Car tyre size guide and measurement

29.05.2019
Car tyre size guide and measurement

What is TYRE SIZE ➤ Tyre size GUIDE ➤ Tyre size COMPARISON ➤ HOW tyre size is measured ✅

When you have to upgrade tyres for your vehicle, for example, a passenger car or a truck, you should have a clue in their sizes. Otherwise, there remains a risk to purchase something incompatible and waste your money.

What is tyre size?

It is a set of literal and digital denotations which are mentioned on the side of the tyre. What are they intended for? An explanation is quite simple: such codes are car tyre dimensions.
The image of such symbolic denotations can be displayed as P195 / 55 R16 87 V., at first sight, everything seems complicated for understanding. Yet, in fact, everything is not so hard though you will have to remember some things. This tyre size guide will help you distinguish what each digit and letter mean, and also tells you about the main measurement units.

The meaning of denotations on a tyre. Let’s look closer to the aforementioned example. In this case, the decoding is as follows:

1) The number 195 is the denotation of tyre sectional view width. It is measured in millimetres and means the distance between the inner and outer plates of the tyre.
2) 55 gives us other information which is important as well, more specifically – it is the aspect ratio which is also called the profile. This specification is measured in percent to the width of the profile. The value in the mentioned example corresponds with 55% of the tyre width – 195 mm. Consequently, the higher aspect ratio percent is, the higher the side plate height is.
3) The next denotation means the type of construction. So, the ‘R’ letter should be read as ‘radial’. So, the tyre has radial construction.
Note: almost all current automobile tyres have radial construction.
4) The number that follows construction type denotation represents the tyre rim diameter. This value of tyre size measurement is represented in inches, and most commonly they write not an “inch” word but the quotes marks after the number. In our case, the wheel has 16” diameter.
5) Load index – the last digital denotation. This parameter shows the maximum load this vehicle tyre is able to stand. This value is usually measured in kilograms. In our case, the index is 87. This means that the tyre is able to carry 545 kilograms.
6) The last symbol defines the maximum speed which can tyre can run without harm for itself. The table hereinafter specifies possible values.

 

Speed symbol

Miles per hour (maximum)

Kilometres per hour (maximum)

N

87

140

P

93

150

Q

99

160

R

106

170

S

112

180

T

118

190

U

124

200

H

130

210

V

149

240

ZR

150+

240+

W

168

270

Y

186

300

 

How tyre size is measured

Usually, vehicle tyres producers which are located in India, America and most parts of Europe display tyre size in inches. One more often used variant is denotation in the metric system. There is nothing complicated in reading tyre size set in inches, however, the other system is less simple. Still, you can make sense of it if you have to.

Note. Measurements in inches are also true for bike owners: calculations contained hereinafter also fit them.

Measurement in inches

Let us take a denotation 33X12.50R15. The first number specifies the overall diameter which is also the height of the whole tyre, so in our case, this value equals 33”. The second number displays the width. Usually, it is not a round number. In our case, the tyre has 12 and a half inches width.
Obviously, not all tyres can be installed on any vehicle. The last value is needed to understand the model the driver needs. It represents the size – diameter – of the wheel where the specified tyre can be placed on.

Let us understand metric denotations

As we have written before, metric measurements are far more complicated. In most cases, you will have to use a calculator to get the correct values and select a compatible tyre.
Let us take that our tyre’s specifications have the following denotation: 285 / 75R16.
In the case of the metric system, the first number is the width of the tyre displayed in millimetres. One-inch equals 25.4 mm. Thuswise, when you have to translate millimetres to inches, you have to divide 285 into 25.4. Here we get 11.22 inches.
The next number, as it was said in the very beginning, is the height of the side plate and means the percent to the width. In our case, it is 75%. If to count in millimetres, this value is equal 214 mm. In case the calculations are required in inches, this value equals 8.41”.
When you get overall tyre diameter, you can specify two measurements of side plate size and the wheel diameter. It is the last number in our example and in measured in inches in any case. With that in mind we can crunch such numbers: to calculate size plate size we have to multiply its height – 8.41 inches – by two and add the wheel diameter. The arithmetic formula will look as follows: 8,41 х 2 + 16 = 32,82.
That metric denotation of 285 / 75R16 tyre size is identically equal to the tyre denoted in inches of 32,82 / 11,22R16.

Remember! Not all tyre producers set their sizes in inches or millimetres.

Tyre size comparison

When you have to increase automobile’s traction while driving on snow, dirt, and stones, you need special tyres developed for off-road driving.
In this case, you have to take into consideration that such tyres are subject to upsizing. Consequently, they have a higher side plate coefficient in comparison to standard tyres. Moreover, tyres for off-road driving are equipped with a much deeper protector. All of these increase the car’s passing performance on hostile terrain.

