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My Impala has same size engine, it's gauge is pretty accurate, I've had some 35 mpg 60 mile trips (holding it to 55 mph). Doesn't really matter if guage is accurate or not, if it give consistant numbers it will allow you to calculate percent change. I don't know the RPM off hand but would speculate it's the same gearing as yours.
Cold weather is pull both of our number down, last 3 tanks on the Cobalt all under 40 mpg.
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