Quote:
Originally Posted by captslow
For the 16" you need to pick a tire with wider side profile so that the overall diameter stays roughly the same.
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Not quite true. In order not to throw off your speedometer, yes, you do need to keep the diameter the same. This can be done by changing the aspect ratio of the tire in relation to the tread width.
For example: stock size is 215/45R17, going to a 16 inch rim you could choose a
225/50R16, 245/45R16, 205/55R16, 195/55R16, etc. with minimal difference in reported speed (as the diameter of each of the four example are similar).
For winter use one would want to go with a narrower tire as opposed to a wider tire as the narrower tire will help with cutting through snow. Because the diameter of the new wheel/tire won't be exactly the same as stock (it is possible to come really close but very difficult to get 100% the same), you can expect some difference between what the speedometer says and what you are really doing.
If the spedometer shows 100km/h on the stock wheel, choosing a 205/55R16 tire will show 100km/h but actually be going 101.1km/h. The 195/55R16 would show 100km/h while actually going 99.3km/h.