct. But we all know they are not.

 

[andyinluton]

Even a fill to fill calculation is not going to be accurate unless the distance covered is accurately recorded.
With even factory spec wheels having slightly differing sizes the chances of that being right are virtually nill.

 

[Ozzy Pete]

Full to full is the only true calculation. Providing you don’t change the parameters.

 

[WelshGas]

Full to full is the only true calculation. Providing you don’t change the parameters.

Or use the Parking Heater.

 

[andyinluton]

Full to full calculates fuel used accurately but not distance covered.

 

[Ozzy Pete]

Full to full calculates fuel used accurately but not distance covered.

Like I said, Providing you don’t change the parameters. Full to full over a number fills gives you the most accurate reading possible. Wether you do 100 miles or 500 miles it’s still fuel consumption.

 

[andyinluton]

Like I said, Providing you don’t change the parameters. Full to full over a number fills gives you the most accurate reading possible. Wether you do 100 miles or 500 miles it’s still fuel consumption.

How do you work out MPG when you know how much fuel you have used but you don't know how far you've gone?

 

[ardm17]

Think I'm going to stick with calculating SPG - Smiles Per Gallon

 

[Tim T]

How do you work out MPG when you know how much fuel you have used but you don't know how far you've gone?

But I'm not really after a figure that's "accurate", close will do. To be useful they just need to be comparable.

In my experience I have never found that tire wear or any of the other "variables" that have been mentioned really make any appreciable difference, (being self employed I used to log every fuel receipt against mileage). But the switch from summer fuel to winter fuel stands out a mile, so does the difference between supermarket fuel and one of the bigger named brands, (so did the extra miles when the chip fat used to go straight in the tank).

We all know speedometers over read, (therefore so must odometers), so I know even my 38mpg is probably nearer 36mpg. But it still gives a very consistent figure from tank to tank. Which kind of puts the VW average figure even further from the truth.

On a modern computer controlled engine, with modern emission standards fuel used is probably just a data logging exercise, the coasting function is controlled by the throttle positionand the same computer... It wasn't that long ago that VW programmed it's engine management system to recognise if it was on a emissions evaluation test and adjust the fueling accordingly. I would've thought that took some fairly accurate monitoring of certain parameters... ;-)

 

[Cali Phillip]

Or use the Parking Heater.

Beat me to it, I can see in variance when the parking heated has been used more or less on a fill up.

 

[Ozzy Pete]

But I'm not really after a figure that's "accurate", close will do. To be useful they just need to be comparable.

In my experience I have never found that tire wear or any of the other "variables" that have been mentioned really make any appreciable difference, (being self employed I used to log every fuel receipt against mileage). But the switch from summer fuel to winter fuel stands out a mile, so does the difference between supermarket fuel and one of the bigger named brands, (so did the extra miles when the chip fat used to go straight in the tank).

We all know speedometers over read, (therefore so must odometers), so I know even my 38mpg is probably nearer 36mpg. But it still gives a very consistent figure from tank to tank. Which kind of puts the VW average figure even further from the truth.

On a modern computer controlled engine, with modern emission standards fuel used is probably just a data logging exercise, the coasting function is controlled by the throttle positionand the same computer... It wasn't that long ago that VW programmed it's engine management system to recognise if it was on a emissions evaluation test and adjust the fueling accordingly. I would've thought that took some fairly accurate monitoring of certain parameters... ;-)

I wouldn’t really trust VW software 100%. We all know what they did. All vehicle manufacturers give false impressions on what your fuel consumption should be. Every single car I have owned doesn’t give a fuel consumption like the manufacturer states.. Not just Petrol and Diesel but the same applies to EVs. Accept you are not going to get the manufacturer figures and you won’t be disappointed.

 

[WelshGas]

I wouldn’t really trust VW software 100%. We all know what they did. All vehicle manufacturers give false impressions on what your fuel consumption should be. Every single car I have owned doesn’t give a fuel consumption like the manufacturer states.. Not just Petrol and Diesel but the same applies to EVs. Accept you are not going to get the manufacturer figures and you won’t be disappointed.

If you drove your vehicle under the exact same conditions the manufacturer did to get their quoted fuel consumption readings you would get the same. However, you don’t , you drive in the real world , varying ambient temperature, load, tyre pressure, wind and weather etc: etc. In fact, if you did match the Manufacturers figures there is probably something wrong.

 

[Ozzy Pete]

If you drove your vehicle under the exact same conditions the manufacturer did to get their quoted fuel consumption readings you would get the same. However, you don’t , you drive in the real world , varying ambient temperature, load, tyre pressure, wind and weather etc: etc. In fact, if you did match the Manufacturers figures there is probably something wrong.

If I could manage to get almost 50 mpg average from a Two and a half ton van I would be ecstatic. I’ve tried absolutely everything to get figures even remotely approaching the VW quoted figures and sadly fall way way short. But like I said it’s not just VW it’s ALL vehicle manufacturers.

 

[WelshGas]

If I could manage to get almost 50 mpg average from a Two and a half ton van I would be ecstatic. I’ve tried absolutely everything to get figures even remotely approaching the VW quoted figures and sadly fall way way short. But like I said it’s not just VW it’s ALL vehicle manufacturers.

But did you drive according to the Official Testing Procedure.
There have been changes since, but nothing really significant.
The Testing Procedure is carried out exactly the same way by ALL manufacturers and is only of use comparing one vehicle model against another . It has absolutely no scientific relationship to Real World Driving.

How do manufacturers calculate mpg?

Fuel efficiency. If it’s high on your list of priorities when it comes to choosing your next car, then you’ll be studying its official mpg stats wi...

leasing.com

How is the mpg test done?​

New cars are subject to the New European Driving Cycle (NEDC) procedure, which can be used to find out a number of figures which enables manufacturers to find their “official” mpg. The NEDC was created to proved consumers with fair, unbiased data, but recent reports show that few cars can deliver their official fuel consumption figures in the real world.

To understand how the figures are calculated it’s necessary to look at the procedure itself. So the playing field is as level as can be, every test is carried out on a rolling road, with an ambient temperature between 20C and 30C. The engines can’t be run beforehand either. In fact, nothing is left to chance.

The boring bit…​

The urban cycle is completed first, and is meant to replicate driving in a built up area. It’s done by accelerating and decelerating from 0mph to speeds of 9, 15 and 20mph. This is repeated three more times and the entire cycle lasts 780 seconds, taking the car 2.8 miles.

The next step is the extra urban cycle, that’s meant to equate to driving on faster roads. Firstly, the car will gently accelerate to 43mph, maintain its speed for 50 seconds and then decelerate to 30mph.

It sticks at 30mph for 69 seconds and then accelerates back to 43mph for a further 50 seconds. Finally, it accelerates to 60mph, maintains speed for 35 seconds and then accelerates to 75mph for a further 10 seconds.

The car then slowly decelerates to a standstill and the cycle ends. The figures consumers will be most interested in are not the urban and extra-urban fuel consumption figures, but the combined consumption that is worked out from the two.