Manual calculations are often not correct because people estimate and round off. They do the calculations from memory. They use different pumps at different times of the day. They will use the same pump, but from the other direction. 25.3 gallons turns into 24 or was it 23.5? 423 miles turns into 400 miles or rounds off to 450 miles. People will swear they didn't do any of that, but they do.
10 people report their mpg from self calculations and 10 report the mpg from the truck computer. You are going to lose your money betting on the people who do the manual calculations. All 10 of those manual calculations people will swear their math was correct. Most will reflect the persons belief.

 

Mine is off by 2-3 MPG. I usually show anywhere from high 18's to high 19's driving city and highways (20's+ if I am driving longer distances on the highway). After 6 fill ups logging them in Fuelly (no estimates or guesses, I fill in exactly whats on the odometer and my receipt), it calculates 16.5 MPG. I don't understand why it is so off. I did put bigger rims/tires 265/60/18 to 275/60/20's, but I did the wheel/tire adjustment in Forscan. My speedo shows the exact same speed a multiple GPS apps, but my MPG sucks

 

It's actually not as good as it gets. There is a thread on here about how to recalibrate the fuel economy calculation: https://www.f150forum.com/f118/fix-i...inutes-421912/. I used one year's worth of exact calculations. Any overfill / under-fill becomes a wash after that much time, and so in my case I believe I obtained a very accurate constant for the recalibration.

 

We just purchased an Acura for my wife. Have only ran three full tanks of fuel through it but all three hand calculations have the fuel meter within a few tenths of hand calculations.

No one will convince me that Ford isn’t capable of programming the software to be much closer to accurate then they are. The fact that nearly all of us are finding the fuel consumption meter to be optimistic indicates to me that it is not by accident. I’ve yet to read anyone on these pages that has an inaccuracy that reads LESS then actual.

I suspect, but have no proof, that there is some sort of regulation on how accurate these meters must be and that Ford has found the tolerance and taken advantage of it.