You want to fit a larger diameter wheel and/or change the tyre width. This tire size calculator will tell you the circumference of your original tyre, the circumference of the new tyre and the millimeter and percentage difference in the circumference between the two tyres.

Here is the tire size calculator.

Enter the dimensions of your existing tyre and the dimensions of your proposed tyre to see the difference in the circumference in millimeters and %. Ideally you want the two circumferences to be identical but up to 3% may be okay, personally I think 5% is too much.

Tyre Profile

The 40, 60, 50 number you see on tyres is the profile. If you fit a wider tyre you will probably need to fit a tyre with a lower profile ie go from 60 to 55 for example. If you use a wider tyre with the same profile the circumference of the tyre will increase.

Wheel Diameter

If you increase the size of the wheel, the profile of the tyre should generally be reduced to keep the circumference of the tyre the same.

If you increase the diameter of the wheel and fit wider tyres, the dimensions of the new tyre will be even more different from the original tire. You may have a 60 profile at the moment, and this could change to a 40 profile, if the dimension changes are large enough.

Why Keep the Circumference the Same?

Two main reasons. Speedo accuracy and suspension geometry.

If the circumference is larger the speedo will under read and vice versa.

If the centre of the wheel is a different distance from the ground some of the suspension geometry will change.

Both these things are a negative usually.

Image Sources

Casio calculator JS-20WK in 201901 002 – Calculator – Wikipedia

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