Car & Driver: The Truth About EPA Estimates

Since the late 1970s, the U.S. Environmental Protection Agency has certified the fuel-economy projections of some 450 million new vehicles sold in this country.

The projections are there by law and appear boldly on the window stickers of new vehicles, for example, CITY MPG 16, HIGHWAY MPG 25. They appear authoritatively, almost like a pledge from the federal government, and motorists have put a lot of faith in these numbers.

The EPA figures also determine whether an automaker meets the required fuel-economy averages for a company’s entire vehicle line, so the numbers are a very big deal. As such, you might expect the federal government’s facility to be about the size of, oh, the Department of Agriculture and loaded to the brim with persistent bureaucrats.

While the public mistakenly presumes that this federal agency is hard at work conducting complicated tests on every new model of truck, van, car, and SUV, in reality, just 18 of the EPA’s 17,000 employees work in the automobile-testing department in Ann Arbor, Michigan, examining 200 to 250 vehicles a year, or roughly 15 percent of new models. As to that other 85 percent, the EPA takes automakers at their word—without any testing—accepting submitted results as accurate.

The Truth About EPA City / Highway MPG Estimates - Feature - Auto Reviews - Car and Driver

An interesting feature which may go further to explain why the estimates are, indeed, only estimates - and why it's a lot easier to 'beat' 2008 estimates than earlier ones.

 

I just thought it was an interesting article, that I came across on Autoblog. I didn't realise it had been posted already!

The UK Vehicle Certification Agency writes about our tests here. You can see from their graph that the test is a lot more artificial than the EPA tests which appear to have actually been recorded from real-world driving at some stage. That chart doesn't show the required gears for manual-transmission cars, but each segment dictates which gear the car should be in.

As you can see, the testing is all relatively low speed. There's only an acceleration to 75mph, a short hold for a few seconds, then a long deceleration, right at the end. We desperately need higher-speed tests to reflect what drivers do. Which? magazine (equivalent to Consumer Reports) adds a constant 70mph segment to the mandatory tests, under the same conditions, which drops the 2G Prius from 65.7 to about 56mpg.

As near as I can tell, the numbers quoted are straight off the rolling road, with no compensation (the EPA reduces the city figure by 10% and highway by 22%). It's more scientific, but you're a lot less likely to get the quoted figures. The test results are actually recorded in litres per 100km (L/100km) and rounded to the nearest 0.1L/100km. This rounded result is then converted to miles per imperial gallon for the UK market, since we're the only ones still using legacy units. (Petrol is sold in litres, so an mpg display is pretty useless.)

The Gen 2 Prius got a figure of 65.7mpg; the MFD is showing 62.2 over the last six months. My calculations are closer to 60 for the same period, but I've had some tanks come out over 63. My last tank was a short fill (pump cut off early) which skewed the figures somewhat - it came out at 71.3! Lifetime average is 56.9.

My previous car, a 2001 Ford Focus 1.6L with 5-speed manual, got 40.9mpg on the test. My spreadsheet says my lifetime average was about 37.8mpg.