18 wheeler trucks or semi trucks use diesel fuel to operate and they contain 2 diesel tanks that are around 150 gallons each.
So each semi truck holds on average of 300 gallons of diesel fuel to fuel the semi truck so they can usually go quite a bit of distance before refueling.
The average semi truck gets between 5 to 7 miles per gallon of diesel fuel so they require lots of diesel on board to make their trips.
With an average miles per gallon of 7 miles per gallon and 300 gallons of diesel fuel in both tanks the semi can go up to 2,000 miles until they use all the 300 gallons of diesel fuel.
However no semi truck driver goes non stop for 2,000 miles because that would be impossible and they usually will refuel when they stop and need to refuel before they run out.
Semi Truck drivers are limited by law to the amount of hours they're allowed to drive for safety reasons so they must stop at the limit of driving and rest until the next day before driving again.