Fluctuating MPG

I assume you're using the dashboard readout then? I'm not convinced that it's all that accurate - it takes readings from a number of sensors and comes up with a "MPG" figure. The shorter the trip, the more inaccurate it is.

My last 2 cars were the same - over a tank were within about 5% of actual fill calculation, but single trips seemed like they could be 20% out.

 

Would you mind logging every trip for a while so we can look for a pattern?
Helpful would be:

date
trip direction
trip length
trip average speed
trip mpg​

Since roughly half of the power required to drive 62 mph is used to overcome aerodynamic drag, your mpg variation of nearly 20% could be caused by a light breeze of just 5 mph. In my opinion, this is the simplest, most common, and most likely explanation.

I think the next most likely cause would be a sporadically dragging brake. That can be checked by driving at least five miles at highway speeds and coming to a stop with minimal braking, and measuring the temperature of the brake rotors with an infrared thermometer.

What is your mpg in the other direction?