Did you tell him what gears you have and what diameter tires? He had mine within 5%. I had to use my GPS to figure out it wasn't exact. He can fix the PCM if it's off. You might have to remove the tailhousing off of the transmission and count the teeth on the speedometer reluctor if he doesn't get it close on your next tune. Sometimes people will do funny things to their cars to keep the mileage low on the odometer. Grinding off every other tooth on the reluctor will do that, cutting the miles in half on the odometer.