How to calculate tire size?

As a general guideline regarding backspace, you want to keep the rim centered where the factory rim was centered — that's where the most clearance is. So if the factory 4.5" rim has 3" backspace, and you want to install a 7" rim, split the difference (7 minus 4.5 equals 2.5, 2.5/2 = 1.25) and add it to the factory backspace to get the backspace you need (3 + 1.25 = 4.25). If the backspace is too small, the rim will sit farther out than stock, which is likely to cause clearance problems on the lower front corner of the opening with a wider-than-stock tire (on a Barracuda). For example, I had a set of original TorqueThrust mags, 14x7 SBP, but they only had 3.25" backspace when mounted — the widest tire that would have cleared would have been a 215 (maybe), and I already had 215s on my factory steel wheels — I wanted 235s.

The other problem is that a lot of cast 14" rims won't clear the calipers with proper backspace — this is true of many slot mags.