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This really opened my eyes when I read it. For years I had wondered how the EPA did the testing and came up with the numbers they post on the window sticker. This was pulled directly from the EPA website describing their testing protocol. Fuel Economy - On-road Vehicles and Engines | Cars and Light Trucks | US - EPA. What really stunned me was that the "highway testing" was done to the equivalent of a 10 mile trip at an average speed of 48 miles per hour. Really? Does anyone else see this as completely out of touch with how Americans drive on the highway?

At any rate, here is a partial quote from the EPA website and I encourage you to read the website for yourself by clicking on the link I provided above.

"How are Vehicles Tested?

Vehicles are driven over identical driving patterns by professional drivers in controlled laboratory conditions on a dynamometer, which is like a treadmill for cars. The conditions that occur during driving, such as wind drag and inertia are accounted for on the dynamometer. There are two types of tests that are conducted: city and highway tests.

The city test is approximately 11 miles long and simulates a stop and go trip with an average speed of about 20 miles per hour (mph). The trip lasts 31 minutes and has 23 stops. About 18 percent of the time is spent idling (as in waiting for traffic lights). A short freeway driving segment is included in the test. The engine is initially started after being parked overnight.

The highway simulates a 10 mile trip with an average speed of 48 mph. The vehicle is started "hot" and there is very little idling and no stops.

How Are the Label Estimates Calculated?

Fuel economy estimates are calculated from the emissions generated during the tests using a carbon balance equation. We know how much carbon is in the fuel, so by precisely measuring the carbon compounds expelled in the exhaust we can calculate the fuel economy.

After the vehicles have been tested, the results are adjusted downward to account for conditions that occur on the road that can affect fuel economy which don't occur during laboratory testing, such as cold temperature, aggressive driving, excessive use of power-hungry accessories, among others. The city is adjusted downward by 10 percent, and the highway by 22 percent.

The equation for calculating the city or highway average fuel economy, given in miles per gallon (mpg), is:
FEave = (total sales / [(sales1/FE1)+ (sales2/FE2) + ...+ (salesn/FEn)]
The calculation for combined fuel economy weights the city at 55 percent and the highway at 45 percent using the following equation:
FEcomb = 1 / (( .55 / city FE) + (.45 / hwy FE))"
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