wheel back spacing and offset

Seasons greetings and happy new year to everyone. Have a wheel question. We were talking with US Mags about wheels (15 x 7 front and 15 x 8 rear) for our Sebring, the guy asked me if I needed back space or offsets? We're using a 5 bolt hub front and rear with 4.5 between centres, doner car was 89 mustang LX. We checked the bolt pattern with a 1995-97 rim and it works perfectly. So not sure what US Mags is looking for?

Here are a few links to explanations:

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Just remember that a wheel is typically 1" wider overall than the listed width.  Example: An 8" rim will be 9" from outside edge to inside edge  Backspace is measured from the back edge of the wheel.  A tire that has a 10" section width will add 1/2" to the total backspace of the 8" wheel/tire combo.  A tire with a 12" section width will add 1" or half of the tire section width. Some websites like Tire Rack.com will have listings for various tires that indicate section width and a range of acceptable rim widths for a particular tire. Summit Racing has this info too if you look up a specific tire.

When you order rims, you need to consider the tire section width you intend to run or you may have a fitment problem.

In the rear of the car, take a straight edge and lay it across the wheel mounting surface on the car and measure to the back of the fender well.  This will tell you how much total backspace you have available.  You will need to subtract about an inch from this number in the rear to allow a little clearance.  1 1/2" would be better.  

In the front you will need room for the tire to sweep as you turn.  If you have stock MII struts on the frontend, these will most likely be the tightest spot.  Turn the wheels all the way to the end of the steering rack travel and put the straight edge across the wheel mounting surface and measure to the strut rod.

Once you have the maximum allowable backspace you have, you can work out exactly how wide of a tire section width you can run.  From there you can figure out the rim width you need and what your rim backspace needs to be to get it under the car.  It helps to draw it out to scale.

Thanks Hotrod for that very detailed reply, appreciate it. According to the manual we were going to go with a 15 x 7 front on a 215 65R15 tire and for the back a 15 x 8 rim with a 255 60R15 tire. A standard setup according to the manual. But I will do the measurements as you suggested and review the links too. Thank you.