Tyre Plus Sizing & O.D. Calculator.

Calculation comparing tyre overall rolling diameter for Radial Passenger, SUV, MPV, Light Truck

Please enter the width, aspect ratio and rim size for your current tyre and proposed new tyre.

Why? It is important to ensure that the speed rating and load index of the new tyre/s are the same or higher than the existing O.E.M tyre/s. This will ensure that the individual carrying capacity x 4 (load index) of each tyre (at the correct cold pressure inflation) is sufficient enough to handle the G.V.M of the vehicle (+ passenger load). The speed rating must be sufficient to legally handle the engine capacity & speed attainable by the vehicle. Failure to heed these facts may and can lead to premature tyre failure.

Why? Should the tyre width exceed the width of the rim width, three major faults will occur. The first is the tread of the tyre will distort to the extent that the tread will not be able to make proper road contact across the tread area. The second visible fault are the side walls of the tyres will not "stand straight" and will lean in from the shoulder of the tyre relative to the bead flange, which seals the tyre to the rim. The third problem being the bead of the tyre will not seat or seal itself correctly onto the rim and this can cause "bead rock" or chaffing of the bead of the tyre, against the rim flange. Sudden air pressure loss may also be experienced. (Should the tyre width be narrower than the rim width, the reverse of the above will occur). Premature tyre failure will occur.

WHEN FITTING DIFFERENT TYRE SIZES TO SPECIFICATION, CONSULT YOUR VEHICLE SPECS WITH RESPECT TO ON-BOARD COMPUTER (E.M.S)

Why? Many newer vehicles have engine management systems installed. These E.M.S will often have been programmed to monitor tyre overall diameters and will pick up the differential if the new tyre/s exceed specifications. Often, the handling of the vehicle can be disrupted or compromised, and the E.M.S could shut down the engine, perceiving that a problem exists. Prior to fitting the larger tyres, please consult with your local vehicle agent in this regard, should you have an E.M.S on board.

Why? On fitting new over-sized tyres & possibly new rims, it is highly advisable to ensure that there is sufficient clearance between the front inner suspension and new tyre/rim assembly, as well as the outer bodywork/wheel arch. Before dropping vehicle onto the road surface and whilst the vehicle is still on the wheel lift, spin the front wheels to ensure that the inner side of the new rim does not catch on the brake calipers. Remove the vehicle from the lift and turn the steering wheel full lock, left and right, ensuring full turning clearance. Load a heavy weight (a few persons) into the rear of the vehicle to ensure that the new wheel/rim assembly does not catch on the rear wheel arch. Failure to conduct these tests could result in badly damaged bodywork and severely damaged tyres and or rims.

South African tyre markings are based on the European Tyre and Rim Technical Organisation (E.T.R.T.O.) markings.

Size markings

The size markings of a tyre denote the physical dimensions of the tyre and indicate the configuration or shape of the tyre casing.

185/70 R14 82T. The significance of each number and letter is as follows:-

• 185 mm in width
• /70 = Profile - percentage of width. In tyres where the aspect ratio is not shown the profile = 82%.
• R = Radial.
• 14 = Rim diameter in inches.
• 82 = Load Index per tyre at the speed rating.
• T = Speed Rating

Some off-road tyres use the above type size markings while others still use the older American markings, for example- 31/10.50 R15 LT. The significance of these markings are as follows:-

• 31 = The outside diameter of the tyre in inches.
• 10.50 = The section width of the tyre, in inches.
• R = Radial Construction.
• 15 = Rim diameter in inches.
• LT = Light Truck.

A common problem facing off-roaders is interchanging between these two size systems without interfering with the outside diameter where a variation of only 2-3% is acceptable.

Example

A common example would be changing from a 265/75 R15 LT to a 31/10.50R 15 LT: Determining the outside diameter of the 31/10.50 R15 LT is easy; convert 31 inches into millimeters. (31 inches = 787.4 millimeters.)

The outside diameter of the 265/75R 15 LT is calculated as follows:

• The section height of the two sidewalls added to the diameter of the rim.
• By using the same "inches to millimeters" conversion for the rim diameter we find that 15 (inches) x 25.4mm = 381mm.
• The sidewall heights are:- 265mm
• Section width x 75% (the aspect ratio) x (two sidewalls):
  • 265 x 75% = 198.75
  • 198.75mm x 2 = 397.50mm
  • Rim Diameter + 381.00mm
  • Outside Diameter = 778.50mm

The difference in diameter between the two tyres is 8.9mm or 1.13% which is acceptable.