I would never trust the fuel gage to determine mpg. I know you are just checking consistency, not going for absolute accuracy, but it wouldn't be much harder to do it right. Note the gals used to fill-up (completely) and divide by the trip meter reading, reset the meter for next time. Occasionally, on a long trip, compare your odometer to GPS or highway mileage markers and calculate a "scale factor" for "actual miles".
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