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 below for a visual tyre size comparison of your stock wheel and tyre size versus your new wheel and tyre combination.

Got a comment or question regarding the current version of the tyre size calculator? Click here to leave a comment or ask a question.

Vesion 2 of the tyre size calculator is in development, if there’s anything you’d like to see in version 2 then please feel free to leave a comment here: Tyre Size Calculator V2: Wishlist.

## 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:

- the ride height will change, the car body will be further from the ground.
- the top speed will increase slightly and the speedometer will under read.
- the car will accelerate 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.

## Tyre Width Equivalency Table

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

Rim width | Min tyre width | Ideal tyre width | Max tyre width |
---|---|---|---|

5 .0 inches | 155 mm | 165 ou 175 mm | 185 mm |

5.5 inches | 165 mm | 175 ou 185 mm | 195 mm |

6.0 inches | 175 mm | 185 ou 195 mm | 205 mm |

6.5 inches | 185 mm | 195 ou 205 mm | 215 mm |

7.0 inches | 195 mm | 205 ou 215 mm | 225 mm |

7.5 inches | 205 mm | 215 ou 225 mm | 235 mm |

8.0 inches | 215 mm | 225 ou 235 mm | 245 mm |

8.5 inches | 225 mm | 235 ou 245 mm | 255 mm |

9,0 inches | 235 mm | 245 ou 255 mm | 265 mm |

9.5 inches | 245 mm | 255 ou 265 mm | 275 mm |

10.0 inches | 255 mm | 265 or 275 mm | 285 mm |

10.5 inches | 265 mm | 275 or 285 mm | 295 mm |

11.0 inches | 275 mm | 285 or 295 mm | 305 mm |

11.5 inches | 285 mm | 295 or 305 mm | 315 mm |

12.0 inches | 295 mm | 305 or 315 mm | 325 mm |

12.5 inches | 305 mm | 315 or 325 mm | 335 mm |

Existing size 205/45/16 New size 195/65/15

Your calculator says speedo at 30mph (existing) will be 32.2mph (new)

Your calculator says speedo at 60mph (existing) will be 64.4mph (new)

Wiltheyfit.com says the reduction in rim size decreases actual speed reading

speedo at 30mph will become 27.9mph and

speedo at 60mph will become 55.9mph

In view of discrepancy, please explain which is correct & why

Thanks Mike

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