Prejudice and preference aside, engineers, scientists and the federal government say there's little need for premium.

The main advantage of premium-grade gas is that it allows automakers to advertise a few more horsepower by designing and tuning engines to take advantage of premium's anti-knock properties. But auto engineers generally agree that if you use regular in a premium engine, the power loss is so slight, most drivers can't tell.

"I go back and forth, and I'm hard-pressed to notice" whether there's regular or premium in the tank, says Jeff Jetter, principal chemist at Honda Research and Development Americas. He drives an Acura designed for premium.

Import brands, especially, use premium fuel to distinguish their upmarket models. Most Toyotas, for instance, are designed to run on regular or midgrade, while the automaker's Lexus luxury brand prefers premium. Same with Honda and its Acura luxury line.

Premium, in fact, sometimes is worse fuel than regular. It resists knock because it's harder to ignite than lower-octane fuels. As a result, some engines won't start as quickly or run as smoothly on premium, notes Gibbs, the SAE fuel expert.

High-test does have a potential fuel economy benefit. It is slightly denser than lower-octane gas, meaning there's a little more energy in a gallon. But the small difference is hard to measure in real-world use, and that same density can contribute to undesirable buildup of waste products inside the engine.

No data show that engines designed strictly for regular run better or longer on premium. [yes I see the word strictly, prove the Fit isn't in this category.]

The Federal Trade Commission, in a consumer notice, emphasizes: "(I)n most cases, using a higher-octane gasoline than your owner's manual recommends offers absolutely no benefit. It won't make your car perform better, go faster, get better mileage or run cleaner."

There is "no way of taking advantage of premium in a regular-grade car," says Furey [chemist and fuels specialist at General Motors].

"There is no gain. You're wasting money," insists Jim Blenkarn, in charge of powertrains at Nissan in the USA.

"No customer should ever be deluded into thinking there's any value in buying a higher grade of octane than we specify," says Toyota's Paul Williamsen, technical expert and trainer.

"The regular fuel will burn properly and the premium fuel will burn properly and therefore there is no reason you should pay the extra money." William Green, a chemist at the Massachusetts Institute of Technology.

But for standard cars on the road today, purchasing premium gasoline is simply paying a premium for a fuel that delivers no added benefits. "If you think you need it," Green says, "you're being very eccentric."

Myth No. 3: Going Premium
The Lie: "More expensive fuel offer better fuel economy, because it's higher quality!"

The Truth: The media has confused this issue terribly. Post-1996 model-year cars virtually all have knock sensors. If the octane rating is too low, the computer will roll back the ignition timing a few degrees to compensate. This will reduce peak engine power, and also increase fuel consumption. So, in some sense, this myth gets it right--if your car is supposed to run premium. One of our long-term test cars in the PM fleet delivers an impressive 25-percent better fuel economy running on premium than it does on regular, although our test was hardly done under rigorous conditions. I'll leave the math for an exercise, but I calculate the price differential between 87- and 91-octane fuel at more like 6 percent--at least in my neighborhood. Your mileage may vary, but it's worth trying to see what happens in your car. If you top 6-percent better fuel mileage on premium, it may save you money to run it.

Of course, older cars, built in the days before knock sensors, may be damaged by running too poor a grade of fuel. Spark knock can actually burn holes in pistons, so burn substandard fuel in non-knock-sensor-equipped cars at your peril.

But wait, there's more! Is your car supposed to run on Regular? There are more BTUs (energy) in regular than in higher grades. You may very well get better miles-per-gallon from regular.

Q What octane-rating gas should I use in my car? It calls for premium, but I get slightly better fuel economy on regular, if you can believe it.

A The nominal octane requirement for any engine is in the owner's manual, or probably even placarded inside the gas-filler door. Most modem, post-1996 vehicles with knock sensors can burn lower grades of gas without the danger of detonation knocking holes in the pistons. The engine-management computer will richen up the mixture and retard the timing enough to make the knock abate. This comes, no surprise, at the expense of power and economy.
Your vehicle, for whatever reason, is happy burning lower-grade gasoline without knocking. What is knocking? When the temperature of the gases in the combustion chamber is high enough, the fuel-air mixture ignites spontaneously and prematurely (before the spark plug fires). It makes a knocking noise, which can damage an engine in a short period of time. Higher-octane gas is more resistant to igniting under these conditions.
Why, then, do you see higher mileage on regular? In the fractional distillation of gasoline from crude oil, the higher-octane-rated fraction has less potential chemical energy, providing less power. Racers have always known that using the lowest-octane-rated gasoline that doesn't melt the pistons provides the most power.