As a general rule, you should be replacing your tyres about every six years. If you drive a lot, that number will be insufficient. Also, the types of roads that you drive on will determine if and when you need new tyres. Paradoxically, driving on highways is actually less damaging to your tyres. Your tyres degrade due to friction. The friction of starting and stopping that is common in city driving is more damaging than the constant driving on highways. There is no good rule of thumb for changing tyres unless you look at them first. Here is what to look for.
Recently, tyres in Leeds have had indicators of when they need to be replaced. If you look closely at your tyres’ treads, you’ll see a small bump in between the tread lines. This bump is your indicator. As the tyre ages, the indicator will let you know when you need to replace them. If the bump is flush with the tread, you need to replace your tyres. Alternately, if it is close but not quite there, it’s probably a good idea to replace them now.
If you don’t have a full-sized spare tyre, replacing a fairly worn but not completely finished tyre will give you the opportunity to have a spare tyre in reserve if something should happen to your tyres while you’re on the road. Of course, you’ll need to buy a whole new rim for that to work, but you will save some money over the long run.
You can also just look at your tyres. If they are wearing unevenly, you should have them rotated. If they are worn past the point of saving, you should have them rotated, replace the worn tyres, and have your tyres balanced. Tyres should wear evenly when they are properly balanced.