resetting mfd on gas up

I reset the MFD every fill-up and leave the car "on". I pull out onto a highway on pretty level ground with a 50mph speed limit which then drops to 40mph through the city. I can see phenominal numbers immediately or it'll start in the 40's and rise to "significantly" above average in 5 minutes or so. Now at 2 pips this tank is down to 61mpg which is still significantly above average. This station is on the way home so I've already driven 15 miles in the evening.

 

After a reset and you only have <5 miles on the display, any change in the driving MPG will have a big effect display MPG. If you are calculating the average of 5 numbers any one of them can make a big difference in the answer. If you average several hundred numbers it takes a lot to change the answer.

Large swings in the MFD display average are very normal after a reset. Every mile will generate less and less of an affect on the MPG display.

Bob

 

I've had 3.6L/100km displayed at 13km but I average 4.2L/100km currently. This is because I fill up on flat ground with a warm engine but I live on a hill.

 

I use the same system as Tony, which means my displayed mileage is back up in just a minute or two. Usually I get my best mileage at this point, but inevitable short trips and other sub-optimal driving eventually takes a toll and brings my mileage back down to its normal average value in a few days.

Tom

 

Typically I can exceed my tank average within minutes of fill-up, because I have plenty of glide opportunity for long distances right after leaving the pumps. So the average gets skewed immediately. After that, it takes a round trip to work (100 miles) to steady the numbers near my tank average. But I get increasing numbers as the quantity decreases (lighter car) to establish the final and typical average of 60.X MPG. This car is awesome.

 

It depends on where you live and where you fill.

I live in the mountains at 9000' and work in town at 5000'. If I fill at home, my average pegs out 99.9 MPG for the trip down to town and stays there for about 40 miles. If I fill in town, my average is around 38 by the time I get home. Then over the course of the tank, it varies up and down. Going down the hill the MPG works its way up and vice a versa. Only when I re-fill at the same elevation as I originally filled do I have a good idea of my actual mileage.