* Your owner's manual: This is the most reliable source.
* The sticker inside the driver's side doorjamb: This sticker often lists the fuel tank capacity.
* A reliable online parts database: Websites specializing in car parts often have specifications, but be sure to specify the exact trim level of your 1999 Dodge Ram.
Without knowing the specific model and configuration of your 1999 Dodge Ram, I cannot give you a precise answer.