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Dear Tom and Ray: Because I don't think about cars at all (unless there's one coming at me at a rapid speed), this question may be very dumb. My sister says that different brands of gas, even though they're the same octane, will produce different amounts of mileage. She says she can buy one brand of gas and get from Pittsburgh to Carlisle, Pa., on a quarter of a tank, while another brand will burn half a tank going the same distance over the same roads. She attributes this to the octane rating being an average and says that some companies cheat with their numbers. Is she full of gas?
-- Sandy
Tom: Full of gas? No. Full of something else? Sounds like it to me!

Ray: While there are very slight variations in the octanes of gasolines you buy, it's nothing you would ever be able to measure with the equipment you're using (for example, a 1992-era Ford Escort and a dashboard gas gauge).

Tom: Right. There's no way that different brands of gas of the same octane could produce mileage differences this great. My guess is that when she "fills up" the tank, sometimes she fills it more "up" than other times.

Ray: The dashboard gas gauge is a very rough estimator. Even though the gauge may read "full," there may be room for another couple of gallons of gas in the tank itself. And that would explain why she sometimes arrives at her destination with the gauge reading half-full.

Tom: Other times, when she really tops it up and puts in as much gas as it will take, it's only down a quarter when she arrives.

Ray: Dashboard gas gauges are good as relative measures. They tell you when the tank is pretty full, and when it's pretty empty. To use them for anything more precise than that is like using your kitchen scale to weigh molecules of radioactive plutonium (which our lawyers have asked us to remind you not to try at home).

Tom: If she wants some slightly more accurate numbers, Sandy, tell her to fill up the tank (and stop when it automatically shuts off the first time), drive to Carlisle and record the number of miles driven. Then fill it up again the same way, do the math and figure out how many miles per gallon she got. That method is not approved by the federal bureau of weights and measures either, but it'll be more accurate than what she's doing.

Write to Car Talk in care of The Buffalo News, P.O. Box100, Buffalo, N.Y. 14240. Tom and Ray can't answer your letter personally but will run the best letters in the column. Their radio show airs at 6 and 10 a.m. Saturday, and 5 p.m. Sunday, on WBFO (88.7).

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