What saves more gas, driving in a higher or lower gear?

I'm sure this is one of those questions that there's no "simple" answer, so let me specify. Say I'm just cruising along going around 30, making it so I could put my car (98 ford escort SE, btw) in 3rd or 4th and both would run smoothly. Which saves more gas? I'm pretty mind-numbingly dumb when it comes to cars, so save any BS you're going to give me. Best answer goes to the person who clarifys it the best.

The higher gear uses less gas

Lower gear always. Generally don't pass 2500 rpm and you'll save a lot

Higher gear is more efficient.

You need to load your engine so as little power as possible is wasted spinning the engine, and that means using the highest gear that has enough torque to maintain your speed. Some older engines had carburetor problems at low RPM but those should have been scrapped by now. Efficiency is largely about impedance match, and mechanical impedance match means the engine and the ~road are evenly matched.
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https://en.wikipedia.org/...er_theorem

On level ground Higher gears, On hill climb lower gear. Try to keep the tach between 2000 and 3000 rpm.

In a higher gear, the engine is moving more slowly and thus will use less gas - you may have to down shift when you encounter a hill to maintain speed.

Automatic? Use 'D'.

Based on some of the answers here there's some confusion. To put it simply, the higher the gear the slower the engine needs to turn and the less fuel you use.

That is why transmissions have a 4th gear or overdrive.

Driving as slow as you can while maintaining the highest gear.

This may change a bit for acceleration, but when you are simply cruising down the road, the higher gear will save a bit of gas. This is because the engine will be turning at a slower rate and will therefore be burning less gas and air (the engine is set up to always burn about the same amount of gas in each cylinder every time the spark plug fires, and how often the spark plugs fire is directly related to how fast the engine is turning. Without throwing a bunch of technical terminology at you, the faster the engine is turning (higher RPMs), the more gas you will burn. As you have probably already seen on your tach, the engine will turn at a lower RPM in 4th gear at 30MPH than it will in 3rd gear. Hard acceleration will also burn more gas (at least in part because the engine is revving higher), so if you accelerate gently and put the trans in a higher gear when cruising, you should get better gas mileage and also slightly longer engine life (because when it strains more, it wears out faster, like in hard acceleration).