READING A RADIAL TYRE
P = Tyre Type (Passenger)
215 = Section width in millimetres
65 = Aspect Ratio
R = Radial Construction
15 = Rim diameter in inches
95 = Load index (see chart)
S =Speed rating ( see Chart)
READING A CROSS PLY TYRE
525/550 = Section width in inches
17 = Rim diameter in inches
4 PLY = Ply Rating
90 = Load index (see chart)
P =Speed rating ( see Chart)
TYRE TYPE P= Passenger car , LT= Light Truck
Is the measurement from sidewall to sidewall, at its widest point.
On Radial Tyres this is always a single measurement
On Vintage X ply tyres this can have multipliable sizes.
ASPECT RATIO/SERIES NUMBER
The height and width relationship of the tyre. This number is determined by dividing a tyres section height by its section width. For example: the section height of 205/70R14 is 70% of the section width which is 205.RIM DIAMETER The diameter of the rim at the bead seat. The most accurate method to determine the diameter is to measure the circumference of the rim at the bead seat and divide by 3.14LOAD INDEXThe load index is an assigned number ranging from 0 to 279 that corresponds with the load carrying capacity of the tyre. Most car passenger car.
For MOT requirements in the UK, it is not necessary to fit a tyre with a speed rating to match the original vehicle fitments or the maximum speed of the vehicle. Tyres must be suitable for the purpose for which the vehicle will be used.Insurance companies might say that a vehicle should be fitted with the tyres having the speed rating recommended by the car manufacturer: but even if you fit Z-rated tyres for speeds over 150 mph they will not insure you if you drive in excess of 70 mph in this country!
The vehicle manufacturer has to recommend a tyre with a rating to match the maximum speed of the car because when it leaves the factory he does not know exactly where the car is going or for what purpose it might be used. You might want to use the car in a country where higher speed limits apply than the UK or take it to a race track at the weekend.
The tyre speed rating (i.e. S) is the maximum speed for which the tyre is rated. For example, the S rating identifies speeds up to 112 mph
(1) Tyres marked "VR" within the size designation ( 235/60VR15) are designed for speeds in excess of 130 MPH
(2) Tyres marked "ZR" within the size designation ( 235/60ZR15) are designed for speeds in excess of 149 MPH
Tyres marked with a speed index ( 235/60ZR15 98Y ) use the above chart Y would be up to 186 MPH
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