keep the tires you choose properly inflated and don't jack rabbit start from each light or stop sign. Relatively speaking, a constant speed will probably give you better gas millage on the highway, but obviously hills, weather conditions, and traffic conditions plays into things too. Also, since I live in the north where there is snow, the colder weather make you use more gas for some reason, maybe because it takes more time for your car to reach proper operating temperature. Snow tires seem to also, but I'm not sure about that.
I know in the winter here, a somewhat narrower tire will help with traction in snow as you will have more weight on a smaller foot print than on the summer tires. That being said, a tire that's to big or to small will effect your speedometer. perhaps others here can explain that part to you better than i can, as i have not modified my car in any way such as that and don't want to give you any wrong info.
