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

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Why is this important?

Matching the combined wheel and tire 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.

Tire Size Guide

Ride Height

Take the default example in the flash tire size calculator above, here we have a stock tire size of 165/55R14 and a new tire size of 215/45R17. There is a total difference of 8.82cm in the two wheel and tire diameters, if we divide this by two we get a difference in radius of 4.41cm. If we ignore tire 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 tire 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 & tire size. If you look at the above default example again you will see that with a 215/45R17 wheel and tire 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 tire 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 tire 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.

Tire Width Equivalency Table

You may also find the following table of recommended tire widths for given rim widths useful when changing your tire width.

via pneus-online.co.uk