Lifted trucks almost always experience poorer fuel economy due to the extra drag created by the exposure of under-vehicle components, not to mention the additional rotating mass of the tires & wheels. Even the width of the tire and the tread pattern can add to the wind resistance.
As for the MPG, the Evo/Gryphon uses the MAF sensor reading to calculate the amount of fuel being used. There are times when it might be slightly inaccurate, such as under WOT acceleration, but these situations are infrequent and "weighted" so that they are given proper consideration.
The thing to remember is that it isn't the final word in fuel economy... A pen and paper is. As Jack said, it should be used more as a means of measuring trends. For what it's worth though, I've found it to be more accurate than the MPG calculation in our Expedition.
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Power Hungry Performance - The
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