Data Collection

 
POPULATION OF INTEREST
    The U.S. Environmental Protection Agency (EPA) and U.S. Department of Energy (DOE) produce the Fuel Economy Guide for U.S. car buyers, only the cars included in this guide will be the population of interest for this project. The Vehicle Certification Agency (VCA) in the U.K. does similar testing to the EPA but since several manufacturers that sell cars in the U.K. and Europe don't exist in the U.S. it would be a waste of time doing full comparisons. So by simply comparing the best of both lists we should be able to get a good average. The best diesel and gasoline cars the VCA tested will be compared against the best cars the EPA tested. Trucks, vans, and SUV's will not be compared, neither will station wagons.
HOW DO THE EPA AND VCA ESTIMATE FUEL ECONOMY?
    According to the EPA they estimate fuel economy using the following guidelines:
"Each vehicle has two fuel economy estimates: A city estimate that represents urban driving, in which a vehicle is started in the morning (after being parked all night) and driven in stop-and-go traffic. A highway estimate that represents a mixture of rural and interstate highway driving in a warmed-up vehicle, typical of longer trips in free-flowing traffic. These fuel economy estimates are based on laboratory testing. All vehicles are tested in the same manner to allow fair comparisons.
This Guide provides annual fuel cost estimates for each vehicle. The estimates are based on the assumptions that you travel 15,000 miles per year (55% under city driving conditions and 45% under highway conditions) and that fuel costs $2.83/gallon for regular unleaded gasoline and $3.06/gallon for premium. Cost-per gallon assumptions for vehicles that use other fuel types are discussed at the beginning of those vehicle sections. The fuel costs were determined in advance to allow time for printing fuel economy labels and the Guide and may not reflect current fuel prices."If you want to know in much more detail about how those lab tests are conducted you can go to http://www.epa.gov/nvfel/ for honestly, too much information.
    The U.K. follows these guidelines which are a little more straightforward:
"Official fuel consumption test procedures have been in use since the 1970's. EU Directive 80/1268/EEC (as last amended by 2004/3/EC) describes the tests which all new cars on sale after 1 January 2001 have been required to take.
FUEL CONSUMPTION TEST  (Directive 80/1268/EEC as amended by 2004/3/EC)
The current test has been agreed internationally and provides results that are more representative of actual average on-road fuel consumption than previous tests. There are two parts: an urban and an extra-urban cycle. The test cycle is the same as that used to determine the official exhaust emission classification for the model of vehicle in question. The cars tested have to be run-in and must have been driven for at least 1,800 miles (3,000 kilometres) before testing.
Urban cycle
The urban test cycle is carried out in a laboratory at an ambient temperature of 20°C to 30°C on a rolling road from a cold start where the engine has not run for several hours. The cycle consists of a series of accelerations, steady speeds, decelerations and idling. Maximum speed is 31 mph (50 km/h), average speed 12 mph (19 km/h) and the distance covered is 2.5 miles (4 km). The cycle is shown as Part One in the diagram below.
Extra-urban cycle
This cycle is conducted immediately following the urban cycle and consists of roughly half steady-speed driving and the remainder accelerations, decelerations, and some idling. Maximum speed is 75 mph (120 km/h), average speed is 39 mph (63 km/h) and the distance covered is 4.3 miles (7 km). The cycle is shown as Part Two in the diagram below.
Combined Fuel Consumption Figure
The combined figure presented is for the urban and the extra-urban cycle together. It is therefore an average of the two parts of the test, weighted by the distances covered in each part.
The fuel cost of driving 12000 miles is calculated using the combined fuel consumption figure and an average fuel price which is assessed each year. Currently it is 88p/litre for petrol, and 100p/litre for diesel and 51p/litre for LPG."
You can click here for more information on the regulations and testing procedures in the European Union.


SAMPLING METHOD
    Instead of comparing every car on the EPA list, I used a systematic random sample to choose the cars and reduce bias as much as possible. First I separated them according to their EPA Vehicle Class. From each class of vehicles I broke it down according to engine type, and from there each car in the list was given a number. To make the amount of data manageable I only took 5 cars from each engine size, when five were available. I then went to http://www.random.org/intergers to generate a set of five random numbers from zero to however many cars were counted in that category. Then the median car in the list of each class and engine size was picked. Depending on what the comparison is the data could be bias for either the U.S. or the E.U. because not all European cars were counted, only the best. Also the U.S. will show many more V6, V8, V10, and V12 power-plants which could potentially skew results.

VARIABLES MEASURED
  • Engine Type
  • Horsepower
  • Fuel Type
  • EPA Vehicle Class
  • City MPG Estimate
  • Highway MPG Estimate
  • Transmission Type
  • Annual Fuel Cost Estimate
  • 0-60 mph, sec.
  • Price
  • Country of Sale
MEASUREMENT ISSUES
    Because of the need to maintain strict comparability of results achieved by the standard tests they cannot be fully representative of real-life driving conditions. Firstly, it is obviously not practicable to test each individual new car; thus only one production car is tested as being representative of the model and may therefore produce a better or worse result than another similar vehicle. Secondly, there are infinite variations in driving styles and in road, car and weather conditions, all of which can have a bearing on the results achieved. For these reasons the fuel consumption achieved on the road is unlikely to be the same as the official test results.  Prices for European model cars had to be converted, as did annual fuel cost, this could also affect numbers slightly.