I have a some-what odd question here. In my owner'* manual, it is called for that 87 octane can be ran fine in my engine, but 91 or higher is recommended for best performance. During the summer, I don't notice really any performance difference between running 87 octane, or 93 octane. BUT, in the winter time, I notice a huge difference between running regular and premium. Shouldn't I be noticing a larger difference in the summer, or am I wrong? Does this make any sense?

Its probably because they put all that crap in the gas for winter.

Could also have to do with the colder denser air. There'* a lot of factors that change from summer to winter. It'* tough to nail down a single one that is the cause.

I'd think it has far more to do with air temperature, but that'* easy to test if you switch back and forth on similar days.

I don't know what they put in gas for winter but I loose 5 mpg. Actually Custum I would think the 91 would be better in summer with the heat.

You're also warming up the car more ( I assume).

Colder air is denser, and requires more fuel then the same throttle position then say a 90* day.