I have an 07 base 350z.. Last night I filled up my tank with premium gas as I always do, I went to reset my Miles per gallon gauge and it started acting funny ever since. When I'm at idle it shows that I'm getting 6-10 miles per gallon, when I start driving it starts jumping all over the place ( 12-25 )...before I would have it steady around 17 and average speed of 25-30, now my average speed shows 17 or less. I took it to the mechanic to get it tested with the smoke machine to make sure it wasn't an intake or vacuum leak and nothing came up. The car runs smooth and normal but the gauge keeps going crazy. Has anyone had this issue ? It's driving me insane

 

When your car is on but sitting still it reads it as 0. So it will average your speed accounting for that 0mph reading.

 

It's calculating a moving average, so it changes all the time. If you check it at short intervals over varying conditions, the readout will vary widely. If you drive for a long time under mostly unchanging conditions, those factors will have more weight in the calculated average, and the readout will vary less. This is basic math, right?

http://en.wikipedia.org/wiki/Moving_average