Navara tyres

Submitted: Wednesday, Apr 20, 2005 at 19:29
ThreadID: 22234 Views:3896 Replies:2 FollowUps:1
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A question to all you learned people out there.........The series 1 Navara has 255/70/16 tyres & series 2 has 265/70/16. I own a series 1 & the tyres are due for replacement. Am I legally allowed to go one size bigger than those on the series 275/70/16, as the 2 vehicles are essentially identical, or can I only go one size bigger than those on my series 1 ie 265/70/16. The RTA website doesn't help very much

Ta !
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Reply By: Ants - Wednesday, Apr 20, 2005 at 19:37

Wednesday, Apr 20, 2005 at 19:37
You are legally allowed to go 15mm increase in diameter from the biggest size listed on the placard on the vehicle....

So, check to see what it listed on the placard or in the handbook as possible sizes.

You might find that they list all tyre sizes fitted to all models within that range...
AnswerID: 107582

Reply By: D-Jack - Wednesday, Apr 20, 2005 at 20:50

Wednesday, Apr 20, 2005 at 20:50
Saw this on another forum the other day, think it was courtesy of Jackablue (if it is the same one on this forum) courtesy of another author. I found it very informative, here tis.......thanks to the original poster on whatever original forum it was on. Hope you don't mind Jackablue.

The information presented below may not be acceptable to all state regulators. If you are unsure of how your state regulator may interpret their rules, contact them and obtain a response in writing. Keep it handy in case it is required by your insurer.

The information below assumes you will retain 16" rims, although this is not necessarily required by regulation.

We don't know all state regulations yet (pitch in if you can help with facts). However, NSW, QLD, VIC and SA all have a basic limitation on the overall diameter and width for replacement tyres.

Paraphrased, and discarding the reference to reducing sizes, the regulations go like this:

"The maximum increase in overall diameter (OD) that may be fitted is 15 mm greater than the largest wheel & tyre combination provided by the vehicle manufacturer as indicated on the tyre placard affixed to the vehicle.

The maximum increase in overall width that may be fitted is 26 mm greater than the largest wheel & tyre combination provided by the vehicle manufacturer as indicated on the tyre placard affixed to the vehicle."

The the standard (and only) wheel/tyre combo offered on the Aussie Pathy (to date) is indicated on the placard as a 245/70/16 (on 7jj rims). To covert any such tyre code into an OD in millimetres simply substitute the respective values into the following equation:

(Width mm x profile % x 2) + rim mm = OD
(245 x 0.70 x 2) + 406.4 = OD
343 + 406.4 = 749.4 mm OD

This equation provides what I refer to as the "nominal" OD of a tyre.

Therefore, the nominal OD of a 245/70/16 tyre is 749.4 mm. Add to this the allowed increase of 15 mm and the nominal maximum allowable OD is 764.4 mm. So what? What sized tyre can I fit?

Using these nominal sizes and converting back to a code, you may fit a 245/70/16 or a 255/70/16 or a 265/65/16. That's it.

However, ...

the regulations do not refer to "nominal sizes, nor to the European code width/profile/rim (245/70/16). The regulations do, however, refer to "Overall Diameter", which can only be stated in millimetres or inches, and a specific size increase of +/- 15 mm. I have presented the argument to NSW RTA that the ambiguous nature of the regs leaves only one approach - to use actual diameters measured in millimetres - and this was accepted by them.

So, now we can ignore nominal sizes and codes and look to actual OD data. This means you can fit any wheel/tyre combo, regardless of coding as long as the actual OD in millimetres/inches, as stated by the manufacturer, is within the allowable maximum OD. OK, great. So how do we work that out now?

Simple: add 15 mm to the largest 245/70/16 tyre available. I found one that was stated as being 755 mm so the new maximum allowable OD is 770 mm. But it gets better. Because we are now using actual sizes, not nominal, you will be able to find a 245/75/16 that is less than 770 mm (remember though, you can't have it both ways - you cannot fit any 245/75/16 - it must be less than 770 mm OD). So, now you can fit a 245/70/16, a 255/70/16, a 265/65/16 and some 245/75/16 tyres.

This is where the story definitely stops for SA, and perhaps also for VIC and QLD (although we have not tested the next part of the story in those two states).

For NSW, the story continues, and gets better again :happy:

I had my heart set on a set of Cooper ST 245/75/16 LT tyres. These measure-in at 775 mm and I wasn't going to take "no" for an answer. Following a lot more research, I discovered that manufactures have an allowable manufacturing variance in the product dimensions of +/- 3.0%. This variance forms part of the tyre manufacturers' specification guidelines and is also part of the Australia Standards Association guidelines. It also explains the large variance in actual OD of tyres sharing the same code. For example, a 245/70/16 tyre can actually measure between 739.1 mm and 759.7 mm OD.

I took this information to the RTA, and claimed that if a manufacture can make a 245/70/16 tyre that is 759.7 mm in OD, that is the number referred to in the regulations as "the largest" and I should be able to add 15 mm to it to get a maximum allowable OD of 774.7 mm (which I rounded to 775 mm, of course ). I received written (email) confirmation that I could apply this argument, and the associated maths, and I promptly purchased and fitted my Cooper STs. :happy:

So, according to the above experience, in NSW you may fit a 245/70/16, 255/70/16, 265/70/16 and any 245/75/16 tyre to your Pathy, as long as the stated OD does not exceed 775 mm.

Phew, I said I'd keep it brief.

Oh, and remember to ensure both load rating and speed rating regulations are adhered to also and note these differ between states.
AnswerID: 107593

Follow Up By: marklynn - Thursday, Apr 21, 2005 at 07:17

Thursday, Apr 21, 2005 at 07:17
Glad you kept the reply short & to the point...thanks for that....I wouldn't have wanted to read a long , drawn out response !!!!

FollowupID: 364526

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