MPG display screwed up

My display that shows the running MPG per tank full has gone wonky. I was cruising along down to about 1/4 tank left showing 18.9~ cumulative MPG for that fill up and all of a sudden when I shut down and re started up it showed 12~ MPG. It then started to climb pretty quickly to about 18 MPG (4 mile trip). Shut down and started again and it showed 14 MPG which again started to climb quickly.

Anyone seen this. I'm out of warranty and very hesitant to take to the dealership as this could turn into a nightmare in expenses.

 

Are you 100% sure the display is on average per tank, not average per TRIP?

 

I'll look into that but would assume if that's the case, the starting MPG at start up would be zero and not some higher number. I haven't refilled since this started, but when I did before, it always read zero and went up from there.

 

Are you parking on hills? Noticed my tank average will reset if it detects any additional fuel: aka the fuel float is a little higher on certain inclined parking spots.

 

Good thought, but no hills. Nothing has changed with my daily driving and this just started a few days ago.

For my mileage I do the math as you suggested at fill up and that's the number I report to Fuelly. For the past 4 1/2 years this function worked as it should. Reset to zero at fill up, and as miles are driven the number climbs (no matter how many shut offs and restarts) and then settles down when it gets enough miles on it. It only changes a bit if I'm stuck in heavy stop and go traffic or a drive through due to the lower MPGs being produced and added in the the total number driving it down a bit.

 

I'm going to the gym a little later which is 4~ miles and if I can manage while I'm driving I'll try to record it.