OK I measure from the ball (the bulge) of the sidewall to the wheel well lip on each side & need 3/4" clearance on each side then I need that(the outside measurement) amount more rim width for the outside and on the inside I need that(inside measured amt) more rim width which would be an increase in backspacing of that (measured) amount?. EDIT So If I have 2&3/4" measured on the inside and 3&3/4 on the outside & my test rim is 4&1/2 wide w 3" backspacing I need a 9&1/2" rim w a 5" backspacing?.