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Metric Tyre Sizing
Metric Sizing is the most difficult to understand in the three main sizing types as it mixes both Metric and Imperial calues and puts a percentage of the width in to determine the profile just to really confuse people, it has been adopted as it allows for the largest numebr of variations across tyre sizing. Read more below to figure out how it works or visit our Online Store Here.
Tyre Width
As shown in RED above - this is the width in millimetres of the tyre (the section that has the Tread Pattern on it).
As shown in GREY above - this is the divider, although it should really be called a multiplier becuase if you look at standard symbols that reflect mathmatics (especially in computers), you would expect that the slash would mean to divide, however in metric tyre sizing it means to multiply by a given percentage that follows in the case shown above this is 45% or 0.45.
As shown in BLUE above - The Tyre profile is 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. The profile is the height of the black visible band or the side view of the tyre. IF you are calculating the overall diameter of a tyre you must multiply this measurement (in this case 105.75mm) by 2.
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.
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 235/45R17s as shown above come with between 6-8mm of tread as this wears down the Overall Diameter will decrease by a further 12-16mm when both sides are accounted for. Its due to thsi very nature that all this is 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 Width * Profile ) * 2) + Rim Size * Inch to Metric Conversion) * Bead width and Vehicle Weight Allowance.
Or in the case above..
(((235 * 0.45 ) * 2) + (17 * 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