good motoring
Understanding tyres
H. Kishie Singh

In days goneby you just asked for a tyre for your Fiat, Ambassador or jeep. No attention was paid to the size of the tyre that the manufacturer printed on the side wall. If I remember correctly, the jeep tyre size was 600x16. Simple and yes N.D.M.S. meaning non-directional mud and snow.

Now every tyre manufacturer offers a number of tyres for the same vehicle, with information printed on the side-wall, the materials used in the construction of the tyres. The tread pattern is explained. H/T for Highway Terrain, A/T means All Terrain. This tyre is recommended for 4x4 or A.W.D. vehicles which are likely to be used off-road. If it is a unidirectional tyre, an arrow will show the direction of rotation. Tyres with asymmetrical tread will have "outside" printed on the side-wall. This tells you how to mount the tyre on the rim. The most confusing information is the size of the tyre, expressed as 235-60R-16. The 235 is the section width of the tyre, expressed in millimetres. The next figure 60 is referred to as the aspect ratio. It is the percentage of the tyre width.

This is the distance of the tyre from the rim to its outer circumference. It decides the height (ground clearance) of the car. So 60or 0.6 of 235mm is 0.6x235mm=141mm (or 5.64 inches). This is followed by a letter 'R, denoting radial tyre. It is the easiest to understand. Then comes 16. It is the diameter of the rim size, in inches. Itís quite easy to understand. Millimetre, percentages and inches. The same tyre will have an option: 235-70R-16. Everything remains the same except the aspect ratio. This raises the height of the vehicle. By how much? 70 per cent or 0.7 of 235 is 0.7x235=164.5mm or 6.58 inches. With the aspect ratio at 60 per cent, the height was 141mm or 5.64 inches. There is a difference of 6.58" minus 5.64" equals 0.94 inches or 23.5mm. The vehicle will be higher by this much. If you opt for a larger size tyre, the problem is: Does the tyre fit in the wheel well of your vehicle? Turn the steering wheel from lock to lock, does it move freely? If not, you will have trouble with this tyre. 

Some car owners move from the 235mm (tyre width) to a larger size, to 245mm, 255mm or even more. The manufacturer considers parameters like the weight of the car, the power of the engine and how this power is transmitted to the the road via the tyres. Larger tyres, unable to do this, will change the handling dynamics of the car. Cornering, braking and take off will be affected. If tyres stick out of wheel arches, it ruins the look of the car.