Dash readout V's actual
 

I appreciate the average MPG dash readout is optimistic for almost every car out there, my car generally reads 2 to 3 MPG higher than the actual calculations. But I fuelled up a day or two ago and there was an almost 5 MPG difference. I checked all my figures and everything is correct, but I've never had such a big discrepancy like that. Any ideas why this is? Could the fuel pump have been over reading slightly? Any other ideas or suggestions?

It's because you did not fill the tank to the same level on each consecutive fillup. Even if the dash were exactly 100.00% accurate, it would never match your calculation exactly because of the inherent inaccuracy in filling up to the same level each time. It's impossible to do.

That's the thing, I do fill to exactly the same level. Even though it's not recommended by some, I carefully and slowly brim the tank until fuel is sitting about 2cm from the top of the cap. Ive always wanted as accurate figures as possible, so I try to be consistent. But this is the first time such a big difference has been noted.

Very rarely is my car's calculation more than 1MPG over the actual. The few times it was actually higher I think it was due to a bad automatic shutoff on the pump making trigger happy. That was born out by the subsequent pump calculation.

Was the fill up with the high discrepency one of your better ones? If the gauge is off by a certain percent, better tanks will be off more.

Most of these gauges are monitoring injector pulse length for fuel consumed.

No it was just an average tank. Funny though, its the second most amount of fuel I've put in apparently, which doesn't make sense as I filled up a day after the warning light came on, and I usually leave it much longer. Felt like I didn't let it run as low as I do most of the time, which leads me to believe the pump may have been over reading.

Just filling to the brim isn't a guarantee of getting the same amount of fuel in. Wayne Gerdes of CleanMPG uses a jack to tilt the cars in order to make sure he lets all the air out of the tank. He got 10 gallons over the F150 rated tank size in.

10 gallons, you mean 10 litres? I'm not going to that extreme ha! But I try and be consistent with my methods, I tend to drive with the warning light on as long as possible and when the gauge is as far down as it can go. Obviously garages are built on level ground so overall my method remains the same on every tank. That's why I'm a tad confused by the almost 10% difference. I guess it will average out, maybe on my next tank when I brim it, I might get less in and perhaps I'll understand what's going on.

Nope, a full 10 gallons. 2015 Ford F-150 – The 23 Gallon Tank That Holds 33+ Gallons - CleanMPG Forums
The engineers design fuel tanks with a head space to account for expansion of fuel, and it appears the difference between the standard gas tank on the F150 and the larger, extended range option is where the vent comes in. It enters at a lower level and you get a larger void in the filled tank.

Fuel station, and hopefully all, construction aims for level, but climate conditions at time of cement curing and settling afterwards means they won't be perfect in the real world. Then there is having inclines for rain drainage to consider.
So the only way to be sure of getting the same level of fuel, without using a jack, is to only fill at the same station and same pump.
Another thing to consider is the temperature of the fuel. Here, whole sale fuel deliveries take temperature into account to adjust the volume. If the station recently got a delivery, the fuel may not have completely cooled off in the underground tank yet.

Jeez there's so many things that could affect it. I'm just thinking back, when fuelling up, I noticed the fuel seemed very frothy with lots of bubbles in it, maybe there was air in the fuel? I'm going to stop thinking about it now and juts accept it, I'll see what happens on my next fuel up next month.

Yeah. I've been trying this even if the station/pump is not at the ideal point of my commute loop. Now most of the MPG variable is based on if I break out of the commute loop and drive time temperatures.

I've been with Wayne on drives. Being 6'4" and 240 lbs I always seem to be bouncing on the back bumpers to get more fuel in.

The computer MPG on my car has been 7.5% optimistic for my entire ownership (5500 miles so far) but I have seen some 'blips' as high as 12% out. I put this down to gremlins in the recording system. My MPG via fuelly is pretty consistent, so I'm happy its not more serious than a gremlin. I'm keeping tabs on the average discrepancy as it is possible to enter a correction factor into my computer, to make it read closer to real measured figures.

Well I ran the tank down as far as I dare go, I squeezed the most fuel in I've ever got and the dash MPG was about 2-3 Mpg higher than the fuelly calculations. Safe to say then that it's working fine, and the discrepancy was probably due to the fact that I used a backstreet filling station with a pump that maybe hadn't been calibrated in a while? Who knows, but at least I know the cars calculations are working ok, the 2-3 mpg optimism being perfectly normal of course.

Draigflag: I don't expect the in-dash fuel-economy meter to be accurate, and mine isn't. My Audi Q5 3.0 TDI's display is very optimistic. According to a close friend of mine who is a retired Naval Aviator, test pilot, and rocket scientist (no joke), it's costly to build and implement an accurate fuel flow meter, so car manufacturers don't. They just give you a "pert near" reading as guide. Even though it displays precision to 1/10 of a liter per 100 km or 1/10 MPG, it is certainly far, far less accurate.

Although I have no way of knowing what's really happening on your end for sure, I believe the similarity between your computer read-out's MPG and your actual MPG was likely a pleasant coincidence while it coincided.

You can add a correction factor to your AUDi's mpg display. GOOGLE VAGCOM trip computer mpg correction. This can be done on all VAG cars using current version of the infotainment unit.

luckypants: Can you tell me where I can get VAGCOM cable and required software? I'd like to get into these VAGCOM mods.

It can't be that difficult to devise an accurate reading, Scangauge seem to have a pretty accurate computer model, why can't they use that on other cars for more accurate readings?

