MPG ECU calculation sample size

[kenski]

Anyone know how far back a sample size the ECU takes when calculating your mpg? In other words, if I've never reset it - ever, then does it calculate over the entire life of the car or does it top off somewhere like the last 100 miles or something?

 

[1engineer]

I think it goes off of resets and here is why:
I towed a 6X12 trailer this past weekend 800 miles. I was running (on the readout) about 19.6 mpg. I reset it for towing and ended up getting 12.3 mpg over that milage. I then have since reset it and it has gone right back up to 19 or so and the miles to empty went up proportionally. So, armed with that knowledge I would think that if you never reset it it eventually would be very close to your overall mpg. Unfortunately, that would not be a very good indicator as that would take into account burning fuel while sitting still such as warming up in the morning or traffic jam stops. It would give total gallons consumed over a milage but it would not be a real good indicator of actual fuel efficiency.

 

[1000MPH]

Resets would be my guess too.
Not resetting seems like a waste of it's usefulness. I reset all the time depending what I'm testing: trips, tire pressures, driving techniques, etc.
If you want a good indicator of lifetime mileage efficiency use the range.