The
ScanGauge instant MPG display updates once a second at the default refresh rate (you can increase to a faster rate if you want.)
My experience: if your load is stable, the MPG reading is also very stable. Never found it to jump around.
And my preffered highway technique is "driving with load", so I have quite a bit of experience with the instant MPG readout.
---
On a related subject: EcoModder is now selling
ScanGauge II's directly. I've updated the first post in this thread, plus there's a new thread on the subject here:
>>
ScanGauge II fuel economy gauge now available directly from EcoModder