The data is just a bonus from the system that actually controls the engine, no second system. Germany required fuel consumption meters in cars way back in the 1980's.
If only the indicated MPG went down, that means the car "thinks" it's using more gas than it really is. That would be consistent with lower fuel flow, and that could be due to lower fuel pressure or some restriction of flow in the injectors. But, the O2 sensors make sure the car doesn't run too lean.
I read somewhere that too low fuel pressure can actually cause excessive fuel flow. The only explanation I can come up with for that is that the fuel pressure is used somehow to shut the fuel flow off in a pulsed injector.
My Cobalt only has one MPG display, not a separate OBC MPG and Trip Computer MPG. I always reset the MPG and trip odometer when I fill up. But, I'd like to know my MPG's in between fill-ups. So, I made a(nother) spreadsheet. I enter the starting and ending MPG and trip odometer mileage to calculate the MPG on an in-between-fill-up trip. Since there's some uncertainty because mileage and MPG is only reported to one decimal place, there is an uncertainty window.
I took a friend to the airport, 44.1 miles by the time I got home. The Cobalt got 30.0 +- 0.4 MPG. Under perfect conditions, I've hit 40 MPG in that car. Amazingly, I've seen 38 MPG in Frau Putzer's G01 X3 xDrive 3.0i. But, not with her driving it.
Last edited by Autoputzer; 09-06-2019 at 11:59 PM.