The distance you'll be able to drive on 18 gallons of gasoline depends on the miles per gallon that your vehicle uses.
For example my truck gets an average of 15 miles per gallon so on 18 gallons I could drive 270 miles before I used the 18 gallons of gasoline.
If your vehicle gets 20 miles per gallon you could drive 360 miles and if your vehicle got 38 miles per gallon you could drive a distance of 684 miles on the 18 gallons of gas.
To figure out how far your vehicle can go on 18 gallons of gas just take your miles per gallon and multiply that amount times 18 and you'll get the miles you can drive.
For example 15 miles per gallon x 18 gallons of gasoline is 270 miles.