[aerohead]

I re-ran the numbers with these conditions :
1) the OEM configuration is good for 41.07-mpg.
2) the 100% grille-block, mirror delete, rear skirts, wheel covers, and burst edge extensions produce an estimated Cd 0.1820, good for 47.76-mpg.
3) adding a 12-mph tailwind produces 59.49-mpg.
4) adding the reduced air density of 4,000-foot elevation brings us to a maximum allowable 62.17-mpg.
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Somewhere, we've got to account for the missing 16-mpg.
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The eastern trip, from Beaverton to Boise is 438.5-miles, and almost exactly 5-hours and thirty- minutes., at 80-mph. A lot can happen to 'outdoor' conditions within the span of 5 1/2-hours.
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If you had '150-miles' left on the tank by the time you rolled into Boise, that means that your 'range', at 80-mph, was 588.5-miles to fuel exhaustion.
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588.5-miles, divided by 11.9-gallons fuel capacity = '49.45-mpg' *
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We need to reconcile the discrepancy between the 78-mpg reported by the ' G.O.M.', and the actual road trip range of 588.5-miles, which 'should' have been 928-miles to 'vapors.'