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Imperial Tyre Sizing
Imperial Sizing is far more common in America than other countries. It is by far the easiest tyre size to wrap your head around as it is measured the same as if it were a square. In many ways it would have been good if Europe had of adopted a similar way to size tyres when it moved from Classic Sizing to Metric Sizing, however, they had to go and mix metric, imperial and percentages into tyre sizes. I think the Americans definately had a better system here. We currently stock a handful of Imperial Sizes which can be viewed on our Online Store
Tyre Height
As shown in BLUE above - The Tyre height is the overall height of the tyre including the rim cut out and the sections that are visible of the tyre that goes aroud the rim or the black band that can be seen on all cars when driving. When calculating overall diameter this needs to be multiplied by 2.54 and then vehicle weight needs to be factored in. In this case it will be 78.74cm * 0.98.
As shown in GREY above - this is the divider, in Imperial Sizing, the divider reflects older measurement standards, there is not fancy maths here, it is just to show that it is measures the same as a square. In this case it reflects the 'by' in the following statement '31 inches by 10.5 inches'.
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) In this case it is 10.5 inches or 26.67cm.
As shown in GREEN above - This designates the Construction Type of the tyre, Radial are a requirement on most 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; In this case the rim size makes no different to overall diameter . The Rim size is in Inches. 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.
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 Speed Rating Charts and a Speed Ratign Calculator.
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 9-11mm of tread as this wears down the Overall Diameter will decrease by a further 18-22mm 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..
(Tyre Height * Inch to Metric Conversion) * Bead width and Vehicle Weight Allowance.
Or in the case above..
(31 * 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