Stock Sucks
+1y
take the width (275mm), multiply the ratio by 2 then divide that by 100 and then the width (35*2=70) (70/100=.7) (275*.7=192.5)
Then take that number and divide by 25.4, then add the wheel diameter(192.5/25.4=7.57 (7.578+20=27.57
27.58 is the overall diameter of a 275/35/20 tire
26.73 is tht overall diameter of a 285/30/20 tire
these are just averages as every tire co. will be a little different, but these are within 16ths of an inch b/w manufacturers