How much is a gas cap?

Depends on car type

Most cars will travel 350-400 highway miles on a tank.

Smaller car with better mileage will have a smaller tank than a larger car with lower mileage.

As a guide....

Economy, compacts 10 - 13 gal

mid Size - 15-18

Full size - 18-20

Luxury and SUVs 20-30

== == An average gas tank holds about 15-18 gallons of gas. somewhere around 15