can't speak the 5vz in mr2's.
i can say that in he camry/es world that all mz blocks get 1mpg average and 2mpg highway better than the 3vz-fe's, and the latest vvt-i's slightly better thn that (but almost none can actually average 30mpg highway. A/T gearing is a hair too high)
be the mpg increase from engine tweaks, transmission / torque convertor tweaks, or overall slightly decreasing .cd VS frontal area over time i can't tell ya. not something i would honestly worry about myself.
personally i feel if people worry about >5mpg in performance cars they shouldn't consider any swap. grab an old cheap stock NA or a miata or something & stick with 30mpg+.
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