USING SIZE 235/35-19 FOR EXAMPLE The dimensions of a 235/35-19 tire are 9.3” wide (section width) & 25.5” tall (outside dia) The section width is 235mm wide (There are 25.4mm to an inch) STEP 1) 235mm divided by 25.4” = 9.25” (section width) STEP 2) Multiply 9.25 (section width) by .35 (aspect ratio) 9.25 x .35 = 3.24” (section height) To find the outside diameter Multiply 3.24 (section height) x 2 then add rim diameter STEP 3) 3.24” x 2 = 6.48” + 19.0” = 25.48” round out to 25.5”  (outside dia)