Are the same size tyre the same size?

[Zed]

Well this has got me stumped, I seriously thought a particular size tyre no matter the brand would be the same rolling diameter


Well I found the same brand and the same size but different tyre to be different rolling diameter.
Check out these web sites the particular size I was comparing is in the LT construction 265/75/16.
The A/T3 states 803mm
The S/T Maxx 813mm
And the tyre calculator pickle has kindly posted in the FAQ section says 804mm

http://www.coopertires.com.au/index.php?page=tyres&tyre_id=14

http://www.coopertires.com.au/index.php?page=tyres&tyre_id=14

http://www.etyres.co.uk/tyre-size-calculator.htm

[dbomba]

They can vary quite a bit from brand to brand.
Daryl

[Zed]

Why though?
Sizing on a tyre should be

265 = width of tyre
75 = % of width
16 = rim size

Same principle for any size tyre or brand I'm lead to believe
The insurance companies, transport department and other goverment authorities stipulate and enforce not going over the OEM by X amount (most states i think are 25mm) so having 10mm difference in size can easily put you over or under. Make a good arguement if any one has every had insurance claims knocked back for tyre size or a traffic fine.

[cjmarsh81]

I do not understand how those tyre sizes can be different?

They are either not measuring from the same place or flat out lying about the size of the tyre.

I would have thought the ACCC would have cracked down on tyre manufacturers if the tyres were not in fact the size stamped on the side

[luke.k1983]

I think that it's because Cooper (and other brands) put more rubber on it thus creating deeper thread and lasting longer. It is still 265/75R16 but has those 5mm extra rubber on it.

[1993 NJ LWB]

I think that it's because Cooper (and other brands) put more rubber on it thus creating deeper thread and lasting longer. It is still 265/75R16 but has those 5mm extra rubber on it.

I think that this is basically it. They are the same size before tread goes on. Different brands and tread types different overall size.

[cjmarsh81]

If they are measuring the size of the tyre before the tread is added that is just crazy.
That means people's speedo's are going to be out depending on which brand of tyre they purchase.

This may be the case it just seems a little odd to me.

[Rendrag]

We're only talking 10mm here though, it's not really that much - that's less than the difference between new and bald, on my D694's!

[Merts]

If they are measuring the size of the tyre before the tread is added that is just crazy.
That means people's speedo's are going to be out depending on which brand of tyre they purchase.

If your 2007 VRX speedo is already out by as much as mine is, a few mm difference in the actual diameter of your tyres isn't going to cause any problems.

A tyre like a cooper STmaxx has a tread depth of about 14.5mm when it's new. Worn down to minimum legal depth the tread is 1.5mm. So, if the starting diameter of the new tyre is 813, it will reduce by 2x13 to 787 at the end of its life. The average diameter (half worn) would be 800mm (pretty close to the nominal calculated size of 804.

BTW, the maximum upsize in most states from original is 15mm not 25. Some places including Vic have adopted the latest national standard which allows 50mm increase.

[cjmarsh81]

True the speedo in my Paj is already out.

[Zed]

If they wern't so picky at booking somebody there wouldn't be an issue at all.
but 2+2=4 so why does 75% of 265 have 3 different anwsers?

[Merts]

If they wern't so picky at booking somebody there wouldn't be an issue at all.
but 2+2=4 so why does 75% of 265 have 3 different anwsers?

75% of 265 will always be the same answer. The issue is whether the tyre is manufactured to be the calculated diameter when it's new, half worn, or completely worn.

The difference is so small it makes no difference in normal car tyres with smallish tread depths. It only becomes relevant when you are dealing with tyres with very deep tread depth.

As far as I know, the allowable upsize is based on the calculated diameter (ie, rim diameter + width x profile% x 2). You aren't going to be booked or have insurance denied if the calculated diameter is within the allowable limits.

Also, effect on speedo readings will be less than 2 or 3% for the diameter differences you are talking about. Most cars read over by more than that anyway. (If you are going 100 the speedo will be reading 95 for example)

So, none of this actually matters!

[Zed]

75% of 265 will always be the same answer.

Aparently not, look at my first post and you will get 3 answers

[Merts]

Aparently not, look at my first post and you will get 3 answers

Did you even read either of my posts?

I've got to ask. Were you really interested in some feedback on your original post?
And seriously, giving yourself a 'clap' for that last comment?

[dean]

Not an issue at all.
Yes all companies may vary slightly but it's only a few mill.
Dean