4 replies to this topic

[01smartc]

Hi all, quick question.

How much smaller in diameter would a 205/20-13 tyre be than a 205/50-13?

I.e if you layed them ontop of each other how much smaller would it be?

Chris

[Ethel]

the /** number is the height of the sidewall as a percentage of the width so a 205/50 is 102.5mm. Can you get 20 series tyres

[01smartc]

yer you can get 20. Its for racing slicks you see.

Anyone know the diameter of some 205/20-13 slicks? Or whee i can find out?

Chris

[Jupitus]

Ever been confuzzled by the numbers on the wall of the tyre? I was for ages, but got it figured out, so here is a little walkthrough!

Tyre sizes are referred to with 3 main measurements;

1) The width of the tyre (in mm)

2) The diameter of the tyre's inner circle, i.e. the dimeter of the wheel it will fit on (in inches!)

and

3) The ratio of the width of the tyre to the height of the sidewall, i.e. inner diameter - outer diameter difference.

For example, my tyres are 175/50 R13.

This means they are 175 mm wide. The sidewall is 50% of 175 mm, i.e. 175/2= 87.5mm high. The inner diameter of the tyre is 13 inches.

So, when dealing with wheel sizes it can be useful to know what the actual size of the wheel and tyre will be combined... lets compare my wheels and tyres with some 12" wheels...

My tyres and wheels:

I have 13"x7" Wolfrace slotmags.

The tyres are as above, so
175mm is 175/25.4 = 6.89 inches width, i.e. close to 7"".

To calculate overall diameter I use the following formula:

Inner diameter + 2*((widthmm*(ratio/100))/25.4)

i.e. 13 + 2*((175*(50/100))/25.4)

or 13 + 2*((175*0.5)/25.4)

or 13+ 2*(3.44)

= 19.88 inches.

If I wanted 12 x 5 inch alloys, I would need to fit tyres such as 145/70 R12. This gives a size of:

12+2*((145*(70/100))/25.4)

= 19.99 inches.

So although the wheel diameter is smaller, with the available tyres I end up with a bigger overall diameter!

[pantera2075]

There are two points to bear in mind here:

1. The actual size of the tyre when fitted to the car - this matters if you have problems with them rubbing the arches etc.
2. The rolling radius. Different tyres of the same nominal size will have differing rolling radius (affecting how far the car moves for every turn of the wheel) depending on the construction of the tyre, tread pattern, pressure etc. This is important for gearing and speedo calibration etc. The tyre manufacturer should be able to give you this info.