[greenskeeper]

If you do cumulative mpgs over multiple tanks, the difference closes significantly.

You really need at least 3 or more tanks to get a true idea of mpg.

Also you need to consistently fill to the same level in the tank each time.

 

[greenskeeper]

I wonder if it would be different if the fuelly app could connect to the obd to measure fuel coming out of the injectors.

Or Just take the indicated readings and take away about 1mpg. The fuel tank/vapor recovery system is like filling up a balloon. The ECU doesn't have all of the variables to correct for that. It thinks it's using less gas than what you are putting in.

Why Your Trip Computer Isn’t Giving Accurate MPG Readings

And how to fix it. Cars' on-board fuel-economy meters can be inaccurate up to several percentage points.

www.caranddriver.com

re-filling the tank to the same level consistently is the problem when comparing ECU estimate to hand calculated