one of the most commonly asked questions i am going to try and answer the best i can
the way to measure the diameter of a wheel is to measure the diameter between the inside faces of the wheel, not the exterior face you see
as tyres come in metric sizes 205/40 etc and wheels come in imperial sizes etc its best to convert the hweel size to metric as well
as 25.45 mm = 1 inch
there fore:
13" = 330.85mm
14" = 356.30mm
15" = 381.75mm
16" = 407.20mm
17" = 432.65mm
18" = 458.10mm
19" = 483.55mm
Tryes are measured in percentages
205/40 profile tyres means the contact area with the road when the wheel is perpendicular is 205mm, the side wall is 40% of this area
205 / 100 = 20.5 x 40 = 82.0mm / 2 = side wall depth in this case 41mm
The aim is to get the alloys and tyres as close to the original diameter as possible to avoid major arch work of course after going over a certain size this is not possible but the lower profile of sidewall the less you have to do IN THEORY i can turn round and say it will fit because manufacturing tolerances aren't that tight
so here we go common sizes
tyre size > sidewall depth > rolling diameter
13":
155/70 = 54.250mm = 385.100mm(15.13")
185/60 = 55.500mm = 386.350mm(15.18")
14"
185/55 = 50.875mm = 407.175mm(15.99")
195/50 = 48.750mm = 405.050mm(15.92")
195/45 = 43.875mm = 400.175mm(15.72")
15"
195/50 = 48.750mm = 430.500mm(16.91")
195/45 = 43.875mm = 425.625mm(16.72")
16"
195/45 = 43.875mm = 451.075mm(17.72")
205/40 = 41.000mm = 448.200mm(17.61")
195/40 = 39.000mm = 446.200mm(17.53")
17"
205/40 = 41.000mm = 473.650mm(18.61")
18"
215/35 = 37.625mm = 495.725mm(19.48")
19"
215/35 = 37.625mm = 521.175mm(20.48")
HTH Tony
