The computer is doing the same math to calculate MPG’s that we do, just in real time. How accurate is your speedometer? If that is accurate, the MPG display should be accurate.
Tire size and tire wear will affect how accurate the speedometer is. I get just over 21 MPG with a 2004 V8 sport.
For improving mileage;
Correct tire pressure.
Tire alignment.
Good wheel bearings.
Drive train fluids in good condition.
Lighter tires used less energy to get going.
Extra weight in the vehicle.
How you drive.
Air resistance i.e. roof racks.
Clean air filter and throttle body.
Clean injectors.
Good spark plugs.
Clean vehicle, mud and dirt is weight.
Derek V