Scangauge only has tools at its disposal which are already present on the vehicle. There are no fuel flow meters on cars, and all MPG meters only use the pulse width from the car's ECU to the fuel injectors as a measure of fuel flow. So the scangauge and all other MPG meters (in production vehicles) are limited by the accuracy of this signal.

The cables are available on Ebay, but not all are fully connected, so I think it is buyer beware. The Ross-Tech software is best bought as an official copy otherwise you might find all the menus in German.Ross-Tech: Home. This software is often referred to as VCDS, so another good term to use when searching.

I have found the SEAT forum very helpful in this regard, particularly for the MK3 Leon which has the same infotainment system as current Audis. You might like to search on SEAT Cupra.net - SEAT Forum for some help. I have come across some good Audi and Golf forums when researching the same sort of issue.

I do not have the VAG-Com software, I paid one one for the SEAT forum members who has the software and cable to make my mods.

Here is a video demonstrating making the change to the MPG display of the trip computer. https://www.youtube.com/watch?v=RY8nfw6CTgI to show how it can be done. I'll be getting my Leon corrected to be closer to my measured fuelly average soon. (my car shows a long term average of 61.7 UK mpg, fuelly says 56.7, needs an 8% correction or thereabouts!)

HTH

Some people here are reporting better/more accurate readings from the Scangauge than the cars computer, otherwise, what's the point buying one just to get the same info the cars computer gives you?

Scangauge uses the same inputs as the car's computer to measure the fuel used, but the set procedure involves calibrating with a couple of fill ups from the station.

Not all cars allow you to see that data. And the scangauge does indeed have "calibration parameters" which are nothing more than scaling coefficients, but some folks use them to make their scangauge line up with their empirical data.

I see. Here's the thing I couldn't work out, car gives current MPG, trip MPG, average speed, distance, range but the one that interests me is "fuel used". Surely if I brim the tank every time, once the "range" reading goes blank (usually about 80-90 miles) I could use the fuel used (measured in UK gallons to one decimal place eg 10.3 etc) reading to work out roughly how much is left. I might start making notes of the "fuel used" reading and compare it to the fuelly calculations. If it proves accurate, I can then use it as a guide once the range is empty.

Try Amazon for this. You will surely get the discounted offers there.

Could your information be a reason that burning E0 seems give a slightly more accurate reading of mpg than the use of E10 gives, since E0 averages 8%, 8%, 7% & 5% better mpg over decades, than E10 in my 4 cars?

No, ethanol content wouldn't have anything to do with the accuracy of the fuel flow calculation.

I've also got an issue with indicated versus real mpg. In September I traded my Audi A1 1.4 (petrol) which I'd owned for 22 months from new, and covered 40,000 miles in that time. During that time I very carefully monitored real consumption and compared with that indicated by the in-car computer. It NEVER differed by more than 2 mpg, and was usually bang on.


My new car is another Audi A1 (1.6 TDI) the fuel consumption is (unsurprisingly being a DERV,) far better, the main reason that I bought it, however the difference between the indicated mpg and the real figure is huge – anything up to 12 mpg.Has anyone else seen similar?Is this another attempt by VW Audi to mislead the public?

Hi there. Do you brim the tank consistently? Only reason I ask is that when I fill my car, it clicks off, then it clicks off again, but the truth is, if I gently squeeze the trigger, I get a lot of extra fuel in. Only when I can see fuel sitting in the cap am I happy that I refilled to the same level as the previous tank. Generally dash V's actual readout and 2-3 Mpg different, so 12 seems a bit crazy!

Hi
no I always fill until the pump cuts off. I only ever use two or three filling stations, and try and go for the same pump each time to be as consistent as possible. The issue isn't that there's an error between my method and the cars computer, its that two very similar cars (OK different engines, but similar computational software) produce completely different results. I've queried it with my Audi dealer and they say its "odd" - very helpful


P

First, it is likely a percentage off between the two, and in a car with better fuel economy the raw MPG number difference will be larger. But that seems too large to explain this completely.

It could be that the software was written for gasoline. The program is tracking injector pulse width on one of the cylinders to track the amount of fuel being used. The difference in viscosity between diesel and gasoline means a different amount of fuel gets injected. If the computer thinks it is injecting a the wrong fuel, there can be issues.

I also see your fill ups are a bit erratic in fuel economy. Try recording the dash display results in the notes, and we might be able to see a trend on when it is off the most.

As to VW shenaigans, isn't the 1.6 only of the cheater engines there? The dash display being off could be an artifact of the cheat code. Not necessarily delibrate on VW's part.

I actually read that 1.6 TDI in particular needs not only a software update, but the injectors need swapping too. Is it possible one or the other has not been? Has the car had the recall?

I'm finding a bit of a discrepancy in my fuel usage results between what it shows on the car and the Fuelly website. After a trip up north on the weekend the numbers showing on my dash was 7.6 ltrs per 100 kms. In this site (https://pacificwebsites.com/Convert_L...per_Gallon.htm ) it shows that 7.6 per 100 translates to 37 mls per gallon. Yet, Fuelly shows it at 26.7 mls per gallon. Am I doing something wrong or is there a problem with the cars results.???


2015 Chrysler 200 Limited - Build Date 03/15 - Vivid Blue - Black Cloth Interior - 3.6L 6cy -

You might need to check your settings, there is US MPG and UK mpg, they are different measurements.

That chart shows the imperial gallon, not the US gallon which is smaller and might be what Fuelly is calculating for you.