Rolling stock question

[nukefission]

When you convert to new wheels and/or tires, you have to take into account the rolling stock diameter so that your speedometer will remain accurate. This effects your choice of tire width and aspect ratio. I was wondering, why does it have to affect the width of the tire you choose? It doesn't seem like the width has anything to do with the ultimate diameter of the wheel/tire setup. For instance, if I went to a 17" wheel, and chose a 225/45-17 tire, wouldn't that choice be just as valid as say a 215/45-17 tire. The physical diameter ends up being the same, so why should it make a difference with the speedometer?

[B430]

because the number is the aspect ratio, not the sidewall height. so if the width increases to keep the same ratio the side wall height increases. that's why for wider tire you need a smaller aspect ratio to keep overall diameter the same.

[Nick 92 TSi AWD]

Either of those sizes will work, but why buy narrower tires? There's about a 1/2" to 3/4" of room for changes, the speedo will stay pretty accurate up there.