An explanation of tyre sizes, please?

[dave.j.robbo]

This might sound like a silly question, but....

I know how 'standard' tyres are sized, eg 215/50/17

How are tyres marked 30 9.5 15 measured?

Now, I know it's not rocket science, and an assumption would be the 30 is overall diameter (in inches), the 9.5 would be the tyre width, and the 15 is rim size.

BUT, if I measure my tyres, with the above size, it isn't 30 inches tall, it's only 28(and a little bit)...


I've also measured a 31 10.5 15, and that was just under 30 inches,,

Mmmmm

What's going on?!?

Dave

[BUSHWHACKER]

I think the overall diameter depends on what kind of tread you have.
Have a look here, it may explain.....


http://www.1010tires.com/TireSizeCalculator.asp?

[dave.j.robbo]

That site says my tyres have a diameter of 29.99 inches.

Where are my two inches?!?!

(Hey, every little helps......)

[BUSHWHACKER]

There must be a chill in the air!



P S. I'll be up in your neck of the woods on the weekend, we're going to a place called 'MiniMail' a Mini spares place in Kilcot, near Newent.
Any places we could try out our new truck tent around there?

[dave.j.robbo]

There's some lovely forestry commission campsites in and around Christchurch, in the Forest of Dean. Not far from Newent. I'll have a dig around and see if I ca find the numbers...

[Gamedawn]

The tyre size only works when the tyre is on the right width rim, there is an R in the month and the moon is blue with sort of pinky tinges round the edge . According to BFG my 32's are supposed to be 31.7" dia when fitted to a 9" rim - on my 8" rims they measure 31.3" tall (I know 'cos I just measured one for a wheel cover). I would have thought a narrower rim would have made the tyre taller but that just shows what a fool I am

[sumo]

There is n article on tyres in this months (august) issue of 4x4 mart and all is explained in simple turms,if I can understand it ANYONE can.

[Gizmo]

That site says my tyres have a diameter of 29.99 inches.

Where are my two inches?!?!

(Hey, every little helps......)

Possibly on the road LOL

[TheWolfCub]

Maybe it's all down to inflationary pressures....seems to cause the country a lot of issues!

[Southern Surfer]

This might sound like a silly question, but....

I know how 'standard' tyres are sized, eg 215/50/17

How are tyres marked 30 9.5 15 measured?

Now, I know it's not rocket science, and an assumption would be the 30 is overall diameter (in inches), the 9.5 would be the tyre width, and the 15 is rim size.

BUT, if I measure my tyres, with the above size, it isn't 30 inches tall, it's only 28(and a little bit)...


I've also measured a 31 10.5 15, and that was just under 30 inches,,

Mmmmm

What's going on?!?

Dave

Try this:

265/70/15
265 = the tyre width in mm, or 26.5 cm
70 = profile, which is 70 % of the width, i.e. 185.5mm, which is the depth of the tyre from tread to rim.
15 = rim diameter

31/10.5/15 is what is called a 'millimetric' size and is american.
31 = the tyre width in cm
10.5 = profile, which is the depth from tread to rim in inches
15 = rim diameter

The 31/10.5/15 is therefore the size to go for if you want the biggest footprint.

Just replaced my tyres with bridgestones latest A/T - D694, which look the biz, so far untried as i only picked them up last night.

The boys at Black Circles told me this, so if it doesn't add up.............

As soon as i have some pics, i will post an add for a set of 4 shiney 8x15 alloys with good 32/11.5/15 tyres if anyone is interested. Lovely wheels but they stick out about an inch so really only for off roading officer.

hope this helps.

Gaz

[da SLUG man]

Try this:

265/70/15
265 = the tyre width in mm, or 26.5 cm
70 = profile, which is 70 % of the width, i.e. 185.5mm, which is the depth of the tyre from tread to rim.
15 = rim diameter

31/10.5/15 is what is called a 'millimetric' size and is american.
31 = the tyre width in cm
10.5 = profile, which is the depth from tread to rim in inches
15 = rim diameter

The 31/10.5/15 is therefore the size to go for if you want the biggest footprint.

Just replaced my tyres with bridgestones latest A/T - D694, which look the biz, so far untried as i only picked them up last night.

The boys at Black Circles told me this, so if it doesn't add up.............

As soon as i have some pics, i will post an add for a set of 4 shiney 8x15 alloys with good 32/11.5/15 tyres if anyone is interested. Lovely wheels but they stick out about an inch so really only for off roading officer.

hope this helps.

Gaz

no,
31-10.5-15s would be 31"diameter overall - 10.5" tread width - 15" rim


mine are 33-11.5-16
so, 33 inches tall - 11.5 inches wide - on a 16 inch rim

[openback]

no,
31-10.5-15s would be 31"diameter overall - 10.5" tread width - 15" rim


mine are 33-11.5-16
so, 33 inches tall - 11.5 inches wide - on a 16 inch rim

thats what i've allways thought! was getting a bit worried, sounded so certain with millimetric name.

[dave.j.robbo]

So why are my 31 9.50 15's not 31 inches tall?!?!

[da SLUG man]

So why are my 31 9.50 15's not 31 inches tall?!?!

how tall are they??

my 31s were just under, but the 33s are spot on

[da SLUG man]

http://www.miata.net/garage/tirecalc.html

best calculator i've found... shows the difference with a little piccy!