Yes, AC impacts fuel economy

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Indeed, AC impacts fuel economy - though the exact extent depends. Image from Wikimedia Commons.

Indeed, AC impacts fuel economy – though the exact extent depends. Image from Wikimedia Commons.

In case anyone still thought it didn’t really, AC impacts fuel economy and performance. There are other things which can make fuel economy suffer in summertime, but turning on the air conditioning will sap a few miles per gallon.

AC impacts fuel economy due to extra work

Anyone ever hear someone say “I don’t run AC because fuel economy suffers” or something to that effect? It may be increasingly rare for anyone to still think it’s bogus, but there are a few idiots out there who don’t entirely believe it.

Granted there are also people out there who think voting still matters…suckers!

Anyhow, why would an air conditioning unit cause a car to get worse fuel economy? It isn’t as if it uses gasoline. Well, the answer is that it increases the workload of the engine. To run the air conditioner, the engine’s crankshaft and timing system has to run a little faster to turn the crankshaft of the conditioner itself and power the air conditioning system.

That said, it doesn’t add up to much, but it’s still a few fewer horsepower and revolutions per minute that are being devoted to spinning the output shaft of the engine, which goes to the transmission and to the differentials and make the wheels spin. That means it takes a little more work for the motor to net the same performance.

Efficiency, as some may remember from middle school physics, is a factor of the ratio of input to output. Greater input to yield the same output means less efficiency, and thus fewer miles per gallon.

By how much?

Well, the extent that AC impacts fuel economy is not disastrous, but certainly detrimental.

Television show “Mythbusters” conducted a test, according to Discovery by putting 5 gallons of fuel in two identical trucks and driving until they ran dry. One had AC going, one did not. The truck that ran sans AC lasted an additional 15 miles.

As they were pickups, you can assume air conditioning can reduce fuel economy by up to 20 percent; 15 mpg is not far from what most pickups achieve and with 5 gallons in the tank, that’s an extra gallon’s worth.

While a number of studies by various universities and other research organizations have all found universally that running air conditioning does lower fuel economy, the degree to which it does varies.

A Society of Automotive Engineers study (and a similar Stanford University study) both found a 5 to 10 percent reduction, according to The Department of Energy cites another SAE study which found a fuel economy reduction of up to 25 percent was possible in some cases.

However, the SAE has also repeatedly found driving with the window down is worse than running the AC.

All studies found certain conditions exacerbated the dip in economy; hybrids and electric cars fared worse than traditional drivetrains. Speed is a factor as are driving conditions; the AC system is less of a burden on the highway than in urban traffic.

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