ScanGauge MPG gauge--why not 9999?

First, 100 mpg on the dashboard scale does not mean the engine is shut off. There are many times when the engine will be running with actual MPG somewhere between 100 and 300.

Second, your ScanGauge may sometimes have an offset error. Pull up and watch GPH. On the few occasions I watched it, mine frequently stuck at 0.02 GPH, which easily generates faulty MPG readings of 1000-3500. But I don't usually use these ScanGauge displays on the Prius, so haven't tracked down the source of this problem.

 

I just installed a ScanGauge II on a 2011 Prius. I set it to fuel type HYBRID, 1.8L engine. The GPH usually shows 0.02 even when the RPM is 0. But then I noticed when coasting down a hill at around 50MPH, the RPM was around 990 and yet it went to 0.00 GPH! Weird. Maybe it depends on whether the accelerator is being pressed or not.

My previous car was a VW diesel with the same SCII. It always went to 0.0 GPH whenever I took my foot off of the pedal while the car was moving.

Edit: I read in another post about the TPS cutoff setting. I lowered the TPS cutoff closer to idle (16). Made no difference. Apparently it's an SC bug that shows fuel consumption when the engine is off.

 

I see the same behavior. When you're coasting and see RPM around 990 and 0 GPH, you're in Warp Stealth mode. You can press the gas down a little and get a little forward power without starting the ICE and getting fuel flow. If you push a bit harder, you will see GPH>0. You can see on the HSI how much power you are getting without triggering ICE restart.

I suspect that the 0.02 GPH may result from the signal that the Prius is sending to the OBD rather than from an error in the SG.

Unfortunately, the amount of time you spend with the 0.02 GPH number probably affects the SG's calibration. My SG fuel usage varies several percent at every fillup, probably for this reason.

 

It's similarly reflected in the MFD. The MFD overestimates mileage more for me in the winter and one of the few occasions where it significantly understated mileage was when I was descending 1,500 feet over 180 miles and spent a lot of the time gliding.