[SentraSE-R]

Mine says 9999 when I'm in DFCO. If you set your cutoff 4 below your idle TPS, it'll never reach that, ergo you'll never see 9999 mpg in normal driving. You need to set cutoff 4 above your idle TPS.

It's indeed possible that both of your cars do not have DFCO. It depends on your car manufacturer and year. My Toyota-made 2006 Scion has excellent DFCO. My 2002 Nissan had very primitive DFCO. My 2006 Hyundai didn't have DFCO at all.

I own both Scangauge and Ultra-gauge. Neither read an actual volumetric fuel flow rate. Both measure fuel injector pulse duration instead.

You need to calibrate your fuel consumption and speedometer accuracy, IIRC. The former because it's obvious. The latter because it affects distance traveled. To do the former, you have to log your fuel consumed, at least for the first tank or two.