I gained 7% fuel economy with a 50% grill block, a windshield wiper block, and a smooth front and rear belly pan. All of the mods were done with coroplast, think campaign signs. The ones I got had a vynal layer on them that peeled right off, leaving a nice white surface to work with. Doing an upper grill block is preferable (leaving the lower one open/partially open) as it screws less with your aerodynamics. Pop your hood an look at where there is a radiator, in my car, there was only a radiator on one side, so the other side got the grill block.
Your taller tires would work well with better aerodynamics, since the lower load seen (due to improved aerodynamics) means less power needed (so taller gearing is preferable). Just remember to not go TOO high, as you still need to be able to accelerate. Also, remember that you are going to be screwing with the speed reading you are getting. My speedo is off by 6% due to the larger tires I am running.
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