I've just purchased a 2013 Focus, and it's my first with the MPG display. I've noticed several things that affect mileage here in Southern California:
1) Humidity and temperature. I'm getting 42 mpg on the freeway, but I had it up to 50 mpg one early cool, clear morning on the desert.
2) If I end up stopping when I've gained altitude, there's a difference going back. I had to think a while, because it might look flat, but it's not.
3) Sidewinds lower my MPG. So do even slight headwinds vs. tail winds. A slight elevation loss with a tailwind makes a difference.
But I think the suggestions about checking the calculations are valid. When I get mine on the open road, I plan to use the Highway mileposts between here and San Francisco to really test the odometer, and I will keep track of the actual gallons I put in. I did this once in my girlfriend's Toyota Highlander, and the computer MPG of 21 was actually 19 over about 600 miles.
Mike Hartfield, CPA
Small Business CFO, Taxes & Accounting • Quickbooks Advisor
Office: 661/309-4681 • Cell: 818/433-5359
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