Yeah, eye popping numbers are doable in perfect conditions.
I once did 104 MPG US / 2.3 L/100 KM / 44 KM/L in my 1.0L Firefly (Metro/Swift) for a tank. Summer weather, almost 95% driving on empty rural roads, using pulse & glide. To put that number in perspective, my average speed was something like 40 km/h. Though my top speed ... on some down hills ... was over 80 km/h.
Not practical, but an interesting experiment.
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View my fuel log 2014 Mirage ES 1.2 manual: 62.4 mpg (US) ... 26.5 km/L ... 3.8 L/100 km ... 74.9 mpg (Imp)