A tire manufacturer is interested in testing the fuel economy for two different tread patterns. Tires of each tread type are driven for 1000 miles on each of 18 different cars. In a separate experiment, 18 cars were outfitted with tires with tread type A, and another 18 were outfitted with tires with tread type B. Each car was driven 1000 miles. The cars with tread type A averaged 23.93 mpg, with a standard deviation of 1.79 mpg. The cars with tread type B averaged 22.19 mpg, with a standard deviation of 1.95 mpg. Which method should be used to find a confidence interval for the difference between the mean mileages of the two tread types