Before making the engine kill switch, check how easy your car will DFCO which basically would do the same thing automatically. The Ultragauge or a scangauge can show if your car can DFCO and how long/hard it is to keep it in that mode. My camry I basically never feel it since it does not happen very often, and first time driving my corolla, it was a world of difference, a light off the gas and it cuts fuel off.
The biggest gain so far for me is driving 45mph instead of 60 on back roads (95% of my driving). Lots of aero mods and improve how you drive, and you might hit the 60mpg mark. It should be a good challenge
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