Tire width vs weight and fuel economy

215/85R16 weight=35.7lbs
245/75R16 weight=31.8lbs
The diameter is the same for both. Same tire just different dimensions.
My thought, the heavy but skinny one is going to be better on the highway but it needs more power when you are pulling away from a stand still(city traffic conditions).
The wider but lighter is going to be the opposite.
Am i right?
Also the 215 has a load index of 115 and the 245 has 111,is that factor going to affect the fuel economy on the highway due to the less flex of the higher load index?
Thank you!