I can see it change so it's probably like 20 miles. I used to think it was per tank but it changed too fast for averaging with the last mile with 300 miles on the tank.
I believe it calculates it almost continuously. If you check at the start of a fill up it updates about every half mile. John (Britprius)
From what I can see, if I drive at let's say 45mpg for a couple miles, it will change maybe .1 depending on how low or high the previous mpg was. I know it's always calculating, but is this a rolling previous 20 miles or 50 miles or 100 miles. It's possible it's per tank. Edit: I suppose I can reset it and just keep an eye and see at roughly what miles driven it slowly changes up and down for mpg. That would give me a rough idea. I would think it would be a neat number like 20 30 40 50 etc so might be easy to figure.
It up dates continuously against whatever your average is on the MFD consumption screen. If the MPG figure is not reset it will appear not to update as quickly because any increase or decreases in MPG over the last few miles averaged out over say the last 5000 since the last reset will have only a small bearing on the over all average. John (Britprius)
It seemed to change too quickly to be averaging a couple miles into the hundreds I put on it, but maybe .5mpg isn't much to change.
Yeah it averages over the full tank. Mine changes very rapidly just after I fill the tank (2005 model automatically resets the MPG data each time you fill). I often go down to about 23MPG (10 L/100km) in the first minute as I go up a big hill, and then in the next minute or so it's back up to 50MPH as I hit some downhill sections. Once I get a few hundred km on the tank however, it only changes very slowly.
it calculates constantly the average MPG depending on the distance travelled. So the time interval between changes gets longer as you put more miles because it will have a higher threshold.
