A courier service buys four new cars of the same make and model, and they are driven under similar city and suburban conditions. Each time the cars are filled with gasoline, the car's mileage and amount of gasoline required to top off the tank are noted. At the end of a year, the company manager uses these data to calculate the gas mileages achieved by the cars between their fillups. This data set is given in DS 11.4.2. Are any of the cars getting a better gas mileage than the others? Use a computer package to obtain a graphical presentation of the data set, an analysis of variance table, and pairwise comparisons between the different cars.