erm.. usually on the tyre wall, you can find a triangle pointed
upwards. when ur tyre thread is at the point of the triangle, it is
time to change.
however, you must also consider a few factors.
the car maybe a 2nd car or a car that is hardly used. thus tyre
wear very little. for example, average users travel about 20,000km
a year. but some travel 20,000km in 2-3yrs? but they must still
change tyres even when 20,000km is not travelled because the
composition of the tyre will actually harden and not suitable for
useage. this is rather known as tyre's life span.
facts you should know
an average tyre should last you 20,000 MILES
(35,000km-40,000km)
an average tyre life-span is 2 - 2.5 years. please check your
tyre's DOM indicated on the tyre wall too.
my car is nearing 3 years this august, and ive already clocked
70,000km. im already on my 3rd set. this is just a rough guide for
you. i change tyres more often because mine is UHP tyres which tend
to burn more than normal comfort tyres.
abit more information for you... tyre rotation every 20,000km
for proper wear and tear.. but i do mine every 10,000.
although from the exterior, ur tyres may seem ok, but turn ur
steering full-lock to the right or left to check the wear and tear
of the insides.
hope these info help.