It's so hard to tell what size of gas tank you have without actually looking at it because Rangers come with some many different sizes. For instance, I have a 92 with a 21 gallon tank, but my friend has a 89 with a 15 gallon tank.
However, I just did read a little thing that is interesting. Apparently, in 91, there was a re call on the gas tanks because they cracked.
Also, I just read that the 91 Explorers only came with the 19.3 Maybe that's the tank you have?
Here you go:
Although I don't totally agree because I have run my truck to almost dry and have put 20.7 gallons in mine.
1992 Ford Ranger XLT 4X4
Stock tires, blown engine, aesthetic accessories.
2005 Chevy Silverado 1500 ex cab 4X4 Auto
4.8 V8, No options, other than AC, Four Door
Kenwood CD/MP3/WMA/USB and Sattelite Radio Ready Head Unit, AEM Brute Force CAI with a K&N cone filter, 33X12.50/17's