Not sure tire wear would explain it. Tire wear would affect the odometer in both cases (computer calculation and owner calculation). The variable is how much perceived consumption is occurring. More likely is that the tank doesn't fill to the same level every time. Every pump is different and the filling trigger may go off at different tank levels.
For example, say you drove 350 miles on a tank of gas. If you fill up and get 17 gallons in the tank, your MPG is 350/17=20.6mpg. Now say a different pump was used and put in 17.4 gallons (or you topped it off a little). The same calculation gives 350/17.4=20.1mpg. There's a 0.5mpg difference for ya.
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2013 Cadillac V-Wagon, RIP Z
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