MPG Display Wrong

[ajsl]

The differences aren't nearly as large as you guys are calculating .

The key thing here isn't the rim diameter (17" vs 18") is the total rolling radius including the tyre. For instance you're probably running lower profile 18" tyres.

i.e. on my car I have 18" B6 S4 wheels and tyres to replace my standard 17" ones.

The standard S4 17" wheels come with 225/45 R17 tyres, where as the 18" B6 wheels have 235/40 R18 tyres. So even though they have a larger inside diameter they have a lower profile.

Using somewhere like
http://www.tiresizecalculator.info/

Gives:

Original tire
Sidewall: 3.99 in
Radius: 12.49 in
Diameter: 24.97 in
Circumf: 78.45 in
Revs/mile: 807.61

New Tire
Sidewall: 3.70 in
Radius: 12.70 in
Diameter: 25.40 in
Circumf: 79.80 in
Revs/mile: 793.97

Speedometer reading with new tire is 1.72% too slow.
When your speedometer is reading 60mph you are actually traveling 61.01mph.

So the difference isn't as great as the rim size change would indicate .

[derdle]

We all know your such an honest guy Clive!

[CliveH]

Alex - absolutely right - I'd assumed Paul had taken account of that, but of course he doesn't know the size of the tyres on my 18s

Paul - even so, point well made - you could make an honest car salesman of me yet...

[derdle]

I'd forgottten all about the tyres. Doh! Just goes to show how stressed out I was yesterday and I didn't even realise it.

Sorry guys.

I'll get my coat an go hibernate for the rest of the winter.

Paulzzzzzzzz

[viperbl]

How about with full tread and low tread! this must make a difference as well!

[DavidT]

You will need to change the setting for the injectors to get your mpg closer. Its in the dash I think. Bigger injectors are using more fuel in the same duty cycle compared to the original small ones so the mpg reads too high. Mihnea did mine and got it pretty close.

[viperbl]

You will need to change the setting for the injectors to get your mpg closer. Its in the dash I think. Bigger injectors are using more fuel in the same duty cycle compared to the original small ones so the mpg reads too high. Mihnea did mine and got it pretty close.

As in an adaptive channel that we can do in vag com? or is it a Mihnea only change???

[DavidT]

Dunno, ask him.

[viperbl]

Will do:)

[CliveH]

You will need to change the setting for the injectors to get your mpg closer. Its in the dash I think. Bigger injectors are using more fuel in the same duty cycle compared to the original small ones so the mpg reads too high. Mihnea did mine and got it pretty close.

As in an adaptive channel that we can do in vag com? or is it a Mihnea only change???

Is it not simply this...? http://www.rs246.com/index.php?name=PNp ... ic&t=47768

[viperbl]

Is it not simply this...? http://www.rs246.com/index.php?name=PNp ... ic&t=47768

We've been there and done that on the first page of this thread

Problem was mine was so far out because of the new injectors that this fix was not enough, ie, I was out by over 130%, and this only allowed 115% difference...

[derdle]

You could put some really small wheels (and tyres) on the car to compensate?

[CliveH]

Is it not simply this...? http://www.rs246.com/index.php?name=PNp ... ic&t=47768

We've been there and done that on the first page of this thread

Problem was mine was so far out because of the new injectors that this fix was not enough, ie, I was out by over 130%, and this only allowed 115% difference...

exactly, that's where this all started, but my understanding is that's how to do it, and you can then 'fine tune' after the first 115 (or 85 in my case) adjustment...?

Mihnea/Doug - is there another way?

[DavidT]

No Clive, you are talking about fine tuning with that when what you really need is a major setting change.

[CliveH]

No Clive, you are talking about fine tuning with that when what you really need is a major setting change.

aha, then I have misunderstood - where does one find such major setting change please?