Pinging at idle

[Corey_R]

So in effect the ADR is mandating that the speedo read faster than the real speed.
As I said - the 2% maximum inaccuracy that Topend is stating should be the case is just not appropriate given that the Golf can be upgraded from the standard 225/45R17 wheel to a 235/35R19, and it complies with the regulations of no more than a 2" upgrade from stock.
The 19" wheel will in effect erase that 2% inaccuracy making the Golf not (or barely?) in compliance with the ADR. In addition to that, it would not leave any 'analogue viewing inaccuracy' variation into how the driver reads the speed. Nor would it allow any 'throttle application inaccuracy' for the drive whilst they're driving through any of the traffic light cameras which now also are speed cameras (new in NSW, but been in VIC for quite some time).

So yeah - the ADR doesn't say "it must be 7%", but Topend's request for 2% is just now practical unless the government makes it illegal to change your wheels without also having your speedo recalibrated - which would just be a pain in the arse.

In the end - what does it really matter? If you need 100% accuracy, you use a GPS or laser based unit anyway - and Topend already has that equipment, which is why he knows about this issue!

[pologti18t]

In the end - what does it really matter? If you need 100% accuracy, you use a GPS or laser based unit anyway - and Topend already has that equipment, which is why he knows about this issue!

That's what I was coming to next... Who cares about speedo inaccurracy if you know what that inaccurracy is? Hardly a "write a letter to VWA" issue.

Golf can be upgraded from the standard 225/45R17 wheel to a 235/35R19, and it complies with the regulations of no more than a 2" upgrade from stock.

It should be:

The outside diameter of the wheel and tyre combination must be no more than 15mm over the largest diameter
wheel and tyre combination specified for the vehicle and not more than 15mm below the smallest diameter wheel
and tyre combination specified for the vehicle.

Which if you look at the figure is about the difference between a fully treaded tyre and a bald one of similar original dimensions.

[Corey_R]

From a tyre size calculator (so keep in mind that there can still be slight variations between different makes and models or tyres).

225/45 R17
Rolling Radius 317.15mm
Circumference 1992.71mm
Diameter 634.3mm

223/35 R19
Rolling Radius 323.55mm
Circumference 2032.92mm
Diameter 647.1mm

Differences between Wheel/Tyre 1 & 2
Rolling Radius 6.4mm
Circumference 40.21mm
Diameter 12.8mm
When your speedo reads 100km/hr you're actually at 101.98 km/hr

So there you go - the change from the 17" GTI wheel to the 19" R wheel (or aftermarket wheel) is only 12.8mm, and within the 15mm requirement of the ADR. So if a wheel really was 15mm then it would give an inaccuracy in excess of 2% hence making Topend's request illegal under Australia Design Rules.

Thanks for the info pologti18t. Good detective work.