Winter gas mileage

[Patriot]

My mileage has dropped from about 10.7 average to about 9.0 to 9.5.

I thought it odd and mentioned it to my H2 tech since it's going in for oil change and minor isuues.

He said it might be winter fuel additives that fuel companies are required to add by some states for winter emissions?!?! Anyone else hear of this or have lowering gas mileage in winter months??

I was thinking throttle body but I have no other issues...no bad idle, no surging, etc.

I do have a supercharger but it was added well over 2500 miles ago.

Thanks...