Replacement Tyres from OEM Continentals

[Sydneykid]

Why dont you ring and tell him hes wrong then seeing you seem to be an expert on the subject.

I didnt go to university so have to rely on the legal documents and their interpretation by the people who have to enforce them
Im sure he will enjoy the conversation
His name is Adam

More than happy to discuss it with him.

FWIW I'm occasionally called on as an expert witness, plus I also submit the odd technical paper on new and pending motor vehicle regulations. It has been my experience that the "enforcers" simply enforce what they have been trained to based on the interpretation of the regulations by their superiors and sometimes that interpretation, when tested, is legally incorrect. Albeit not that often, they are pretty good at their job, which you would expect as they do it every day. But occasionally they look at one reg and apply that in all circumstances when there are other regs that they should also consider.

For those that have been following, I still maintain my original position that a 215/45/17 tyre is 100% legal on a Polo Gti. As per 6.1.5 formula Dmax = d + (2H . b) where b = 1.04

Cheers
Gary

[brad]

True and to correct the speedo on our POLO which reads 106 at 100 it needs tyres that are 40 mm LARGER in diameter and therefore illegal
Eg tyres are 195/55/15 diameter 595mm

To get 6% correction they would have to be 195/65/15 which gives 6.3% difference diameter 635mm

I hope thats right but this says it is Custom wheels, rim tire packages Put big size in first

Don't ask me why the numbers don't add up but the speedo on the Octavia was out 10% at 100kph and I went from 632mm to 657mm (25mm difference) which is 4% difference in diameter and the speedo was dead accurate. Go figure.

[brad]

When I worked in the tyre industry and Australia actually made tyres 185 and 195 x 14 Goodyear Supersteels came out of the same mould as did a few others. They were close to the same height and width too.

At the multi-brand place I worked at you could line up 10 different brands of a 235/60r14 and they would vary by 30mm in diameter and 40mm in apparent width. A lot of the width difference was due to the shape of the transition profile from tread to sidewall. It was handy if somebody was scraping tyres due to being lowered and too much rim offset as you could plonk some Yokohamas (i think they were the small ones) on it and all the scraping was gone.

That was in the 1980s so i'm sure none of that happens anymore. ;-p

[Sydneykid]

Don't ask me why the numbers don't add up but the speedo on the Octavia was out 10% at 100kph and I went from 632mm to 657mm (25mm difference) which is 4% difference in diameter and the speedo was dead accurate. Go figure.

On old tyres, worn out but not quite illegal, there can be up to 5% difference in diameter compared to the same brand new tyres, with full tread depth. My Polo on brand new tyres did 100 kph (genuine, via the GPS data logger) at an indicated 105 kph. Over time they wore down and it got up to 109 kph (to do a genuine 100 kph). Put on a new pair of 45 series and it reads 100 kph at a genuine 100 kph. Obviously that will change as they wear, but won't get to the 9 kph difference that the 40 series ended up at.

We also experience diameter growth due to centrifugal force, on the race cars the wheel rpm and GPS speed traces differ at 60 kph to what they do at 270 kph. Plus different tyres grow at different rates, the cross ply tyres was use on the historic F2's grow faster than the radials we use on the F3.


Cheers
Gary