Dont build your motor with fuel economy in mind, there really is no point.
Build your motor to perform well for the HP that you want, I would go with the 8.5:1 pistons, 9 is a little steep for me.
if you want to have better fuel economy, you can order up a taller 5th gear for your car
http://www.teamrip.com/GENUINE_FACTORY_AWD_PARTS.html
(1/3rd way down the page)
10% taller 5th gear, they claim 27-30mpg, the cool part of it, is you can install it really easily, all the other gears are tough to take off.
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