convert tyre size to track tyre size
 

i am using 255/35/18 on the road and some tracks but how can you convert to a race tyre

I assume you mean something like slicks.

They are measured from top to bottom so you need to do this:

Calculate the diameter of the wheel - 18" = 457.20mm
Next, calculate the side wall of your current tire. This is a percentage of the width = 255 * 35% = 89.25.
Finally, you need to double the above as you have a side wall on each side.

So, it works out to be:

18" = 457.20mm
255 * 35% * 2 = 178.50
Total = 635.70

so a 250/640/18 would be a good match then cheers mike