MPG Display Wrong

[viperbl]

Ok, I have new injectors now, and its showed me 34 MPG, when in actual fact I had done 26MPG ...

Is there any way to change this to try and get it to display a more realistic figure????

[CliveH]

Yes - top tip here from Dippy : http://www.rs246.com/index.php?name=PNp ... ic&t=47768

[viperbl]

Nice Clive! Just what I wanted ... but, my friend is now complaining about his 18" wheels on his S3, they are stock 17"

He is asking if we can adapt this??

[CliveH]

Correct me if I'm wrong, but the miles recorded will be the same, whatever the size of the wheels.

So if you record miles driven, fuel used, calculate MPG, and then adapt the DIS following Dippy's instructions, he should be OK?

[viperbl]

It will be right with the driven miles, but, if larger wheels make him go futher than one mile (when the DIS says 1 mile passed) then the actual readings will be wrong...

And Doh, mine was out by 30%!!!! lol!!! I did think 34MPG was a little too go on the motorway the other night! when in actual fact it was just 26MPG, BUT, Vag com will only let me change up to 115, so, 15%, so, guess thats not gonna work for me!

[CliveH]

I take your point on the wheels, but I'm not sure where the sensor is or how it works, so I'll reserve judgment on that one.

As regards the MPG adaptation, I had the same problem, but the other way (16 vs 20), and the maximum adjustment was 85, but I think you can recalibrate - try it after a few more tankfulls...

[derdle]

Bit of useless information time....
A 17" wheel has a rolling diameter of 53.4 inches and requires 1186 revolutions to go one mile.
An 18" wheel has a rolling diameter of 56.5 inches and requires 1121 revolutions to go one mile.

So, if you have 18" wheels on your car instead of 17", then after 1186 rotations the car will have gone 101 yards further than a mile - that is a 5.7% difference. It will also mean that you could be doing 74 mph when the speedo reads 70mph (ignoring other inaccuracies).

As for the mpg reading, did you reset the readings before you set off? The average mpg will be measured over a certain distance and if it wasn't reset it would be calculating an average which includes mileage/consumption before the mods. For instance on a recent trackday my average mpg dropped from 21.6 to 18.7 .. but the true mpg was something like 6 (2½ laps to the gallon!).

Is there a setting within VAGCOM to adjust for wheel size?

[viperbl]

As for the mpg reading, did you reset the readings before you set off? The average mpg will be measured over a certain distance and if it wasn't reset it would be calculating an average which includes mileage/consumption before the mods. For instance on a recent trackday my average mpg dropped from 21.6 to 18.7 .. but the true mpg was something like 6 (2½ laps to the gallon!).

Yep, I did a reset just after filling up, then drove 312 motorway miles before filling up again, on board read out 34.2 MPG average, and it took, 54.59 litres to fill back up ...

I think with my bigger injectors and fuel pump its all out ....

[derdle]

I wouldn't have thought that the change of injectors would have upset the measurement of the fuel (difference between fuel being sent from the tank less fuel returned to the tank)....Really don't know what the answer is!

[CliveH]

Paul - I understand how the differences in wheel size affect rolling radius, distance traveled, speedo reading, etc.

But are you saying that the way distance traveled is measured (and recorded on the car) that with 18" wheels (vs 17") the recorded mileage on the odometer will be 5.7% less since the bigger wheels were fitted?

[rikrose]

The MPG measurement is done by duty cycle of the fuel injectors, not ont amount. If you bung in bigger injectors and tell the ECU about it, then the ECU will know to use a lower duty cycle to get the right amount of fuel in. Unfortunately, the DIS doesn't know about it...

[derdle]

Clive -yes!

Each extra 1 inch diameter of wheel will add 3.1415926 inches to the rolling radius (i.e. the value of pi).

Bit like the guy in lane 2 of a running track will run further than the guy in lane 1 if he doesn't have a staggered start.

Got a client come in so willl have to come back to this later.

Sorry!

[CliveH]

No worries Paul.

I totally understand the concepts, but didn't realise that the location of the sensor is such that the actual recorded mileage is affected

[derdle]

Clive - I'm no expert on these things, but I can't see any other way for the car to measure distance travelled?


Paul

[CliveH]

I'm sure you're right, Paul.

So if I sold my S4 now, I should really declare the mileage as "70,000 recorded, plus 2,280 unrecorded since wheels increased to 18"?