Hi guys,

I need some help with NSW RTA rules and regulations hopefully we have some techincal law gurus to interpret what I feel is a ridculous bit of info for the general public.

Before I start this is what I want to do :

19x8.5 245/35
19x9.5 275/30

I have read through the vehicle standards info sheet , a couple of times, and also spoken to the technical enquiry guys at the RTA.

This is what I have summised:

- Tire size is fine. Falls well within the +/- 15mm diameter provision
- Front rim width is fine given we can go up to 26mm (1") without needing to inform the rta.
- Rear rim width is not fine and will need an engineering certificate.
- Staggered setup is fine as the standard requires the same offset, rim width, tire size, etc, per axel and not on all four wheels.

What I dont understand...

- What the crap is meant by "speedometer accuracy must be maintained"?? even the tech guy couldnt quantify it? with the above I am running at worst a few k's slower than the speedo.
- Is this correct regarding 15mm diameter increase? if so how can you have this rule and then say speedometer accuracy must be maintained? if i increased by the full 15mm surely my speedo would be less accurate than if I increased by say 10mm?

Insurance will be more than happy to include the wheels on the policy...but...as long as the car remains roadworthy.

So the insurer takes no responsibility, the rta takes no responsibility and the onus is on us? can I also come up with the rules??!!

Sorry...frustrating

Thanks for any help you can impart!!

 

Speedo is calculate from the revolution of the tyre.
So the tyre circumference is the critical factor.

Use some tyre size calculator. (google it). It will tell you how much off you are.

Hope this helps.

 

get an engineers certaificate!

your rear is .5" too wide (by the RTA standards anyway)

 

Thanks guys.

I have done the tirecalc, but, what is meant by "maintains speedometer accuracy"...1%, 2%?

Thanks Ezzy, I thought so and will do. Any idea how much it will set me back?

 

back in "my" days.... they started at around $450 and upwards, depending on how many items to be engineered.
You need to get a RTA authorised/approved engineer to certify the wheels & other mods you have (might as well....). then i think you'd need to forward a copy to the insurance company to prove it is roadworthy.

 

lol all for the pusuit of looks 0.5"...what a world we live in...

Thanks Ezzy.

I have another q regarding wheel track but this is more than enough to think about for now.

 

I've printed off the VSI sheet you linked to but don't have time to read it now. Will check it out tomorrow and post anything I can add.

 

Haven't time to search, but I think the Australian Design Rules specify that speedo accuracy must be within +/- 10% of actual speed. My Polo only just qualifies -- stock

 

wow thats quite a variance...I wonder if our speed cameras are that lenient

 

Here is another one...

"wheel track must not increase by more than 25mm beyond the maximum specified by the vehicle manufacturer"

Based on the below diagram and this link (that says max is 1526 (F) and 1531 (R)) I believe I should be right in that respect...red is new, black is current (dashed is rubber).

According to the offset calculator the offset I want will push the rim 26mm towards the outer guard...that should describe all the numbers below



Have I understood the concept correctly? (right side tire)

If so then by my calculations...

New rear wheel track: 1505 (stock) + 7 (203/2 - (241/2 - 26)) + 7 = 1519

Allowed rear wheel track: 1531 + 25 = 1556

?? doesnt look right does it ??

 

like taka said, your speedometer is based on the circumference of your tyres (= rolling diameter). your speedometer is already pre-programmed to assume that for a full revolution of your stock tyres (360 degrees), your car will travel an x distance. Based on this it can then calculate your speed based on how fast your wheels are spinning.

If your new tyres are bigger (in diameter, and ultimately in circumference/rolling diameter), your car will travel further on the same revolution speed, meaning the speedometer will underestimate your speed (i.e., if your speedo shows 60kph, you are actually travelling above 60kph). If your new tyres are smaller in diameter, then it's the other way around (speedo says 60kph, you're actually travelling below 60kph).

This is what is meant by maintaining speedo accuracy, you have to get the new tyres as close as possible to the stock tyres in overall diameter/circumference/rolling diameter.

 

thanks black-8...

so what we are saying here is that to adhere to the spec I need to get a tire in 19" that most closely matches the original...fair enough...

but then why give a 15mm variance on diameter? diameter being directly propostional to circumference doesnt that infer a defined accepted variance in the circumference?

apologies for the rant...but you can see that by giving the +/- 15mm what I assume they are saying is that this is the boundaries within which to "maintain speedo accuracy"...translate that to % and i would guesstimate it might be ~ 5% which is a pretty big influence on the odometer which of course is also affected...

 

25mm of increased wheel base?????

You are screwed Revolver

RNE515 - is there a easier name for us to type?

FYI - you cannot have a rim lower than +38.5 according to those rules. (that is with an 8"). It is too complicated when you factor the width of the rim in. Where to get the exact centre? or at the outside edge? (I guess the latter - RTA just want to discourage you modding)

For VIC, I still think it is right - that if your wheel edge not passing guard, then it is ok.

 

Please refer to related AustRoad and Vehicle Specification Data Sheet. The variance of speedometer accuracy should be at 90% confidential interval, which means error should not more than +/- 5%.
RTA is expecting that if you have parts on car which are different from manufacturer's specification, you must inform RTA and they may organize inspection at your vehicle. Although there is a very detailed engineering specification manual within RTA, it will really depends on how the vehicle inspector think. And once your car become RTA's target, especially if your car get "defects", then it will be on bad list in the future. You may contact the RTA Vehicle Engineering Standard Division for further details, ask them to define the relevant standards on e-mail, and bring alone with you when RTA inspect your car.

 

That's the puppy

 

Thanks wkthyk! I can certainly live with that...I did contact these guys with an enquiry and got the following:

In regards to wheel track, after doing the maths properly, it looks like I will be only off a few mm. When doing the calc I can see where the RTA are coming from in that if you change your offset so that your inner tire walls line up (stock and new tire/wheel) then, in our case, the largest tire you could fit would be a 325...now that will definitely stick out from the guard and there is also a provision for that in the spec.

Link here for those interested...pretty confident it is correct and will live happy in ignorance until I hear otherwise

Maz (just for taka )

 

so the new question is.... how low and how lazy are you?

 

hmm...how do i put this tactfully...how about tac "very" in front lol

 

I've got a very big ruler.

 

if thats the case, it deserves a pic on here

 

Ezzy you old charmer

For sure once I have the rubber fitted...whoops that came out wrong