I've just noticed in the last week or so since filling up my dashboard "tank MPG" is about 8.5 - 10.0. Previously I would get between 19.0 - 26.0 for a tank depending on highway/city driving. Nothing has changed in the last few weeks as far an intake/exhaust, only change was my tire/wheel combo. I was previously on stock 2011 wheel/tires which are 225/40/19 and 255/35/19, new set up is 225/45/18 all around with winter tires.

I noticed today while at highway speeds if I take my foot of the accelerator or put the car in neutral, the current MPG scale does not go higher that about 35 (previously maxes out at 90+). Any insight on what might cause the computer to miscalculate to this extreme? Would it help to reset the ECU? It's not a big concern as I am still getting my normal MPG in actuality.

 

Wondering, have you reset ECU or disconnected the battery recently.

 

I have not. It was recently taken to my local dealership for scheduled maintenance but I cannot imagine they disconnected the battery or did anything to reset the ECU. Even with doing so, I have not driven it to the point of getting that mileage. Reality my mileage has not changed, just the onboard calculation.

 

It's an estimate based on one injector. Are you sure you don't have an air leak somewhere?

 

It is winter. Cold weather will affect MPG. Also winter grade gas. is different from summer grade. In Hybrid cars battery gets less capacity as well making the engine run more. I noticed it when I had RX450h.

 

Reset it. When you are at the mpg screen. Hold down the button and it will reset it.