Fuel-saving tips: a closer look

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NEW YORK (CNNMoney.com) -- Two recent stories we did on gas saving tips resulted in a lot of reader email. And three tips in particular generated some pretty heated debate.

So we decided to take a closer look at them.

We said it made little sense to get up early in the morning to pump gas. Some people say they do it because gas is colder, therefore denser, in the cool morning hours, which means you could get more gas in your tank for the same money.

Recent news reports have cited inequities caused by temperature differences among various regions of the country. It seems drivers in New Hampshire can get more gasoline in a fill up than people in South Florida because the gas in Florida is hotter, so it's less dense.

Gasoline expands by about one percent in size with every 15 degrees in temperature. So filling up with 10 gallons of gas that's 30 degrees hotter would cost you about 80 cents worth of gas at today's prices. That's not a huge amount, but it's nothing to sniff at.

But even that little bit results from a much, much bigger temperature shift than you'd ever find happening in one day inside a thick-walled underground storage tank. The temperature of gasoline at your local service station simply doesn't change that much during the course of a day.

Some readers also pointed out that many gas stations raise prices during the course of the day, making it cheaper to pump in the morning. That is indeed true. But why not pump your gas the night before? When the price of anything rises, the early bird catches the deal, no matter what time of day.

We said that it's a waste of money to put premium gas into a car for which it's not recommended. It'll do nothing but cost more. Nobody argued with that idea.

We further mentioned that in many newer cars for which premium is "recommended," but not "required," it's perfectly fine to use regular gas. It won't hurt the engine.

Some readers said we were offering bad advice. Really, what we're recommending is that you follow your owner's manual instructions.

If the manual says high-octane premium gas is required, you should absolutely continue to use it. Your manual will usually tell if you using regular is an option. If you're at all unsure, contact the carmaker's toll-free customer service number or your dealer's service department.

Beyond that, some readers also said using regular gas in a car for which premium is recommended would result in significantly reduced fuel economy, wiping out any possible financial benefit.

Their argument goes like this: Once engine sensors detect that regular instead of premium is being burned, the spark that ignites the fuel in the cylinder will be slightly delayed. That results in less energy output, which means the engine just has to work harder.

They're partly correct. Premium gas does make the engine perform more efficiently, but where it makes the difference is under high engine loads, such as hard acceleration, according to Rik Paul, automotive editor for Consumer Reports. But during normal, light-load driving, he said, the engine will operate just the same with regular or premium.

Ford Motor Company agrees. "Any difference in miles per gallon is negligible," said Ford spokesman Alan Hall.

While we can all agree that tailgating is dangerous, many readers said it would actually save gas, not waste it.

Many of those readers cited NASCAR racing. It's a routine strategy for stock-car drivers that helps them go faster. It's called "drafting." A driver will pull right up to, and even hit, the rear bumper of the car ahead. That way, the car in front is pushing air aside while the car behind gets to ride in the relatively air-resistance-free zone behind it.

So yes, these readers are correct. At least in theory.

You could get some fuel economy gain from closely following a car in front of you, says Pat Suhy, NASCAR racing manager for General Motors. But to get even the slightest benefit you'd have to be within four to six feet of the bumper of the car ahead. To get a meaningful boost, your cars would need to be almost touching.

On the highway, where you're moving about 90 feet per second, even four to six feet isn't much cushion space. In short - it's pretty dangerous, and there's no way we could ever recommend it.

For argument's sake though, let's look at it more closely. The amount of fuel you save and the distance you need to be from the car in front of you depends on the vehicle you're following.

Bigger and less aerodynamic is better, said Jerry Drummond, an associate professor specializing in vehicle aerodynamics at the University of Akron in Ohio. A tall semi will block more air in front of you than a Volkswagen Beetle, so you wouldn't have to travel quite as close.

But tailgating a big rig is even more dangerous than tailgating a car since your forward view is completely blocked, and the truck driver can't see you either.

At such close distances, you'd also find it hard to maintain a steady speed. You aren't on a race track, and the driver in front of you is unlikely to cooperate with your annoying -- and dangerous - efforts to save fuel.

Any speed reduction by the car you're following would mean you'd need to slow down even more, and then you'd need to hit the gas to keep up. Most drivers will simply move aside to get away from you.

All of this results in speed changes that waste fuel.

Given all the variables - different vehicles, different distances, different speeds, different driving habits - it would be impossible to say how much fuel drafting could potentially save, if any, in real-world driving, says Drummond.

At any rate, any efficiency gain would be short-lived. And so might you. Drafting still works best on the race track, not on the highway.