We all know that turning off lights in unoccupied rooms is not only economically smart, it’s good for the planet. Lightbulbs use electricity. Electricity costs money and its production harms the environment. Turning off lights is a simple win-win.

But here’s a thought experiment for you: How much does your car’s mileage change when its headlights are on? Is it even measurable? Even when your car is idling?

Let’s do some quick math. Based on sources I cite here, a gallon of gas is considered to be the energy equivalent of 34kWh of electricity (34.02, actually. You know this ridiculous false precision is the result of politics, but this is the number that MPGe is based on.) That’s the amount of energy needed to light 340 100-watt incandescent bulbs for an hour.

But to make the comparison fair, we have to factor in the energy required to get the gas into your tank, and the energy lost to bring the power to the socket in your wall. The math I used in the previous episode resulted in a gallon of gas requiring 5 gallons of gas to reach the pump (refining takes a lot of energy), while it takes 2kWh of additional electricity to bring you the 1kWh you burn. We then divide the gasoline energy by six, and the socket electricity efficiency by three, and that brings us to a slightly-more-honest conversion factor of 68kWh of wall-socket electricity per gallon-in-tank of gasoline. Let’s call it 70 to avoid any pretense of false precision ourselves.

That number will move violently depending on where your gasoline and electricity comes from. 70kWh/gallon is just some wild-assed number based on broad averages, but it’s a number less false than most.

Let’s say, to keep the math easy, that you drive a car that gets about 30 miles per gallon (most of us don’t). That comes out to about 2kWh per mile. Roughly speaking, all the lights in your house, on all night, is about the same as driving one mile in your car (if your car is relatively efficient).

One damn mile.

Unless, of course, you’ve converted to compact fluorescent and LEDs in your home. In that case one damn mile could light your house for a week.

Let’s look at things another way, as long as we have our calculators out. The question: how much energy do I save if I choose a car that gets one slim mile more per gallon?

Of course, that depends on how much you drive. But let’s say you tune up your car and inflate your tires and your mileage improves from 30 to 31. And entirely doable scenario.

We’re going to have to go to unsupported decimal points here, so the numbers that pop out are only valid when compared to each other.

70kWhpg/30mpg = 2.33 kWh per mile
70kWhpg/31mpg = 2.25 kWh per mile

Savings: 700 Watt-hours per mile

Improving your mileage from 30mpg to 31 saves about as much energy as you use burning a 70w incandescent bulb for ten hours, every damn mile you drive. Improving from 20mpg to 21 saves more than twice that. Have your checked your tire pressure lately? That simple act could mean a lot. All your other good-energy-citizen choices are dwarfed by your choice of car, and the way you maintain it.

The takeaway here is simple: Yes, please do buy energy-star appliances. Turn off the lights when you leave a room. Be smart. But don’t go the extra mile. Not in your car, at least.