My digital readout for my Gas mileage, the problem I'm having is, when I bought the

car 5 years ago, when I filled the tank full it would give me a read out of 330 or more, lately I have been noticing that the mileage is going down, now it doesn't read over 300 anymore on a full tank, currently it's at 298 when I fill it up full, was wondering why it's been doing that in the past 6 months. I do need valve cover gaskets changed which I just found out lately, does that have anything to do with it, or do you think it could be something else? If someone does or did have this problem, was it easy to fix or was it an expensive solution to fix? Anyone's help in this problem would be greatly appreciated.

Thanks