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I’m trying to figure out if my car’s fuel economy is just bad or if something’s actually wrong. I’ve been tracking my fill-ups and the calculated MPG is consistently lower than the EPA estimate, even with my mostly highway commute.

I’ve been there. I track fill-ups and my calculated MPG sits below the EPA estimate even on mostly highway days. In a long highway stretch I was averaging about 28 mpg while the sticker showed around 32 mpg. I know tire pressure and how full the tank is can move the numbers, and I don’t always hit perfect full-fill at the pump, which probably pulls the math down a notch. Real world isn’t the same as the lab, I guess.

Sometimes I worry something’s wrong, then I remind myself how much the driving conditions matter. Cold starts, wind, hills, and a loaded trunk can drag MPGs down fast even on smooth roads. I try to separate the frustration from the data, but it’s hard not to read every tank as a verdict.

I did a quick experiment where I reset the trip and logged a few tanks after the engine had warmed up. The numbers didn’t jump as much as I hoped, which made me doubt whether the issue is with the car or with how I measure it. I did notice one tank seemed to be a little high after a long coast down a hill, which makes me suspect rounding in the calculation.

Do you rely on the on board MPG readout or do you calculate MPG by miles driven divided by gallons bought?

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