Swapped out dim mfd mpg is way low

How much have you driven the car since you swapped out the multifunction display I don't know if there's a way to correct this I mean what's the correct the computer seeing the miles per gallon That's in the car it's just displayed on the MFD if I'm not mistaken but who knows One day mine will be showing 37 and the next hour later it's 45 I don't think I've ever seen 32 even when my hybrid battery was completely shot and I was running on the gas engine I was still getting 35.5

 

My understanding is that most Multi Function Displays (AKA "MFD") for Prius Gen2 are considered to be "optimistic." IOW, MFD mpg for optimistic units is generally higher than actual mpg (using actual mileage, and actual fill amounts). My MFD is "pessimistic" because the MFDmpg is usually eight to ten percent (8% to 10%) below actual mpg. Thus, i believe that MFDmpg is calculated in the MFD (albeit using data from other locations in system).

 

Well great maybe you'll be able to figure out a way to hook up a computer or a dongle to the MFD and program it

 

37mpg is bad for a Prius so maybe there’s another problem that’s just getting worse.
When’s the last time you changed your plugs?

 

Implicitly you (Another) are suggesting that the replaced MFD is known good. And, thus the reduction in MFDmpg from 37mpg to 32 mpg should be respected as accurate. However, my understanding is that original MFD was replaced with used MFD of unknown accuracy and pedigree. If that is true then thete is a correlation between replacement of MFD with a downward 5mpg change in MFDmpg. Finally, as was noted above, some MFDs are optimistic and some are pessimistic; it appears OP may have encountered a "VERY pessimistic" MFD in this replacement.

ETA: Does anyone know how to reprogram MFD from pessimistic to optimistic?

PS no notes of CEL or rough running, thus hard to imagine a 15% reduction in gas mileage without triggering CEL, or running rough.

 

Yeah do calculated mpg for a few tanks. Might be an actual drop in mpg.

 

Was 37 accurate? Or are they both inaccurate?
Who knows?
Check it the way @Mendel Leisk suggests. If you’re getting less than 40mpg in the summer, it is bad. Look for other issues.

 

Divide the miles you drove by the gallons you pumped.

 

could be one of those low mpg mfd's we used to read about back in the day

 

I believe you might be experiencing this issue