This time we’re taking aim at HowStuffWorks, which is normally a good site to check out if you’re in need of an explanation of some mechanical gizmo. The guy who answered the latest entry in the Question Archive was sleeping on the job, though.
HowStuffWorks’s answer, after a whole bunch of calculations and assumptions: about 406 million gallons per year.
Where did they screw up? Let’s break it down.
A typical headlight bulb uses about 55 watts; sometimes the daytime running lights run at a lower wattage so they use a little less power. Let’s say the daytime running lights use 100 watts since there are two bulbs.
No problems here. That seems quite reasonable, if possibly a marginal over-estimate.
To calculate the energy used, we need to figure out how much time people will spend with their lights on.
And herein lies the enormous erroneous assumption: that having the lights on places sufficient additional load on the vehicle’s alternator so as to cause extra drag on the engine.
I drive with my lights on literally all the time, in both my car (a 1992 Honda Accord EX sedan) and my motorcycle (a 2001 Suzuki SV650S). On the bike, I don’t have a choice — the headlights are hardwired to the ignition. On the car, however, I do have a choice. So it becomes a very simple matter to test this assumption. Drive for a few tanks of gas with the lights on, and a few tanks of gas with the lights off, and calculate the mileage returned by the vehicle over those tanks of gas.
With the headlights (or DRLs) on, we ought to see a noticeable decrease in mileage if their assumption is correct. We don’t. Q.E.D.
If anyone has hard figures for the no-load drag of a typical car’s alternator, as well as for the drag at various loads, I’d be very curious to know just how erroneous the assumption is.
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