Different tires wear at different rates, so rotating your tyres around the car to equalise tread wear is a great way to prolong the life of the tyres and so save you money. You should check with the car manufacturer to see if they give any guidance on tyre rotation.
In general, if you swap the front and rear tyres every 5,000 miles you will get the maximum life out of your tyres.
You should definitely change spark plugs at regular intervals. When the car goes in for a full service the sparks should be changed as a matter of course. Although the old spark plugs may look fine, there is a good reason for changing them. Old sparks don’t fire well and mean your car has lower fuel efficiency, so it costs more to run. Spark plugs are cheap and popping a new set in is a great way to increase the efficiency of your engine.
Manufacturers have a vested interest in claiming that a new car is a good thing. The planet tends to disagree. Even when a new car uses less fuel, emits less waste etc, keeping your old car is usually a greener option since the environmental cost of new car production (and old car scrappage) needs to be added to the calculations.
Check with trusted green websites who have no interest in giving you false information on this topic.
So, the short answer to when you should buy a car is ‘when you have to, and not before’.
