My
Tall tire test found the benefits of taller tires largely offset the negatives of heavier weight, less aerodynamic wheels, and wider tread. My mpg went up over 3000 miles of highway driving with new tall tires, despite those negatives. The keys to mpg benefits from taller tires are their diameter and their weight. The taller tires dropped my engine revs @60 mph down 200 rpm. Their 12 lb. extra weight per tire/wheel unsprung weight had negative mpg consequences. I later mounted 5% taller tires on the OEM wheels without the weight and aero penalties, and confirmed significant benefits even under repeated acceleration (P&G) conditions.
As everyone else noted, speedometer accuracy isn't very important. We use it to avoid speeding tickets, to estimate trip times, and to battle wind resistance.
What really matters in relation to fuel economy is odometer accuracy. and while your speedometer may be 7% optimistic (high), your odometer may be 7% optimistic, 3% optimistic, completely accurate, 5% pessimistic (low), or anywhere else in the ballpark.
My Scion xB's speedometer was 5.6% optimistic with the OEM tires, while the odometer was 3.3% pessimistic (100 miles indicated = 103.3 miles actually traveled). I went with taller tires that are speedometer accurate, but now my odometer readings are 9.9% pessimistic. I checked them on a long trip against my GPS over 60, 80, and 100 mile distances. When my odometer says I've traveled 100 miles, I've actually traveled 109.9 miles. That's a major difference in distance traveled, and therefore in fuel economy.