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Rhetorical- don't need to answer these.
Are you uneasy with the high revving nature of a small displacement sports car engine; do high revs make you anxious?
Could you simply shift earlier?
Do you spend a lot of time on the freeway, enough to trade a 1-2mpg gain for reduced acceleration?
Have you considered other options to improve mileage, such as underbody aero, tires, fluids, etc?
Do you ever drive at high rpm?
Do you ever drive the car aggressively or go on fun canyon drives or track days?
Are you trying to save money? i.e. Spending hundreds or thousands to change drivetrain parts to get 15-20 extra miles out of a tank is a little illogical, like buying a motorcycle to save money on gas.
If you're really after better mpg on the highway, honesty the best way to do this is a taller 6th gear, as it won't affect the other gears that would be used for performance driving. Unfortunately that's not a realistic solution, because I believe it doesn't exist (yet). Also, changing a clutch is relatively easy work, cracking the tranny open and replacing gear stacks is a whole different ballgame.
The next option, which is really your only option, is to change the ring and pinion gears in the diff to a taller ratio. Swapping these two gears will affect every gear, so the spacing will feel the same, just slower. It won't affect the diff operation as it doesn't require the diff itself to be changed. I believe our diff gears are compatible with some other Toyota/Lexus models, so there may be some easy options. Most people want a shorter final ratio for quicker acceleration, but you want a taller ratio... Our cars aren't exactly blessed with an abundance of power, so a taller final ratio will make the car feel slower overall.
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