Wheels for a duster offset and backspace

When I bought my Dart Sport, it already had 18s: 225/40-18 in the front and 245/35-18 out back. I have no idea what backspacing is on these wheels (is there an easy way to tell?). I'm thinking I'd like to run all four of the same size wheels/tires so I can rotate tires and only carry 1 size spare. Ideally I'd probably like 17's but I read that 18's give you more clearance for front suspension items. Could anyone share their experience with one size for all 4 wheels/tires? ThanksIMG_1914.jpg