Speedo is off 10 percent. 10 percent of 18.3mpg is 1.83. Add 18.3 and 1.8 = 20.1 mpg. Doubt it all you want.How are you calculating your MPG? Your onboard calculation is no longer accurate. Your Speedo/odometer are no longer accurate. I seriously doubt you are getting 20mpg with those wheels/tires.