Most cars don't tell you how many gallons are left in the car, just approx how much of a full tank. So mpg doesn't help much in your example, miles per full tan

Most cars don't tell you how many gallons are left in the car, just approx how much of a full tank. So mpg doesn't help much in your example, miles per full tank is a useful number though and from there you can calculate that if you have half tank you only have 300/2=150 miles left. Though most cars will tell you the exact remaining distance to empty in the instrument cluster already so there is no need to do this mental arithmetic anyway.