Speedo correction for larger tires?

Change in fuel economy may be the size of the tires but it is also the result of less efficient tires and or heavier tires. More aggressive tread result in more tire scrubbing and flexing and that hurts fuel economy. More rotational mass with heavier tires also has a negative impact on mileage. But you may also be going 3-6% further than the odometer indicates and need to do a manual calculation with this factored in with the amount of fuel consumed.