Here's one I have not heard or read about.
I am bringing my latest acquisition, a 2002 Borrego (Love the yellow ones!) back from a serious period of "benign neglect".
It had a myriad of issues, too many to list for the moment.........
Working on the A/C today and went for a test drive. As I was fiddling with the trip computer, it gave me an implausible range to empty
figure, so I looked at mpg. 64.5mpg.WOW! so I did a reset, and after the calibration period, it came up with 99.9mpg and
that is where it stayed for the remainder of the trip. So cool to have a Rangie with a 2000+ mile range!.......Yea, Right!
Average speed reading was correct.
Anyone know where it is doing the calculations? I figured Becm, but really have no idea.
It was working when I drove it the 350 miles from Washington to Oregon and showed a very believable Mpg number for that trip.
Since then, I have fiddled with the footwell plugs, and a couple of Becm plugs.
Any insights will be very welcome!
At least the A/C is now working as well!

If the average speed, speedometer and odometer readings are correct, it is seeing you are moving, but it isn't seeing anything from the engine ECU to tell it how much fuel you are using. It thinks you aren't using any. This is done in the instrument cluster. I'm not sure how it calculates how much fuel you are using but it is likely to use the injector pulse durations. The fuel mixture is dictated by the fuel pressure (which should be a constant) and how long the injectors are open for. That will give a calculated fuel consumption figure. I suspect you have disturbed something while playing with the footwell connectors. According to the ETM, there appears to only be one connection from the engine ECU that goes directly to the cluster (tacho signal goes via the BeCM) and that is on a Green/Blue wire on pin 8 of the RH footwell connector.

Ahhhh, Thank you!
I had not mentioned that the old Win7 machine in my shop is down at the moment, so no Rave......
That makes complete sense. I will have a poke at the plugs again.
Solder and heatshrink treatment is on the board, but I am confirming all of the major mechanical and electrical functionality before I dig into the fiddly stuff.
So far, it all looks great. All fluids changed including trans. All came out clear and metal free.