When selecting tyres which were created especially for off-road driving please take into consideration denotation of their size. Their registration numbers differ a bit from the traditional ones described above.

For instance, you will hardly see any literal denotations. As a rule, off-road tyre tagging looks like 35 х 12,5-17. There is no point in detailed explanation for what each number stands for above we have explained what each number means in detail. The only difference is the lack of letter defining tyre construction type. However, you can meet literal denotations for models meant for off-road driving either.

One should know that off-road tyres are usually described with their outer diameter measurement. Usually, everything is calculated in inches. Surely, other parameters of the tyres are important as well. However, it’s the outer diameter that defines the sample for tyres’ size comparison. It allows seeing whether the wheels fit clearance limits inside the car’s wheel arches.
The clearance inside wheel arches is also specified in inches. One should not exceed specified size if he does not want tyres to scratch against wheel arches, mudguards or steering shafts with offensive groaning.
When you buy tyres you have to check them by size compare table. The point is that items with classic metric measurements may be not equal to equivalent sizes calculated in inches. This means that despite diameters of an inch and metric tyres seem equivalent, they may differ from each other.

Let’s have a look at some examples. We have a tyre whose dimensions are 245 / 75-16 and outer diameter equals 30.47 inches. This means that it fits in a 30-31 inches range of off-road tyres.
The best decision here would be the following one. In case the driver has not changed his automobile’s clearance he will rather choose the smaller option of 30 inches. If the vehicle owner has upsized his car and increased clearance, then it would be better to choose a larger – 31 inch – size.

What you have to know either

There exist so-called Run Flat tyres. This modification has been created for being able to run a certain distance when the wheeled camera has lost pressure. That is how tyres of such models provide additional protection.
So, when the pressure in the tyres has decreased, the driver can run the car at a constant speed of up to 80 km/h for a defined distance which equals 50-80 kilometres depending on tyre manufacturer.

Please note that such tyres are intended for certain automobiles that are equipped with a tyre pressure monitoring system (TPMS). You can easily recognize them: you will see RFT abbreviation on their side plates.

A separate classification exists for some commercial automobiles and also vehicles with special carrying capacity. They need tyres for light trucks or truck automobiles. Such models can contain ‘C’ and ‘LT’ letters in their codename. As usual, the first variant is written after rim diameter and the second – before tyre size. For example, 245R15С or LT37х10,5R15.
There also exist special cross-country tyres developed for Mercedes company’s automobiles. They are tagged with ‘MOE’ abbreviation.
Models with such tagging have limited haul operations. Their rigidity rating is less than in RFT tyres. However, they perfectly fit Mercedes automobiles because they provide comfort drive suspension.

Important: it is prohibited to equip automobiles with such tyres with other than MOE tyres.

Tyre Size Chart

In this section, you can see a comparative table with the most widespread tyre sizes which fit this or that wheel diameter.

 

Wheel diameter

Metric dimensions

Inch dimensions

16 inch

125 / 70R16

22,9X4,9R16

175 / 50R16

22,9x6,9R16

195 / 40R16

22,1x7,7R16

195 / 45R16

22,9x7,7R16

205 / 40R16

22,5x8,1R16

215 / 35R16

22x8,5R16

215 / 40R16

22,8x8,5R16

17 inch

125 / 70R17

23,9X4,9R17

195 / 40R17

23,1x7,7R17

195 / 45R17

23,9x7,7R17

205 / 40R17

23,5x8,1R17

215 / 35R17

23x8,5R17

215 / 40R17

23,8x8,5R17

225 / 35R17

23,2x8,9R17

245 / 35R17

23,8x9,7R17

18 inch

125 / 70R18

24,9X4,9R18

205 / 40R18

24,5x8,1R18

215 / 35R18

 23,9x8,5R18

235 / 35R18

24,5x9,3R18

245 / 35R18

24,8x9,7R18

285 / 30R18

24,7x11,2R18

19 inch

215 / 35R19

24,9x8,5R19

235 / 30R19

24,6x9,3R19

245 / 30R19

24,8x9,7R19

295 / 25R19

24,8X11,6R19

20 inch

215 / 30R20

25,1X8,5R20

225 / 30R20

25,3x8,9R20

235 / 30R20

25,6x9,3R20

245 / 30R20

25,8x9,7R20

275 / 25R20

25,4X10,8R20

285 / 25R20

25,6x11,2R20

295 / 25R20

25,8x11,6R20

Now you know what do symbols and letters on tyre side plates mean and how their dimensions are measured. Thank this you will find it easy to choose a compatible replacement for your vehicle.