That's what I thought also but was hoping one of our resident techs could verify.
In switching from OEM 16" wheels to an odd size 17x7.5 (225x50-17) the MPG went from OEM 34/35mpg to 28/29mpg. Speedo reads 3mph faster than GPS.
I'm going to weigh the two to see the difference in rotating mass.
The ECU knows how long each injector is open, how many times they fire, what the flow rate is on them, knows the rpm, miles driven, among other factors. But if your odometer is not accurate neither is your miles per gallon.
A better question is the computer's accuracy when calculating MPG.
I've spot checked it against manual calculations and find it's a bit optimistic. But not by much.
Example... the computer would say 39 mpg at fill up but my calculations said 38.5 mpg. Not a huge difference.
And since my car is rated at 36 mpg highway I was winner because it was over the EPA rating.
I have wonder how so many people have a hard time getting decent gas mileage when every car I've owned has always achieved better MPG than what the window sticker says it should get. I drive the speed limit in city streets but often speed on the freeway. My Hyundai seems very happy cruising at 80mph.
It must also keep track in relation to what the tank level is too. Or rather as with mine, when you fill it the number is reset and I assume a table in the ECU is wiped out to start over. But it is much easier to affect the numbers right after filling up than when you are about empty.
If I fill and then go straight out to the highway and set the cruise to 50 I can get over 40 mph pretty fast if I don't stop. But starting or stopping at an intersect then accelerating will eat into that fast. Not so much once down below a 1/4 tank.