I have never experience any vehicle that had an mpg readout that was better than 7-10% accurate. That the OP claims two other vehicles are within 1-2% is amazing. The best way is the calculation based on number of gallons used and mileage driven. I have seen that vary by as much as 5% as well depending on the pump and it's cutoff. If the OP considers this "being lied to", he is in for a tragically disappointing life. Just drive the damn thing.