When you turn your car AC on, it gets energy from the alternator, which comes from the engine, which uses fuel in the gas tank. The air conditioning won’t work properly without the engine running because the belt that engages the compressor only runs when the car starts. This is the same belt that’s responsible for charging the battery and operating the alternator.
The effects of two methods (windows up and car AC on, versus windows down and AC off) has been debated and tested by several organizations. A leading group, the SAE (Society of Automotive Engineers) performed a number of tests on the effects of air conditioning versus windows down on cars. Results showed that running an AC in a vehicle can actually decrease gas mileage by 5-10 percent.
Both the SAE and Stanford University both recommend traveling with the car windows down if you’re driving on city streets. But when on a highway or freeway, the drag produced by rolled-down windows exceeds the efficiency drop by keeping the windows up and the car AC on. Both agree that a car’s air conditioning should be used when driving at speeds over 45 mph.
With this in mind, it’s best to do your driving in the evening or in the early morning, if possible, because this is when the weather is usually cooler.
Car AC and heating systems are complex. You should never attempt to DIY unless you are a trained mechanic. Rather bring your vehicle to us at Kevin’s Car Repair in Westerville, OH.