my '90 XJ gas tank was 18 gallons... it seems like yours is right... you might be able to get a shot of the factory wheels in a stock photo from http://www.kbb.com/
The 1988 model year for Cherokees came with a standard 13.5 gallon tank or optional 20 gallon tank. I assume the Commanches and cherokees has the same options, as they had alot of things in common.