the holy cow has a tire size calculator that will tell you the size difference and how much of a difference it will make in your speed. you will be going slower than what your speedo says you are. have you considered a different tire brand? I stay to stick with the factory size if you can.
also put those tires to max pressure and do a grill block. that may get you to your goal along with driving style changes. maybe even a scangauge.
*edit* there is a link to his calculator in his signature. just search for him, he isn't that hard to find screen name = "theholycow"
Be the change you wish to see in the world
Yup, there's links in my sig for the size calulator, info on tire pressure, and info on tire width vs. rolling resistance. The OEM-sized 195/70 tires will give you better fuel economy than the narrower, shorter tires of the same model for sure.
That was actually another question I forgot about asking.
Ive considered a grill block but I am concerned about the car overheating.
The summers in Northern Virginia lately have been a bit on the high side ( nothing like it was in Atlanta but still ). Im also looking to move out west. Arizona, New Mexico, somewhere like that.
Is it possible to do a grill block and not have your car die?
And since the object of getting better mpg is to go slower, I think it would be to my benefit for the gauge to be off 2.2% ( as per TheHolyCow's calculator ). Almost like setting your alarm clock 10 minutes fast so you are always up early.
As I understand, grille blocking works in two ways - it causes your engine to reach operating temperature faster so it spends less time running cold, and it affects aerodynamic drag. Considering your weather I wouldn't worry about getting hot enough fast enough...
As for the gauge error, that sounds like a good tool if it will work for you, but remember to calculate in the odometer error when you calculate your mileage.