OK, so there I was in my driveway measuring all the parts of the Hawk so I can make scale drawings and try to see what different paint schemes/accessories will look like and...

what a surprise...my 13" rims are 14" across!

Checked my tires and they are 13" tires...um, OK, I'll try the wife's S-10...her 15" rims are 16" across!

so...uh...can anyone explain this one to me? I just always assumed this measurement would be edge to edge across the hub.

Haha, The tire goes into the lip of the wheel. That is where the measurement comes from.

yours are 13's and hers are 15's

Take the diameter measurement from the inside groove that the tire bead sits on. It’s easiest to measure the inner diameter of the tire after it’s removed from the wheel. If you measure the outer diameter of the rim itself, it will be almost an inch larger than the specific size.
Taken from ehow.com site.