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Old 11-13-2009, 02:00 PM   #25
Registered User
Join Date: Jan 2009
Location: central ma
Posts: 4
My Ride: 330 Ci
Its best to keep the tire diameter as close to stock as possible. This affects your speedo readout.

width in mm x aspect ratio = sidewall height in mm
convert mm to inches
multiply sidewall X2
add wheel diameter to get overall diameter

205 x .5 = 102.5 mm
102.5 / 25.4 = 4.035"
4.035" x 2 = 8.071"
8.071" + 17" = 25.071" dia

plus 1
225 x .45 / 25.4 = 3.986"
(3.986 x 2) + 17 = 24.972" dia
25.071 - 24.972 = .099" difference in dia

You get the idea.

Also tires will present an identical area or contact patch for a given pressure to the road surface, about the shape of your upturned palm. The patch is oriented two different ways. A wide tire (225) will present a crosswise patch while a narrower tire will be oriented 90 degrees or hand inline with the direction of travel. A wide tire tends to build up snow and/or water sooner because of the wide cross section aka hydroplaning.

Not much difference between your choices. You would mostly notice differences in rubber compound and tread design. Always a tough choice though as the manufactures are all very close these days. Good luck.
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