Tyre sizes

Rohan,

Its Bob at the moment

Rohan and Baz - thanks very much for the prompt reply - at least it makes the decision easy !

Bob

Bob, if you can find a suitable (common/popular) larger size that is still legal, I'd go with it. While the speedo reading will change (by about 3%) most vehicles understate the actual speed by a few % anyway and if uncertain, you just drive at 107 kph intstead of 110.

The little bit of extra lift and the larger footprint you get are worth going up in size if you can.

Flappan,

Thanks - Coopers also have this size - they say 806 mm. This gives me back the decision to make just when I was feeling comfortable and lazy.

I don't really understand the measurements - what does the 265 and 275 refer to, I assume the 70 and 75 are 7" and 7.5" rims sizes - i.e. width - is this right ? Where do I get the extra size from ?

Bob the 265 and 275 refer to the width of the tyre 265mm or 26.5cm etc. The 70 and 75 refer to the aspect ratio, or the percentage of the width that the tyre wall is high. For example 265/70 are 26.5 cm wide and the sidewall is 70% of this all the way around. When you add the wheel size (16 inches) you add this to the 70% of 26.5 x 2 (top and bottom sidewall) and you will get the height/diameter of the tyre. Just remember that the aspect ratio is only between the wheel and the outside edge of the tyre so you have to add it twice.

Hope I didn't cofuse you more with my wording.

Charlie

Thanks Charlie - so slightly narrower and slightly taller at 265/75R16 - so is this a good idea or not ?

Bob I went to 305/70 on my 80 series and noticed a slight drop in power from take off. Mine is a td so a petro; might not notice it as much. My tyres are apparently illegal now but at the time in Qld it was allowable to go 50mm taller than standard. Mine are the same height as 285/75's just to give you an idea of what changing the percentages does. On sand these tyres are noticable better infact they grip better in all situations including on road wet/dry. I got the Cooper ST's because of the waranty and had BFG's in the past and was happy with them too but had the pre KO type and heard too many stories of the All terrain KO's falling apart.

Going higher helped my 4wding and have a 3inch lift. The choice is still up to you. I don't think I would ever go narrower but......

Charlie

Bob, slightly narrower gives a slightly smaller contact area. Slightly taller gives a slightly larger contact area. Slightly narrower but slightly taller probably gives you about the same contact area only a longer, narrower one.

I'm a firm believer that off-road, a longer foot-print is better that a short, wide one. Unfortunately for you, there are others that take the oposing view.

Check out this site. It's the best I've seen on explaining (almost) all things to do with tyres.

car bibles

Hey great site - so 275 etc is the width, 75 etc is the diameter - so is there an advantage in a slightly narrower, larger tyre or do I just stay with the stock tyre size ?

Nope - just looked again and it is not that simple - now I really am confused - HELP !!

Look at my follow up above

Bob have been down that track myself and the upshot of it is that you can go to a 265/75 that will be 11mm taller you are alowwed 15mm or you can just put on the 285/75 and take the chance if you go the 285/75 then you will not be legal,your speedo will be out,your low range will not be as low but you will get some good stuff as well which is why i supose you are looking at the bigger tires, me i am still with the the 275/70 at the moment but as soon as the patrol gets back for NZ on go the 285/75 for the Aussi trip because the cooper st c is a 10 ply tire and is rate abour 1500kg per tire.
Regards

Bob check out the coopers st c its a new tire the st and stt are prone to chipping the st c have fixed that

10 Para, the ST-C has attempted to fix that issue but it does so via a softer compound that will wear somewhat faster. The ST-C is principally designed for users that spend 80% of their life on dirt roads. For those that spend that 80% on tar, it would not be a good choice.

BTW, I've had my STs for 10,000 kms, half of which has been off-road, and not a chip or crack yet. (fingers are crossed).

Bob, go with the same size you have now 275/70/16 in the BFG/ATs then there will be no hassle's old one's off, new one's on, and your off on your trip, easy !!!

Baz.

Hi Bob,
A friend of a friend is running them at moment, the feed back is the chipping issue seems fixed, but they are not now wearing well from mileage point of view, so they may have fixed the chipping by making them softer?
Regards
Joe

I don't think I have enough experience in this area to be a very good guinea pig so it looks like the BFG's for us - thanks fellas

Bob, get the 75 profile BFG - 275/75/16.

Rohan,
Thanks - that car bibles site is another good one - unfortunately BFG does not appear to make a 275/75/16 and according to my calculations (which I can now sort of do) it would be just above the legal limit - I am starting to feel really conservative about this issue as Baz suggested and will probably just put the same size back on - maybe I will get more adventurous with a bit more experience. We have another couple of months before we have to make a final decision so I will keep an eye on the posts - thanks for the help - I appreciate it.

Bob, a 275/75/16 is only 1 mm larger than allowed and you'd be bound to find one that is less than your 818 mm limit.

Any 265/75/16 will be well within the allowable limits.

I'd still be inclined to go with a 265/75/16, in any of the brands/models mentioned above. One of the reason the Cooper ST is so popular (and the reason I chose them) is that they fit somewhere between an AT and an MT.

You can read a lot more about these and other tyres over on the OL forum too.

John,

My research indicates that most states (certainly my state - SA) have referenced their motor vehicle standards Acts to adopt the National Australian Design Rules. These indicate that the outer diameter of any replacement wheel/tyre combo must not exceed the outer diameter of the largest listed wheel/tyre combo on the wheel/tyre placard affixed to the vehicle, by 15mm. I have heard that it used to be 3% but that now it is just a flat 15mm.

The example is my Prado where it is difficult because of the limited availability of 17" tyres for off-road use. The tyre placard says 265/65R17. Mathematically the outer diameter (OD) of this size tyre is ((256mm x 0.65) x2)+(17" x 25.4) = 776.3mm. The recommeded Cooper replacement is A/T's in 265/70R17. Using the maths the OD for these is 802.8mm but Coopers brochure lists them as actually being 31.4" (797.6mm). This is still an increase of 21.3mm which would be technically illegal.

In the face of this I asked my tyre dealer what the story was and he said he would have to check as he was not sure but he had fitted quite a few of these and had no problems. Maybe he actually knew but did not want to lose the potential sale????

My Toyota dealer also showed me another Prado ready to be delivered to a customer where the dealer had done the fitting of the Cooper A/Ts as a pre delivery option. Again I asked "is this legal" and the response was "as far as we know, yes".

Go figure!

As to your last question, tyres are considered to be at the end of their working life at a tread depth of 1.6mm.

Steven

Steven, thanks for your comprehensive reply. The 15mm is often quoted but as to the height or axle height or diameter there is usually a missunderstanding I think.

The tread depth is always of course for water evacuation and in some circumstances like in some muds and clays the Coopers would be too shallow at that tread depth of 1.6mm one would imagine.