Article description: by Enderwiggenau
Category: Brakes, Wheels and Tyres
Date added: Aug 11 2008, 06:55 PM
Article starter: Joshua
One of the things many people find difficult to get a handle on are those
pesky numerical codes on the sides of our tyres. What's it all mean, and why
does it matter??
Generally speaking, a cars speedometer is calibrated to a particular rolling
diameter of tyre. So if you have the wrong diameter tyre your speedo could be
reading higher than the actual speed you are travelling, or worse, reading
Here is what I have on the front of my LX:
205/65R13 84 S
'205' - tyre section width, from edge to edge on special test rim.
'65' - profile, series or aspect ratio.
This is - (sec. height/sec. width)x100,
where the sec height=(tyre diam - rim diam)/2.
'R' - Radial construction, as opposed to 'C' - cross ply.
'13' - diameter of rim (in inches) that tyre can be fitted to.
'84' - load index, based on the weight the tyre can carry.
'S' - Speed rating of tyre.
Changing Rims Size
A 195/55x14 tyre is approximately equivalent in rolling diameter to:-
195/50x15, or 195/45x16.
When changing to a wider section tyre, the profile also needs to be reduced.
205/65x13 -> 225/60x13.
Tyre Speed Ratings
Tyre speed ratings are a safety factor based on the highest speed acceptable
for use over sustained periods of that particular tyre design/compound.
R - 170km/h
S - 180
T - 190
U - 200
H - 210
V - 240
W - 270
Y - 300
Z - over 240km/h, generally a road/racing tyre.