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Well I'd say your automatic is killing most of your efficiency as I've driven an auto 1ZZ Corolla before. The 2ZR is somewhat more efficient than the 1ZZ but you're right it's not a massive increase, you'd want a smaller displacement to get the maximum friction reduction.
Unfortunately if you really want to keep the automatic you're pretty screwed because the E140 Corollas and the automatic Yaris were still on 4 speed autos. The auto has a nice cruising gear but the gear spacing is horrendously wide (will hold lower gears longer because it can't shift down) and the torque converter doesn't like to lock.
The larger Toyota engines come with 5 speed autos and recently 6 speed autos, but you'll get a bigger engine so it'll probably burn more fuel anyways.
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