When Something Sounds Too Good To Be True -- or Legal

Dear Car Talk:

Years ago, the difference in price between different grades of gasoline was 10 cents a gallon. Now there's a dollar difference between regular and premium where I live.

On the other hand, a bottle of octane booster is about $4. If premium is recommended for my car, is it OK to add a bottle of octane booster to regular gasoline during each fill-up?

I figure that could save me about 15 bucks per fill-up. But looking at different bottles of octane boosters, one states "barely street legal" and another states "not street legal."

What is this in reference to? -- Doug

Your plan sounds almost too good to be true, Doug. You know why? It is.

First of all, you may be reading the claims on the bottles incorrectly. Octane boosters tend to brag about how many "points" they'll raise your octane. But points aren't the same as octane numbers. Each "point" is a tenth of an octane number.

So, if you buy a product that says it'll raise 5 gallons of 87 octane gas by 7 points, you won't get 94 octane. You'll get 87.7 octane. Disappointing, right?

Of course, the other problem is the "street legal" stuff. In many cases, the main additive used in those products is something called MMT. That's methyl cyclopentadienyl manganese tricarbonyl. But you probably knew that.

The Environmental Protection Agency has set a limit to how much of this stuff can be added to gasoline, because there's some suspicion that it might lead us to grow, like, three heads. And not smarter ones.

Other octane boosting chemicals have environmental issues, or have the potential to damage your engine or emissions equipment, and void your warranty. And, think about it -- if there was a cheap, easy fix for raising octane, wouldn't Exxon already be all over it?

So, you may wonder, why is this stuff on store shelves at all? Well, there are exceptions to the EPA's rule; these octane boosters are allowed to be sold for use on race tracks and for things like lawnmowers, chainsaws and really powerful men's body hair trimmers.

But if that doesn't describe your use, it's best to just fill up at the pump, Doug.

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