Fill-up time: regular or premium?

Fill-up time: regular or premium?

By Russ and Tiña De Maris

“If a little is good, a lot more must be better!” It’s a common equation to a lot of problems, but as you’ve no doubt experienced, “It ain’t necessarily so.” And so it is when you pull up to the pump with a hungry fuel tank. Do you want to pump in “regular,” “midgrade,” or that high-octane “premium” fuel? Is a bigger octane number better?

The “octane rating” listed on a fuel pump is a measure of the ability of the gas to reduce engine knock. “Regular” must have a minimum octane rating of 87, midgrade from 88 to 90, and premium needs a rating of 91 or higher. The higher the rating, the slower the fuel burns, preventing “knock” when engine cylinder pressures are high.

When an engine runs at full throttle, cylinder pressures are naturally at a high level — you can expect a bit of knock then. Towing a rig, particularly when climbing a steep hill, calls for higher cylinder pressures resulting in some ping. But prolonged engine knock can cause damage. Fuel-injected gasoline engines automatically adjust for driving and performance conditions, squeezing the most mileage out of whatever gas your vehicle is burning.

What gas should you use? As a rule of thumb, fuel up with the octane grade your manufacturer recommends. Pumping in premium or even midgrade gas when your vehicle calls for regular won’t improve fuel mileage or engine performance, but will reduce your wallet’s thickness. If you consistently have knocking issues while towing, you may want to consider pumping in a higher-octane fuel. We say “may” with this caveat:

If your engine knocks or pings it isn’t necessarily the gas — it could be you have an engine problem needing work. You could have electronic control systems, ignition timing or exhaust gas recirculation problems. If you’re dealing with an older or high-mile rig, it’s possible you have a buildup of carbon in your cylinders causing the problem. Once engine problems are ruled out, consider pumping in the higher octane fuel. It won’t hurt your engine, and if knocking vanishes, then you may find using higher-octane fuel when towing helpful.

Gas with added ethanol can have an impact on your RVing. Ethanol, or grain alcohol — typically produced from corn — is often added to gas to reduce air pollution. Grain alcohol doesn’t have as much energy per gallon as gasoline, so your tow vehicle or motorhome will indeed use more gas per mile driven. Industry says you may see a 3 percent drop in fuel economy with the use of 10 percent ethanol — many drivers say they lose a lot more. If you fill up without ethanol, or at least less than 10-percent ethanol, you’ll get better fuel economy.

photo: Barbara L. Hanson on flickr.com

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