Fluctuating Gas Mileage

happsJune 2, 2007

Last month I purchased a 1990 Oldsmobile Eighty Eight 2 Door with 54,000 miles. With the tank full, on my first drive around Phoenix with the a/c on half the time and driving normally, I got 13mpg. Then, I drive on the highway at a constant speed of 55mph (cruise control on) for 80 miles with the a/c off and get 20mpg. Then, I drove around Phoenx for 80 miles with no air conditioning and driving like a grandma, staying at the speed limit and at a constant speed as much as possible. I got 20mpg. Unbelievable. First, I get 13mpg in city driving with the a/c on half the time and driving normally, then 20mpg on the highway driving for 80 miles at a constant speed of 55mph cruise control, and now 20mpg around town driving like a grandma with no a/c. I'm totally stumped now. What's going on with this car? I'm not making this up. I'm just perplexed. The only think I can think of is that this car is "breathing and opening up more. Maybe the highway drive "cleaned the car out." The car only has 54,000 original miles.The man I bought it from was in his late 80's who only drove in town to the store and church.

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My guess is your first tank full wasn't as full as you thought it was.

    Bookmark   June 2, 2007 at 1:21PM
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Sounds about normal to me?

    Bookmark   June 4, 2007 at 10:43PM
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Hills one way and not the other? Different gas stations?

    Bookmark   June 9, 2007 at 8:19PM
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Unless you have specialized equipment, 80 miles is too short a distance to compute gas mileage with any accuracy, because it is difficult to fill the tank to the exact same level each time, and the amount is only a few gallons. You can begin to get an idea of the fuel mileage after using 3/4 of a tank. Alway fill to the same level and keep a log of miles and gallons. After a few tanks, you'll have a better picture.

The fuel content does affect mileage. Modern gasolines with up to 10% alcohol has less heating content than 100% gasoline. The reduction in mileage is on the order of 2 miles per gallon.

I suspect that you have a V8 engine. Olds did make a samll V8, a 260 in^3, at one time. It was the smallest automotive V8 ever offered by GM. But you likely have a larger engine. The fuel mileage of large engines are more affected by city driving than a samller one. When a car is waiting at a stop, its engine is still buring fuel, but going nowhere. This drops fuel mileage. There are other factors, too.

    Bookmark   June 14, 2007 at 3:34AM
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city driving sucks up gas.Start and stop and sitting at stop lights.Highway driving you drive at a constant speed means good gas milage.People in the city burn gas and go no place.

    Bookmark   June 18, 2007 at 3:10PM
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