* Check your owner's manual: This is the most reliable source.
* Look at the gas tank door: Sometimes the size is printed on a sticker there.
* Check your vehicle's identification number (VIN): A VIN decoder might provide this information.
* Consult a Dodge parts website: Using your year and model, you might find the specification.
Generally, older Dodge D100s tended to have smaller tanks than newer vehicles. Expect to find capacities ranging from around 16 gallons to possibly up to 20 gallons, but without more specific information about the truck, that's just a guess.