A 15 in. rim will measure 16 1/4 in. dia. and a 16 in. whel will measure 17 1/4 in. dia.
Be careful the offset measurement is another consideration when selecting a rim. The 16 in. rims I am using have a 8mm offset. These is what determines the clearance from the tire to the vertical sidewall of the wheel well and the fender edge.
I can't tell you how to measure offset but any tire shop that sells rims will be able to.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles