I have the On-Board Computer option on my 2005 MCSC. If you have it, you know it provides a read out of a number of interesting statistics. The temperature, average MPH, miles left on the tank of gas and cumulative MPG are all nice to see and monitor. There is an indicator for current MPG (not cumulative). What I noticed is that even if I am using cruise control on a flat road that number bounces all over the place seemingly randomly from 5 MPG to 85 MPG about once a second or so. Almost as if it were resampling and recalculating the MPG too often or just averaging it over too short an interval. I had the dealer look at it and they said something like "oh yah. that's the way it works, just ignore it - it's just an estimate." That didn't really make much sense to me so I was wondering if anyone else who has this option has seen the behaviour. Did your dealer tell you to ignore it? Did they fix it?

 

mine stays put. it shows 30mpg when I know Im getting 35

 

Mine behaves the same way. I have heard the same "explanation" you were offered and have basically decided it's worthless data. I wish they'd just leave it out, since there's no point in seeing random stuff like that...

But I do like the other feedback the OBC provides.

 

Mine only jumps up and down when I go up or down a grade or move the gas pedal. When I hold a steady pressure on the gas on a level section of road it holds a fairly steady reading. If yours is jumping up and down, maybe you're moving your foot more than you realize or you're going up and down on the road.

I agree that it would be more useful if the sampling time would be longer.