Tire size vs MPG - EPA test results
 

The new different trim levels of the new Honda Accord Hybrid come with different tire sizes, and different EPA gas mileage results. The lower trim levels have 225/50 tires on 17 inch wheels, and get 48 EPA MPG. The higher trim levels have 235/40 tires on 19 inch wheels and get 44 EPA MPG.

The wider tires are 0.5" larger diameter, or 2%. The 2% diameter difference does not explain the 9% MPG difference. And 10 mm width difference should not cause enough increased air drag to cause the 9% MPG difference. I suspect that the narrower tires are LRR, and the wider tires are not.

Article: https://arstechnica.com/cars/2021/04...accord-hybrid/

Article claims unsprung mass as well as larger contact patch as the reasons behind it.

You may be right, LRR would certainly make up the difference in a hurry.

Tread design, rubber compound and rubber "stickiness" might as well. Some tires are more efficient than others, even without being LRR. Would assume the sportier 19's are stickier, designed for grip and not for being quiet or long-lasting.