That's odd - I'm using a tire calculator online, and getting the same result, but it seems really....off. I checked using excel, and unless I'm entirely thick (very possible, I haven't had my morning coffee yet), I'm getting a difference in circumference of more like 5%.
Because I'm having a bit of an aneurism about this, if anyone wants to check my math and tell me where I screwed up, please do....
I converted to mm (so everything is in the same form of measurement), and used 25.4 mm in one inch, so:
Old Wheel Dia = 17x25.4 = 431.8mm
Old Tire Dia = 215mm x .45 (aspect ratio) = 96.75mm
Old combo total Dia = 528.55+96.75 = 528.55mm
New Wheel Dia = 15x25.4 = 381mm
New Tire Dia = 185mm x .65 (aspect ratio) = 120.25mm
New combo total Dia = 381+120.25 = 501.25mm
measuring the delta: (501.25-528.55)/528.55 = -0.05165 (or about 5.1%)
No need to calculate based on the circumference, since applying the same formula to both can be canceled out, but just in case you'd like to:
Circumference = 2 x 3.14 x R OR 3.14 x D
(so you'd have 1659.647mm on the old combo, 1573.925mm on the new, same -5.1% difference)
Reference for confirmation of Aspect ratio measurement being in mm:
Tire code - Wikipedia, the free encyclopedia
I'm not trying to be a jerk, I'm just really trying to get how the tire calculator is so different. Obviously the other factors you (Wyatt) mentioned will play a role, so it's not JUST the size change, but it seems like a pretty significant change to me... And if I'm THAT wrong in my assumptions, I'd like to know before I make a tire/wheel combo changed based on those faulty assumptions...
Anyone?
Also, I think I have the same sweater hiding in a drawer somewhere...
NOTE: Sorry, had to edit, copy paste errors in numbers - should be correct now