1. ## Car radio and milage

Two identical automobiles start together on identical paths. One has the car radio on, the other has it off. They go till they run out of gas.
Will the one with the radio off go slightly farther?

2. Theoretically yes, but I think it would be impractical to measure. Truly identical cars is an impossibility, identical drivers much worse.

I once had a 1974 Honda Civic with a 1200cc engine. Idle RPM would drop noticeably when I turned the headlights on. I could even here the revs go up and down in time with a turn signal. But lights are going to draw more current than a modern radio, I'd think.

Proposed test setup:
1. Use the same car for both with and without radio.
2. Start with an accurately measured quantity of fuel fed from a special container.
3. Drive at a constant speed under cruise control on a closed course, a long oval or large circle.
4. Turn the radio up LOUD!

3. Originally Posted by Tom Mazanec
Two identical automobiles start together on identical paths. One has the car radio on, the other has it off. They go till they run out of gas.
Will the one with the radio off go slightly farther?
Theoretically, yes. The radio's power requirement places a greater load on the vehicle's alternator, which in turn puts more of a load on the engine, thus requiring more fuel. It's small demand though, so I doubt you could measure a meaningful difference in the real world over the course of one tank full of gas.

4. The car radio has other uses ...

(It's the 11:01 pm, Nov 19, 2019 entry.)
Last edited by DonM435; 2020-Nov-23 at 06:43 PM.

5. A little googling gave me that 5 amps is typical for a car radio. I found a converter for converting amps to horsepower (I think this is for a 5 amp motor, but whatever). Assuming 12 volts and 90% efficiency, I get 0.065 horsepower. Given that a typical car is 100 to 120 horsepower, this is almost unmeasurable.

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5Amps @12V = 60 Watts. Wow.

7. 60 watts is about the same as one headlight, often 57 watts. There are drivers who avoid using lights at dusk to hopefully save fuel, but it is negligible. Air con uses much more. The car radio may be rated for 60 watts or more but typically uses much less unless you like really loud music. Led lights will use less as they become standard. Less well known is that the air humidity and temperature are more significant in engine power and hence fuel economy.

8. It may depend upon which radio station is tuned in, certain AM frequencies produce much more hot air than others.

9. Mythbusters tested "air conditioning vs windows open" more than once. The first time they tried it, it came out even. It isn't, actually. Drag from windows open would increase by the square of the speed. At low speed, windows open is better. At high speed, AC is better. They'd happened to test at the crossover point, as they revealed later.

5A/60W sounds like an awful lot for a car radio. At least, for a normal one, not a boom car. Maybe that's the size of the fuse!
Last edited by Trebuchet; 2020-Nov-24 at 05:38 PM. Reason: 5 Amps, not Volts!

10. In my youth there was a promotion competition to drive, I think, a triumph herald, fitted with an accurate meter. The tactic is to accelerate at full power, max efficiency and then coast engine off, judging the moment to repeat to average the target 30 mph. Later a petrol project achieved well over 100 mpg doing that. Not very suitable for normal driving. But driving style makes a big difference to petrol engined cars. Radio use not so important! Soon to be a relic of history as hybrids or pure electric take over. The hybrid is like the old herald, the IC engine , whatever is chosen, can run at max efficiency. Even that probably is beaten by electricity generation, even if fossil fuels still have a share. Car radios can also play a small part!

11. Originally Posted by profloater
<snip> But driving style makes a big difference to petrol engined cars. Radio use not so important! Soon to be a relic of history as hybrids or pure electric take over. The hybrid is like the old herald, the IC engine , whatever is chosen, can run at max efficiency. Even that probably is beaten by electricity generation, even if fossil fuels still have a share. Car radios can also play a small part!
Actually, driving style is very important for maximizing efficiencies of hybrid cars too, and there are also sorts of things that people do to try to maximize it (I do some myself). Turning off the radio isn't one of them.

12. Originally Posted by Spacedude
It may depend upon which radio station is tuned in, certain AM frequencies produce much more hot air than others.
Which is why they wouldn't let Howard Cosell do the NFL on radio.

13. Originally Posted by Swift
Actually, driving style is very important for maximizing efficiencies of hybrid cars too, and there are also sorts of things that people do to try to maximize it (I do some myself). Turning off the radio isn't one of them.
Yes, I can see that is true especially with many types of hybrid. Some retain mechanical drive from the engine, while others just charge the battery. Many cities now have electric only zones. I have been looking at second hand electrics, like the Nissan Leaf, with a view to adapting to be hybrid. That is just a hypothetical exercise of course, not legal to drive on public roads. But the radio would not need adaption.

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