I have to apologize in advance because I realize this a stupid question, but I don't know how else to find the right info, but just ask.

On my 1996 Hyundai Excel (ie: Accent), my owner's manual says my original tyres/tires are 155 R 13. From what I can find out about tyre/tire sizes, this means they're 13" wheels? When I actually measure them (excl tyres/tires), they're 37cm (ie: 14.5-15" in diameter).

How do you actually measure wheel sizes, and what do I actually have? They're the standard basic wheels for the entry-level 3-door model, aka "Sprint". The owners manual says that the options are (or were) 175/70 R 13, or 175/65 R 14. I never optioned up.

Reason, is I had my wheels changed over years ago to new alloys, and I still have the original wheels just lying around. I'm trying to figure out how to advertize them to sell them off - as 13" or 15".

Are you measuring the diameter of the wheels or the diameter of the tires? You should be measuring the diameter of the wheels.

When a tire size is 13", it doesn't mean that the tire is 13" in diameter, it means that the tire fits a rim that is 13" in diameter.

I'm measuring the diameter of the wheels, but I didn't realize it was incorrect to measure to the very edge of the wheel from the outside edge. So, while the hubcaps are about 15 inches in diameter, the rim diameter and the wheel diameter aren't the same thing. Hence, by the original tyres supplied (from the manual), they are 13" rims.

http://www.autoamenity.com/FAQ/WheelSize.htm
http://www.raceandroad.com/acatalog/Alloy_Wheel_Size.html

So, mystery solved. Apparently I'm learning.