TYRE & INNER TUBE SIZES EXPLAINED

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Copying and reproduction is strictly forbidden.

Tyre Sizes & Markings Explained... 
There are several methods for giving the size of tyres, with measurements sometimes in inches, sometimes in millimetres, sometimes in both! To help you understand a little better, we have examples and explanations of each method below, from which you should hopefully be able to recognise your own size markings.

It is important not to mix tyre sizes or types on motorised machinery as there can be significant variation in heights and widths between different manufacturers. We have even found that sometimes tyres of exactly the same size marking can vary significantly. As with car tyres, we would suggest you replace your tyres in pairs to ensure optimum performance.

When considering rim diameter, please be aware that it is effectively the diameter of the hole in the middle of the tyre. The measurement from edge to edge of the wheel itself includes the tyre retaining rim flanges, and so will be about 1" greater than the actual marked size.

eg 3.00-4,  3.50-6,  4.00-8,  5.20-10,  6.00-12
This is the most simple size marking. The first part, 3.00, is simply both the width of the tyre and the height of the sidewall of the tyre in inches. The second part, -4, is the rim diameter in inches. So a 3.00-4 tyre would fit a 4" diameter rim, be 3" wide and 3" tall on the sidewall. Therefore, the overall height of the tyre would be 10" (4" plus 3" plus 3").

eg 6-12,  7-14,  8-16,  9.5-18 
Similar to above, but this method does away with the '.00' bit for the first part of the size. Again, all dimensions are in inches. It is worth mentioning that tyres with very similar markings may not be compatible - a 6-12 tyre will be significantly different in size to a 6.00-12. Whilst this may not be important if the application is say a two wheeled rotavator tyre, there can be more serious consequences if you change the tyre size on a four wheel drive tractor which will affect the gear ratios. If in doubt, stick to the exact size that is fitted to the vehicle or recommended by the manufacturer.

eg 4.10/3.50-4,  4.10/3.50-6,  5.30/4.50-6 
This time there are three parts to the tyre size, all in inches. The first, 4.10, is the approximate width of the tyre when fitted to the recommended wheel. The second, 3.50, is the height of the sidewall, and the third is the rim diameter.

So a 4.10/3.50-4 tyre is 4.1" wide, 3.5" tall on the sidewall and fits a 4" rim. This tyre would therefore have the appearance of a low profile tyre. ie it is lower than it is wide.

eg 11x4.00-4,  13x5.00-6,  18x8.50-8,  25x12.00-9 
This method is widely used on turf and ATV tyres and is probably the easiest to understand. The first part is the overall height (from the ground to the top of the tyre) of the tyre in inches, the second part is the width of the tyre in inches, and the third part is the rim diameter in inches.

If you are looking to change your tyre size, the height and width may be selected according to your needs, bu the last figure, the rim diameter, must be the same, unless you are buying new wheels too!

So a 25x12.00-9 ATV tyre would be 25" tall, 12" wide and fitted to a 9" rim.

eg 10x3,  18x4,  260x85 
This method is becoming obsolete, but you may still have tyres marked in this way. Measurements can be in inches or millimetres (look at the examples above, it's fairly obvious which is which). The first part is the overall tyre height, the second is the tyre width.

The rim diameter is not mentioned in this size marking, but it can be easily worked out, because the tyre sidewall height will be the same as the tyre width. So, an 18x4 tyre is 18" tall, 4" wide, and fits a 10" diameter rim (18" overall height minus two lots of 4" high sidewalls). The modern equivalent size for an 18x4 would therefore be 4.00-10.

A 260x85 tyre would be 260mm high and 85mm wide. Converting these dimensions to inches gives us a tyre 10" high, 3" wide on a 4" rim. The modern equivalent size would be a 3.00-4.

eg 145R10,  255/60-10,  155R12,  195/50R13 
This is the traditional method for marking car and van tyres, so is also used quite often for trailer and caravan tyres. The first part is the width of the tyre in mm, the second part, if present, is the height of the sidewall expressed as a percentage of the width, and the third part is the rim diameter in inches. If there is no percentage indication in the tyre size, the tyre sidewall height is taken to be 80% of the width. You may sometimes see a tyre with the 80% in the size eg. 155/80-13. The 'R'. when present within the size, means the tyre is of radial construction.

So, a 145R10 is a radial tyre, 145mm wide, sidewall height of 116mm (80% of 145mm) and fits a 10" rim. A 195/50R13 tyre would be 195mm wide, have a sidewall height of 98mm, and fit a 13" rim. The approximate overall height of the tyre can be worked out by converting both dimensions to either mm or inches, and adding the rim diameter to twice the sidewall height.

It is becoming more common for ATV and turf tyres to be marked up in this manner. They are usually dual marked with their traditional inch dimensions, but if you are handy with a calculator, it's possible to convert from one to the other. For example, a 255/60-10 ATV tyre would be 255mm wide, have a 153mm sidewall height and fit a 10" rim. Converting these dimensions to inches would give us 10" width, 6" sidewall height and 22" (10" rim plus two lots of 6" high sidewalls) overall height. In traditional ATV tyre size marking, this would become a 22x10.00-10.

Ply Ratings and Load Index/Speed Index Markings 
Originally, tyres were marked with the exact number of canvas layers (plies) built into the tyre. This was an indication of the strength of that particular size of tyre. It's fairly  obvious that a 4 ply wheelbarrow tyre will have a different load capacity to a 4 ply car tyre. As stronger materials were developed, the number of layers could be reduced whilst still maintaining the same strength in the tyre. The designation Ply Rating therefore does not indicate the actual number of layers or plies in the tyre, but the number of layers originally required to give the desired load capacity. When working with PR marked tyres, you must use the actual manufacturer's data to determine the load capacity in kg. This value can often be found on the side of the tyre adjacent to the ply rating indication.

The load index and speed index is gradually replacing the PR marking for tyres. The load index is given by a number between zero and 159 in our chart, but can be much higher. The speed index is given by a letter (or letter and number in the case of 'A' speed rating) between A and Z. Typical examples for our type of tyres would be 68M, 55N or  38F. Any ATV tyres we list with this marking will also be 'E' marked and fully road legal. Contrary to the PR marking, the LI and SI give unambiguous and comparable information on the load capacity which can be drawn from the tables below;

As already mentioned, it is important not to mix tyre sizes or types on motorised machinery as there can be significant variation in heights and widths between different manufacturers. We have even found that sometimes tyres of exactly the same size marking can vary significantly. As with car tyres, we would suggest you replace your tyres in pairs to ensure optimum performance.

The author and copyright holder of this material is Tyre-Rite Ltd.
Copying and reproduction is strictly forbidden.