My car is moderately consistent with fuel mileage. I sort of know how many miles to expect out of a full tank. When the gas gauge starts doing its crazy-blinky mode, I stop paying attention to it and I will flip over to the trip odometer. When my trip mileage goes beyond 400, I start thinking about getting gas because I know I have burned through approximately 8 gallons at that point at roughly 50mpg.
I know that's not very scientific, but it's working for me so far. But now that I've said that, I will probably run it out of gas next time...lol.
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View my fuel log 2015 Mirage ES 1.2 manual: 52.2 mpg (US) ... 22.2 km/L ... 4.5 L/100 km ... 62.6 mpg (Imp)