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Truck Tyre Sizing
Truck Sizing (or Classic Sizing) is the oldest type of tyre sizing and goes back to the early 1900s when Pneaumatic tyres started to become more common. It was replaced with Metric and Imperial sizing designations over the last 50 years as sizes needed to be more specific. It is still common to see 11R22.5s, 9.5R17.5 and 7.00R16s on trucks and older vehicles, we keep a large range of this tyre check out what sizes are currently in stock by heading over to our Online Store.
Tyre Width
As shown in RED above - this is the width in inches of the tyre (the section that has the Tread Pattern on it). This will need to be multiplied by 2.54 when calculating the Metric Width (which all Australian Laws are based upon)
Also shown in RED above - This is the Sidewall height in inches. This will need to be multiplied by 2.54 when calculating the Metric Sidewall Height and multplied by 2 to get the Overall Diameter
As shown in GREEN above - This designates the Construction Type of the tyre, Radial are a requirement on msot public roads for passenger or freight vehicles, however certain exemptions may apply to classic, ex-military or special vehicles; these vehicles are commonly fitted with a Nylon cross-ply tyre (sometimes known as RAG Tyres, Nylons, or Bias Ply Tyres - these tyres feature a dash (-) where the 'R' is on the example above. Cross Ply Tyres and Radials have similar construction outside of the base materials. Most Radial Tyres also have Nylon and Polyester Plies in between the Steel Mesh that creates the Radial designation. In some cases the Steel Plies are also reinforced with Kevlar or Carbon Fibre rather than Steel.
Rim Size
As shown in YELLOW above - This shows the Rim Size; amazingly tyre sizes mix both Metric and Imperial measurements (while Full Metrics were available to the Public for a short while in the early 2000s on Europeans Cars and are used in Racing too; they are quite rare in today's market) The Rim size is in Inches above, so to calculate the mertric overall diameter of your tyre you must also convert the inches to millimetres. The Rim is the part in the centre of the tyre for the novices, and the tyre and Rim are two seperate parts.
Load Rating
AS shown in ORANGE above - The Load Rating is a one, two or three digit number that gives the amount of load the tyre can be subjected to safely in normal road use. It is worth noting that this load only applies at a specific pressure, so as a general rule it is more important to know the load rating at the specific pressure stated on the tyre if you are looking at carrying heavy loads. It is usually at the maximum pressure on the tyre that the load rating is effective. So if you are looking at carrying a large load, bump up the pressure to Maximum or a couple of PSI under - You can also Click Here for more information on Load Ratings and to find out why the above example has two load ratings on it rather than one.
Speed Rating
As shown in PURPLE above - The Speed Rating is a Letter between E and Y which designates the maximum speed the tyre can safely be used at, these are genrally quite a bit higher than the speed limits, so unless you are planning on using the tyre on a track, it probably isnt worth worrying about too much. If you want to look it up to be on the safe side - Click Here for our Speed Rating Information page.
Other Helpful Tips
When Calculating the Overall Diameter it is worth also accounting for the Vehicle Weight and the difference between beads and where they are seated. (The Bead is the part that is in contact vertically with the rim when it is mounted) After far too many years in the tyre industry I have found that the Golden Weight Factor is 0.98 for cars and light vehicles and 0.97 for trucks. Tyres may contain steel but it is not their only component, and rubber (or silica in most current cases) bends and moves on the rim in motion. Due to a poor understanding of this concept many try to calculate overall diameter as a static number, this is even more blurred once your tyres start to wear down, most 31x10.5R15s as shown above come with between 8-15mm of tread as this wears down the Overall Diameter will decrease by a further 16-30mm when both sides are accounted for. Its due to this very nature that all the details mentioned here are a guide only and you may get different results between brands and tyre/rim combinations.
The Math
How to Calculate Overall Diameter on Metric Tyre Sizes..
(((Sidewall Height * 2) + Rim Size) * Inch to Metric Conversion) * Bead width and Vehicle Weight Allowance.
Or in the case above..
((( 7.00 * 2) + 16) * 2.54) * 0.98
Just in case you didn't follow the math, Below is a link to our Tyre Calculator which might help - Try out some sizes and see where it takes you.

Tyre Calculator