My fuel gauge is about 3 gallons off. It always takes up to 3 gallons more than is registered to top it off. As an example, today when I filled up, the gauge read 12.5 gallons used and 233 miles; 18.6 mpg. When I filled it, the pump was 14.5 which gave ~16 mpg. It's a little disheartening looking at 18.6 and then it's 2 1/2 mpg less. Is that an acceptable standard or should it be pretty well dead on?