Economy vs. speed - drive slow to save gas? BNW Z3 - 29.7 mpg at 80+ mph

perry lee · 2006-07-01T12:15:21+00:00

Here's a question for the group. For years we have been told "the faster you drive, the more fuel you use," because drag increases on the car etc. etc.My wife and I own a '98 Z3 convertible, 2.8 litre w/a 5-speed manual transmission. We have two different driving styles. My wife gets a kick out of seeing how much mileage she can wring from a tank of fuel. She gently short shifts through the gears, monitors her speed constantly, and believes that a steady, high-gear 60 to 65 mph gets her the best milage of twenty six to twenty seven mpg. My driving style is exactly the opposite, and I squeeze every bit of performance out of it that I can. I go in fast, brake hard and shift at high revs. We took a fairly long trip through Indiana last weekend, where the speed limit is 70 mph, and traffic was moving between eighty and 85, with a few stretches of 90 mph. Imagine our surprise when my wife flipped the computer read out to mpg, and it read 29.7 mpg! This really seems to fly in the face of what I have been taught about speed and fuel efficiency. Does anyone have an explanation? BTW the top was down, and fuel type is always constant. We use Chevron Techron 93 octane exclusively, so that shouldn't be a factor.

Jul 01, 2006 22 Replies

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Geo 20 years ago

The OBC gives spot-on mpg readings vs. manual calculation of miles per gallon. Surprised me, but there it is.

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Geo 20 years ago

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Geo 20 years ago

Huh? Exactly how does an oxygen sensor in the exhaust gas stream respond in any way to the octane of unburned gasoline?

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