Summer gas has a different mixture
Winter gas has higher volatility so it will start the car in subzero weather.
Summer gas cuts back on that stuff so the gas does not evaporate too easily.
Winter gas gives fewer mpg compared to Summer gas
Plus if you fiddle with the mpg gizmo in the dash, you can see how in Winter, a cold start uses a huge amount of gasoline before the engine is warmed up (compared to very little in Summer weather)
((This is why they say start and drive right off in Winter. Wasting a ton of gas idling to warm up the engine is just wasting gas.. And the engine warms up faster if it is being used instead of idling))