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Finding Your Tyre Size


HOW TO READ THE INFORMATION ON YOUR TYRE SIDEWALL

TYRE WIDTH/ SECTION WIDTH: Expressed in mm, this is the width of the tyre from shoulder to shoulder. In some cases it is possible to improve grip by fitment of a wider tyre, however, there are limitations such as the rim width and also suspension and body clearance.

ASPECT RATIO/ PROFILE: The profile or aspect ratio is expressed as a percentage of the section width, a 165 section width tyre with a 65 profile will have a shallower sidewall than a 205 width tyre with the same profile ratio.

R: Most modern tyres have an R designation after the aspect ratio, this denotes a RADIAL construction.

RIM DIAMETER: This is the wheel size expressed in inches. It may be possible to change the rim size on a vehicle by adapting the section width and aspect ratio to maintain as close a match to the loaded radius, this is the distance covered by one complete rotaion of the wheel under the weight of the car.

LOAD INDEX: The load index references a maximum permitted load for the tyre. Some vehicles may have restrictions noted on the tyre detail sticker on the door shut or fuel flap stating "THE TYRES FITTED TO THIS VEHICLE SHALL HAVE A LOAD INDEX NO LOWER THAN xx). This is as important to correct fitment as the width and aspect ratio.

SPEED INDEX/ SPEED RATING: A tyres capacity to resist heat build up at speed is expressed as a letter, With the exception of H rated tyres which fall between T and V rated tyres, as a general rule the further up the alphabet, the higher the speed rating. It is possible to upgrade a tyres speed rating to a higher specification but reducing the speed rating should not be done. An exception to this may be in the fitment of winter tyres where driving styles should be adapted and speeds lowered due to the reduced grip on the roads.

ADDITIONAL DETAILS:

8PR and 6PR tyres: Certain small commercial vehicle tyres will have 8 plies or 6 plies meaning they are suitable for commercial vehicles due to their increased weight bearing capacity for the equivalent tyres size.

C tyres: Another way of expressing a commercial tyre is with the use of a letter C.

Dual load index: Some vehicles have a twin wheel setup on the same corner, where a tyre has two load indexes the higher number is for single tyre fiment, the lower is for twin tyre fitment, bearing in mind this is the load per tyre so the overall load capacity is just below double.

RUN FLAT/ EMT/ RUNONFLAT/ DSST/ SSR Tyres: It has become OE fitment on some vehicles for run flat tyres to be equipped. The absence of a spare wheel allows for a better luggage capacity although in some cases ride quality is sacrificed. It may therefore be desirable to retofit standard tyres however a spare wheel and jack/ brace should also be considered when converting the tyres in this way.

DIRECTIONAL TYRES: Tyres may have an arrow of rotation on the sidewall, this denotes a directional tread pattern. Usually a V shaped tread design these tyres once fitted to the rim become handed and will only give advantage when fitted to the side of the vehicle it was fitted for. It is not advised to run directional and non directional tyres on the same axle. These tyres generally perform well in wet weather tests due to the treads ability to expell water from the centre of the contact patch to the outside of the tread area.

ASYMMETRIC TYRES: Some tyres will have the words inside and outside printed on their sidewalls. This denotes an asymmetric tyre where once fitted to a wheel can be swapped across the axle if desired. Asymmetric tyres often perform well in noise tests as they can allow trapped air (the cause of road noise) to evacuate to the inner wheel area and under the vehicle where other noises such as the exhaust and drivetrain are located thus not creating external noise that could be heard through the windows of the vehicle.

 

Any questions you may have about tyres and their fitment, our trained and experience team will be happy to help.

Call 01789 774444 or email info@paramount-performance.com

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