That sounds like something is screwy with the gauge reporting. Have you double checked the gauge's accuracy? Also, consider it might be resetting itself in the morning instead of holding the information for long term accuracy. For example, if I reset the gauge while sitting still, fuel economy shows up as something you'd expect from an old big block with dual carburetors. When I start moving, it goes up. If I reset while driving at highway speeds mileage is very high, then it goes down. This doesn't affect my overall mileage for the entire tank which you can double check in the old fashioned method: fill up to where the pump clicks off, reset the trip odometer, drive, use up as much of the tank as you can because it's more accurate that way, then refill the tank again to where the pump clicks off. Note the number of gallons on the pump when finished- rounding up or down to tenths or even whole numbers is ok. Divide your trip odometer mileage by the number of gallons, and you have your miles per gallon to double check your gauge.
Now I wouldn't be surprised if there is a difference of 2-3 mpg or so between both methods. The pump method is not entirely accurate, and neither is the gauge. For example, if you top up when you filled up the first time, and then stop the first time it clicks the second time- you're probably putting in a 1/2 gal or so less. Let's see what that does: 300 mi /10 gals = 30 mpg, but / 10.5 = 28.5 mpg. Also that pump cuts off depending on vapor pushing back on the nozzle, you might have more or less vapor in your tank when you fill up, so even if you stop at the first click the tank is likely to be filled up to a different point. Topping off would seem to be more accurate- but then how much did you drip out? None of it is precise, so don't be surprised, but that doesn't mean that double checking isn't informative.
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