Visual Tyre Size Calculator

If you are thinking of changing your car tyre size or are planning on buying a new wheel/tyre combination then you can use the flash tool from Kouki for a visual tyre size comparison of your stock wheel and tyre size versus your new wheel and tyre combination.

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Why is this important?

Matching the combined wheel and tyre diameter is important as it will ensure your ride height and speedometer readings don’t change drastically. An example of this would be fitting 17″ wheels to a car that has 14″ wheels as standard, there are three main consequences:

  1. the ride height will change, the car will be further from the ground.
  2. the top speed will increase slightly and the speedometer will under read.
  3. the car will acceleration slower.

Read on to see the reason for this in detail.

Tyre Size Guide

Ride Height

Take the default example in the flash tyre size calculator above, here we have a stock tyre size of 165/55R14 and a new tyre size of 215/45R17. There is a total difference of 8.82cm in the two wheel and tyre diameters, if we divide this by two we get a difference in radius of 4.41cm. If we ignore tyre pressures and the effects of adding larger, wider wheels to suspension geometry; then this is roughly how much higher the car will be from the ground.

Speedometer

Then there’s the speedometer, a cars speed reading is usually taken from the transmission and is set based on a number of predefined constants. When you change your wheel and tyre combination you are taking one of these constants and making it variable. The speedometer does not know the new value and so continues to read based on the original wheel & tyre size. If you look at the above default example again you will see that with a 215/45R17 wheel and tyre combination there is a difference of 16.4% in the speedometer reading over the stock setup. As you can see, at 60mph your speedometer should actually be reading 69.9mph!

Acceleration

If we fall back to the default example again, the 215/45R17 wheel and tyre combination has a larger overall diameter by 8.82cm. A larger diameter has a greater rotational inertia which requires more energy to accelerate. Or in other words: the bigger the wheel, the slower the acceleration.

It should be noted that putting a bigger wheel and tyre combination on your car does not reduce your cars power. Your cars power remains the same; however it takes more force to turn your car’s wheels resulting in slower acceleration.

found this great flash for tyre size guide. take a look. might be useful.. from Kouki Tech.

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