There are no hard and fast rules of exactly when you should replace your tires and figuring it out can be a little tricky. Here are a few tips to help you know when you should buy new tires for your vehicle.By MileageThe average tire is good for about 40,000 miles before it starts to become unsafe. However, if the vehicle is particularly heavy, this figure might be reduced slightly. Other factors that can reduce the mileage of a tire include harsh weather conditions and poor driving surfaces. However, as a rule of thumb, check your tires carefully when you start to approach 40,000 miles.Measure the TreadThe tread on vehicle tires should always be a minimum of 1/16 of an inch deep. However, this figure should be doubled if the roads that you regularly drive on are icy or wet. Regular friction between your tires and the road will make the tires wear out over time. You can measure the tread of your tires using a penny, turning the penny so that the queen’s head faces towards the tires. Insert the penny between the tread and replace the tires immediately if the whole of the head is showing.Inspect the Sidewall for CracksThe tires sidewalls often get overlooked by motorists, but this is an important indication of whether or not the tires need replacing. Cracks in the sidewall have the appearance of dry rot and if they are present the tires are old and should be replaced. poor alignment can could the sidewalls to crack quickly, so make sure that the tires are perfectly aligned when they are fitted to help make them last longer.The Air Needs Topping Up OftenSigns that the tires on your vehicle need to be replaced are not always immediately obvious. If you find that you often need to put air in your tires, this is a good indication that you have a leak somewhere in the tires. Although the leak may be detectable by a professional, it is also a good indication that your tires need to be replaced. Check your tire pressure at least once every month to make sure that they are full of air.