Dimensionally, fuel economy is miles divided by gallons. Then, presented with more
than one fuel economy value, an approach to averaging the values is to compute the result by
determining the total miles traveled and dividing that by the total gallons used.
Example: A motorist’s fuel economy log for May shows that 704 miles were accumulated
around town in which the fuel economy was 16 mpg, and one 216 mile highway trip was taken
on which the fuel economy was 24 mpg. What is the average fuel economy for May?
The total miles are 704 + 216 = 920. The total gallons thus are 704 / 16 = 44 plus
216 / 24 = 9 or a total of 53 gallons. The average mpg is 920 / 53 = 17.4 mpg. Notice that the
arithmetic average of the two fuel economy values (16 + 24) / 2 = 20 mpg gives an individual
result that is higher than the total miles/total gallons result.
