cooper's / bfg's

Paul ,
Forgive me, 'cause I'm dumb as dog's hit, and really know nothing of tyre diameters.
Jeff H

Paul C - "265/75/16 won't necessarily have the same OD". You will have to explain that one too me as I was always led to believe that a tyre size is a tyre size? Should they not be all the same rolling circumference? Cheers Tony

I always thought the same until reseaching for new muddies last year. There are various differences between maufacturers.
See below.

Cheers
Paul

Glenn - So in Queensland you can have the same size tyre but drive on illegal tires? Because by what you have posted above puts one of the same size tires 21mm larger and outside the allowed 15mm.

Hope someone can explain this logically as I always thought the Maths for a 265/75/16 should always be the same! Otherwise how do you keep it leagal?

Cheers Tony

Glenn,

The Tyre and Rim Manual specifies a 265/75R16 to have a nominal diameter of 804mm. I agree that various brands and patterns will vary up to say 10mm, but I am sure your BFG was a 265/70R16 which has a nominal 778mm, and may well be larger in the BFG case due to fairly deep tread.

Tony B, you are correct that at present and until Qld adopts the ew National Code of Practice (NCOP) the largest tyre will be 15mm larger OD than the largest optionalfitting for a make or series. In the case of most Landcruiser and Patrols which had an 7.50R16 on the base models (covers the make or series part), you can go up to 825mm as the 7.50R16 is 810mm in diameter. In the case of LC200, the standard tyre is 803 plus 15 = 818mm.

When the NCOP is adopted in Qld, scheduled for July 1 according to QT, then upsizes by up to 50mm will be allowed without certification.

Bear in mind that you can get an upsize certified by an engineer along with a lift provided it meets the bumps stop provisions... but that's another thread...

Cheers,
John

Guys, don't shoot the messenger, I'm just quoting the info these companies put out themselves.
I don't know the ins and outs of all this stuff either.

Here's the links I quoted from:

BFG

Coopers

If BFG have made a mistake, take it up with them.


Cheers
Glenn

Not having a go at you Glenn... thanks for putting in the links as this gives an opportunity to clear the misunderstanding up.

Rolling circumference is different than unloaded circumference. Rolling circumference measures the distance travelled by one rotation of the tyre when inflated to a nominal pressure which will be lower than the unloaded circumference by a significant percentage. There is no standard for measuring rolling circumference as this will vary substantially according to load and pressure so is best ignored. Overall diameter (OD) is commonly quoted by tyre manufacturers and best used for comparative purposes such as the OP was looking at.

In my view (I am involved in the tyre industry), the variations between different brands and patterns are not significant at normal pressures however, excepting in full time 4WD systems. The variation is not so important off road or on gravel for any 4WD due to the loose surface offsetting the marginally different OD's and on road, part time systems can be disengaged. But and in any case, 4 like tyres is the safest and best solution.

For the record, the BFG site you linked to, also showed loaded radius (in mm not inches as indicated). Loaded radius is the distance measured from the wheel centre to the ground for a properly inflated tyre at any load. In other words, if under any load, you should inflate the tyres so that the radius measured from the wheel centre to the ground equals the tyre's published loaded radius for normal running. This is one of the best methods of determining the correct pressure under any given load.

Hope this helps clear up the confusion.

John