when you replace 10% of your gasoline volume with something that is less powerfull - the engine has to use more to maintain the energy demand of moving the car.
pretty simple. But your switch of tires and wheels probably contributed to the significance of the reduction. I would expect around a 6-9% reduction in fuel econ.
ALSO Cessna - I read your websites and found some of their articles interesting. Oddly their own article shows how ethanol produced today (regardless of wet or dry method) makes less energy per gallon than it does to make it. Thus it is still a net deficit.
Now on the bit about conservation: how many of you use LED, CFL light bulbs in your house. Use High efficiency Ceiling fans. Replaced your A/C, Heater, or Heat Pump with a SEER 15 unit.
How many of you go around checking the seals on your house, caulking around your windows etc.
turn off lights when you aren't in the room, or reduce your thermostat settings when you are at work. Not to mention the idea of walking to work or the store - which is commendable.
If guns kill people, spoons cause diabetes.
My other car is a 4 door family car.