So you’ve got that second hand car you wanted and at a great price. What are you going to do with the spare money that’s burning a hole in your pocket? No doubt it’s going to replace those boring standard fit rims with some cool alloys - simple. Well so you thought. The market is full of manufacturers of wheels with so many different styles, sizes and weights that it can be an exceptionally difficult decision to make.

Then once you have decided on a style of wheel a load of questions are thrown at you, “What size are you after? And what profile tyres would you like? Do you know what offset you need? What PCD do you require?”

Often the response will be, “Errrrm?” So what do you need to know?

Let’s start with wheel diameter and tyre profile. The car manufacture has designed and set up your car for the wheel and tyre combination supplied with it. Increasing or decreasing the overall wheel diameter (wheel with tyre on) will affect the following:

Speed reading

Acceleration and top speed

Gearing

Ground Clearance

For example, if you take a 1p piece and move it along a table for one complete revolution it won’t travel as far as a 2p piece doing the same thing. This is due to its smaller diameter and circumference compared to the 2p piece. If you imagine a car designed with wheels the size of 1p pieces and you drove it using the tachometer and held it at 1000rpm for one minute you would travel a distance of 1000 times the circumference of a 1p piece. If you did the same thing in the same car but instead had 2p pieces fitted, the car would travel 1000 times the larger circumference of the 2p piece (i.e. further), but in the same time. As speed is equal to distance travelled divided by the time taken, the car with 2p piece size wheels was travelling faster, however the cars tacho and speedo will have been telling the driver that he was driving at the same speed as with the 1p pieces fitted! This is obviously not true.

That rather long winded explanation is trying to say that if you buy larger diameter wheels then you need to fit lower profile tyres to them such that you maintain the overall tyre diameter that the speedo is calibrated for. If you don’t do this your car won’t be displaying the correct speed information. Any good retailer should be able to advise you on what wheel diameter and tyre combinations will work on your car to prevent any re-calibration being needed. However if you want to know how to do the calculations yourself then you need to understand the information displayed on the side of tyres. Tyre size is designated by three numbers as shown below. The other numbers and letters are used to describe other important characteristics of the tyre such as the maximum speed they are designed for and the max load for the tyre. See the tables at the bottom of this guide for details of what the different values mean.

If your wheel had this tyre fitted you could calculate the total diameter by doing the following:

1. Multiply the first number (tyre width) by the second number (tyre height expressed as a percentage of the tyre width), then multiply this figure by 0.01 to give you the height of the tyre wall in millimetres.

 

205 x 40 x 0.01 = Tyre wall height in mm = 82mm

 

2. Take this number and multiply it by two (this is to take into account top and bottom tyre walls) and divide it by 25.4. This has now converted the dimension from millimetres to inches. Add this to the last of the three numbers and you have the total diameter (wheel plus tyre) in inches.

((82 x 2) ÷ 25.4) + 17 = Total diameter in inches = 20.2 inches

So if you get a wider tread alloy of the same diameter as the original wheel, you will need a lower profile tyre to maintain the same overall diameter.

e.g. You could change a 185/55R15 for a 195/50R15. The first would give an overall diameter of 23 inches and the second a diameter of 22.7 inches. This is a difference of less than 2%, meaning that your speedo reading still has reasonable accuracy. If the difference is greater than 3% then you need to consider re-calibrating.

If you want a larger diameter alloy then you may need to look at different wheel width and lower tyre profile. If you want to go from 15 inch wheels with 185/55R15 tyres fitted as standard to 17inch wheels then you could go for 205/40R17. This would give an overall diameter of 23.5 inches compared to 23 inches, a difference of 2%.

There are other things to take into consideration when choosing a wheel, as well as the diameter. Some of these being:

• Will the wheel and tyre combination fit in the wheel arch?
• Will the new wheel foul your brake discs and callipers?
• How many studs does your wheel require and with what Pitch Circle Diameter (PCD)?
• What Offset do you require, also expressed as ET (Einpresstieffe)?

However you’ll have to wait until our next guide to find out more about that…...