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“Lean burn” as you call it, is engaged a couple of ways. But let’s call it what it is - fuel cutoff during deceleration.
There are a couple ways this happens:
1. Time based, let off gas pedal for x amount of time, fuel is decreased/cut off to injectors. It’s somewhere in the 15-30 seconds range according to temp of car, speed, etc.
2. Charcoal canister purge operation, after roughly 1 minute of steady throttle. Valve opens and vents fuel fumes to intake and burns then off.
Your car doesn’t haven’t a lean burn mode like the old Honda lean burn cars of twenty years ago. A lean burn setup requires a wideband O2 and specific tuning to do it.
Having an automatic trans makes your job even harder. Ideally you need to be at 35mph or below, but trans won’t be in top gear. But 45mph can work.
Good luck with your quest - I think you will fall short and manage around 50mpg on your round trip assuming boat tail, weight removal, super aired up tires, etc. That’s my official guess.
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