Really not that difficult
eg if 195/60 x 14 is the factory standard tye size for your car, and I'd personally check this as it can be critical.
1st no is the width of the tyre in mm
2nd no is the aspect ratio in % - where aspect ratio is sidewall height of tyre/width
3rd no is the diameter of the wheel in inches
So for the above - the outside tyre diameter is
(195 x 60%) x 2 + (14 x 25.4) = 589mm ie sidewall height x 2 plus wheel diameter converted to mm
However, you may of course purchase wheels of a different diameter, but the key is to ensure the outside tyre diameter always remains the same - or as near to it as possible.
So taking the above example, your aim is to achieve 589mm
If you fit 16 inch wheels, then clearly you require 16 inch tyres to suit - do NOT muck about with this - but you must reduce the sidewall height to try to return to the original diameter.
Try 215/40 x 16
Overall tyre diameter here is (215 x 40%) x 2 + (16 x 25.4) = 578mm which is close
Try 195/45 x 16
Overall tyre diameter here is (195 x 45%) x 2 + (16 x 25.4) = 582mm which is closer
Try 205/45 x 16
Overall tyre diameter here is (205 x 45%) x 2 + (16 x 25.4) = 591mm which is closer still and probably your best bet if fitting 16 inch wheels instead of your current 14 inch.
Obviously you cannot go too wide as your wheels may stick out too far - an MOT fail - will look awful and you may experience some fouling on the arches when turning.
Speedo reads under if fitting larger tyres and vice-versa for smaller. 10 mm either way on diameter makes little difference however.
Do also check that your new wheels are within the offset range, and that the counter sunk centre bore of the wheel also mates up - ie similar to your old wheels.
Crystal?
