I wouldn't really expect a this sort of MPG gauge to be especially accurate. I wouldn't expect it to be as accurate as others in this thread are saying it is. If that is so, it may be pretty good for what it is.
I think the purpose of a gauge like this is to help drivers develop better driving habits. And the only real way to know what mileage one is actually getting is to calculate it based on miles traveled between one fill-up and the next.
That being said, since we have been driving around with the gas that was in the 2015 Mirage DE we bought last Wednesday, and didn't fill up the tank for the first time until today, I won't be able to calculate mileage until I fill up again. We took a 100-mile joy ride today, and watched the mpg gauge just to see how well it thought we were doing. It got up to 46.5 by the end of our trip. Even if it's not precise, it sounds pretty good.
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View my fuel log 2015 Mirage DE 1.2 automatic: 43.0 mpg (US) ... 18.3 km/L ... 5.5 L/100 km ... 51.7 mpg (Imp)