I started trying to find the exact difference between winter blend fuels and summer blend (there are over 20 different blends in the USA at any given time).
Biggest things I've been able to confirm, winter blend has a lower evaporating point (generally represented in the form of RVP (Reid Vapor Pressure) rating.) Most winter blend fuels use simpler and lighter additives such as butane which is more volatile (affects RVP), but has less energy content than Octane. The additives in winter mix are generally cheaper, and require little maintenance to switch over from summer blend. This combined with less driving during winter months is why winter gas is cheaper.
Summer Blend gasoline generally contains heavier additives, to help reduce volatility. The major purpose of this is so that the gasoline doesn't evaporate before reaching the cylinder, preventing vapor lock, and reducing emissions. The additives for summer blend are harder to implement in the refinery process, so the gas companies tend to do their yearly maintenance at the same time as the fuel switch. The downtime, and more expensive additives are why every spring we see a spike in fuel prices.
The EPA estimates the energy content of Summer fuel is 1.7% greater than winter fuel, which has a small impact on fuel economy.
How Stuff Works, Fuel Blends
2019 Insight EX, Aegean Blue (Owned by my someday wife)
General Automotive knowledge, Degree in Autobody, worked in Electronics for the past 10 years.
Retired Professional Car Audio Competitor. Avid knowledge buff, always researching something.