[wyatt]

Originally Posted by vrmilionzx

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 height = 215mm x .45 (aspect ratio) = 96.75mm
Old combo total Dia = 431.8+(2*96.75) = 625.3mm

New Wheel Dia = 15x25.4 = 381mm
New Tire height = 185mm x .65 (aspect ratio) = 120.25mm
New combo total Dia = 381+(2*120.25) = 621.5mm

Fixed that for ya.