1/5/2024 0 Comments Visual tire size comparison![]() ![]() Now let’s take a look at what differences these changes in alloy and tire size actually create. This is of course achieved by having a 235/40 tire size as opposed to a 235/45 tire size which equates to a 12 mm (0.47 inch) difference in tire wall height. There is a slight difference in the overall circumference but they are very similar so it does not have a big effect on the speedometer. The 18″ alloys have a 235/40 R18 tire size = overall circumference of 79.92 inches.The 17″ alloys have a 235/45 R17 tire size = overall circumference of 79.49 inches.The car comes with several wheel sizes including 17″ and 18″ options. Let’s use a 2022 Ford Focus as an example. The reason that I’m explaining all this is because it’s actually the change in the tires aspect ratio (profile height) when going from a 17″ to 18″ alloy which makes a difference, as well as the alloy size itself. ![]() In each case, the profile height is 40% of the first number (205 or 225 in this example). In this example, the 205/40 tire has a profile height of 82 mm, whereas the 225/40 tire has a larger profile height of 90 mm. The aspect ratio is a percentage of the width so means nothing unless you have that first number. For example, a 205/40 tire does not have the same profile height as a 225/40 tire. It’s important to remember that the profile height is an aspect ratio, and not a measurement that can be taken in isolation. Here are a couple of diagrams to demonstrate this point. However, in most cases car owners will compensate for a larger alloy by having a lower profile tire, to avoid issues with the speedometer by keeping the overall wheel size the same. Changing the overall size of the wheel will impact the speedometer.įor example, if you switch from 17 to 18 inch alloys and keep the tire profile height the same, then the wheel will take fewer rotations to cover the same distance so your speedometer will say that the car is going slower than it actually is. You don’t always have to match the overall wheel size completely to the stock size, however you do need to make sure that it will fit. Whenever you change the size of the alloy, the tire size must also change to ensure that the whole wheel comfortably fits into the wheel arch. Your wheel as a whole is of course made up of the alloy, and the tire. Before we jump into the comparisons between 17″ and 18″ alloys, we need to be clear on the connotations of changing the alloy wheel size. ![]()
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