How come every vehicle's MPG computer is off?

[IndyFan]

My Jeeps and our Ford Transit are all optimistic. GM vehicles I have owned were optimistic, too.

I´m wondering if it can be adjusted on either Jeep Wrangler, the JK or JL, because I drive them both.

 

[Gene K]

I always measure mileage like this:

- Fill to max.
- Set Trip B to zero.
- When it's time to fill up again, fill up to max, see how many miles are on Trip B compared to how many gallons I had to fill up to get to max.

I've done this countless times on all kinds of vehicles and the car's computer is always a few MPGs (2-3) better than what it is in actuality. Why is this? If it's just an estimate, I get that, but how come it's always in the favour of showing better mileage than you're actually getting?

I also wonder when people list their MPG figures, if they're actually calculating by hand, or if they just rely on the computer.

PS. I never pump after the first click, so it's not like I'm filling up the hose.

My 5.0 Ford's have been pretty accurate. My '11 Mustang actual read low for part of the year.

My F150 3.5 Ecoboost was the typical 6% off before I put it in Engineering Mode and calibrated it. Now it's usually within +/- 1%.

 

[DGXR]

I think it's hard for computers to determine exactly how much fuel mass is coming out of the injectors. Depends on fuel pressure, temperature, density of fuel, etc. Also, modern fuel blends contain less energy than straight gasoline, not sure how that would factor in but may be part of it. Then consider features such as DFCO and AFM, it's a pretty complicated computation so in the vast majority of vehicles it will be off at least a little. Naturally the error will favor fuel economy