Read the manual…
Every car owner’s manual specifies its tyre size, load rating and speed rating. As a rule of thumb, always replace your tyres with tyres of such specifications.

But it you must…ignore the manual
If you wish to change to low profile tyres anyway, then make sure you maintain the diameter and section height of your tyres. Otherwise, the new tyres will affect your car’s gearing, speedometer reading and mileage meter reading.

Bigger tyres, for instance, will record lower speed readings. This means that while your speedometer gives you a reading of 110km/h, you could actually be driving at a speed of 130km/h. Sad thing is, a police speed trap will also read 130km/h.

Tyres of the wrong section height will also shift the centre of gravity of the vehicle thereby overloading the tyre.  

How does one get it right?
Simple. Just remember this formula: -

Aspect Ratio= Section Height/ Section Width x 100
So, in order to maintain the section height, an increase in a tyre’s section width should be accompanied by a decrease in its aspect ratio.

As a rule of thumb, always add 20mm to the section width if you want to change from an ‘80’ series tyre to a ‘70’ series tyre. For example, a 155/80R is equivalent to a 175/70R 13 tyre.

What else to look out for?
When changing tyres, check the following:

• The replacement tyres should carry at least the same load rating and speed rating. The change should not affect your car’s body/ suspension clearance. Nor should it breach any term of your insurance contract.
• To be on the safe side, ensure that all four wheels are fitted with tyres of the same size and construction.
• And of course, your tyre must fit the rim of your wheel.
• If in doubt, consult your tyre dealer.