A Staff Report from the Straight Dope Science Advisory Board

Does using your car radio reduce your gas mileage?

March 29, 2001

Dear Straight Dope:

As a hot-rodder and engineer, I have wondered about this for a long time. Theory says that work costs energy, and that there is no free energy. So, here's the query: does your car go slower when you play the radio? In other terms, how many miles-per-gallon does playing the radio cost you? Those may be the easy ones: what about the heater? (the heat, not the fan, that is)

Ah. Confusion. Cecil has trained us to sniff it out and alleviate it immediately or be sent back to The Box. Don't ask about The Box. For those who know, no explanation necessary. For those who don't, none will do.

I admit I found this one a little confusing myself. Initially I thought playing the radio had no effect on gas mileage. My reasoning went like this.  Electrical power for your car is supplied by the alternator. Essentially a small generator, the alternator is powered by a fan belt driven by the engine. As long as the engine is running, the fan belt isn't slipping, and the unit itself is in working order, the alternator creates electrical energy, whether you have the radio, the heater fan, or any other electrical item on or not. Outflow is controlled by the regulator. Some of the flow is directed back to the car's battery, to keep it charged and to replenish the energy you sapped by starting the vehicle. In any case, what you have turned on isn't going to affect the engine's spinning or have any effect on gas mileage. Different systems.

However, Little Ed, who edits these articles, wasn't having any of it. He wrote:

My understanding of what you are saying is that the alternator is spinning, and thus generating electricity, regardless of whether any of that juice is actually being used. This is not the case. With no load, the alternator is simply freewheeling. Once a load is placed on the alternator, it becomes harder to turn, and the engine must do more work (and consume more gas) to make it do so. You have experienced this yourself if you have ever used an exercise bike such as a LifeCycle. These bikes drive a generator that operates the internal electronics. When you increase the level, additional load is placed on the generator and the pedals become harder to turn. Same deal in a car.

This caused a (female) smart-aleck on the Straight Dope Science Advisory Board to comment: "Oooh, I love it when guys butt chests about cars."

Okay. I talked to five folks more knowledgeable about this stuff than I am, and boy, what a debate it turned into. Two insisted that using your car's electrical goodies had absolutely no effect on mileage, no way no how. One said he wasn't sure. Two agreed that the load increase would certainly make the alternator work harder and become more difficult to turn (so, um, it would seem that both myself and my original source of info--my father--were wrong. Look, I admitted it), BUT--BUT--both insisted that even in a fully loaded hooptie with the big speakers and the lights running around the license plates and the purple light glowing underneath, the effect on gas mileage would be negligible.  One even ventured a guess of about "1/4 a mile per gallon, if you had the rear defroster, the blower motor, the windshield wipers, etc., going." The latter two guys seemed the most sure in their convictions. That's what I get for trusting the wisdom of my father. I wonder if I've been getting sex wrong all this time as well.

But you wanted an exact answer. Local resources having been exhausted, I turned to the Web and found the following: 

Gas mileage . . .  is the reciprocal of the force exerted by the engine on the car [which] can be represented by a sum of seven kinematic elements: 

F = ma + (P0 +Tw)/v + mg(mcosq + sinq) + (rAf + m/x0)v2/2 

The contributions are due to acceleration, auxiliary power usage such as radio, AC and headlights, transmission drag, rolling friction, hills, wind drag and stopping (http://usna.edu/Users/physics/schneide/Buick.htm). 

Does that clear things up for you? Well, maybe the Car Talk guys are more your speed. Addressing the question you raise, they write:

Ray: . . . [A]ny device in the car that uses energy; the air conditioner, the headlights, the portable juicer, and even the radio does reduce your gas mileage by some amount.

Tom: But the amount of energy consumed by the radio is so miniscule that it makes absolutely no difference whatsoever. Remember, you can run a radio for months on a little pocket battery! So it's really not even worth thinking about.

Maybe not the radio. However, here's something from a page entitled "101 ways to improve your gas-mileage": 

7. Use electrical appliances carefully. The worst one is the rear window heater, this takes about 3 to 5%. Also remember to turn off fog lights when not needed any longer.

This contradicts the claim of one of my guys that running all the car's electrical equipment, including the rear defroster, reduces gas mileage by a trivial amount. You know what? I'm not getting into the middle of this. I'll let them fight it out.  

Finally, the heater. That's different. Heat comes from the antifreeze, which is warmed in the engine and runs to the heater core, which looks like a little radiator. The heat is a byproduct of combustion. If you didn't use it to warm the car's interior it'd escape to the atmosphere by way of the car's radiator (the one in the front of the car). Unlike electricity, the heat is being generated whether you use it or not, so you don't reduce the car's gas mileage by switching it on, other than what's needed to run the blower motor.

Whew, what a workout. Now go burn some rubber with the tunes cranked.

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