I found it here:
HeyWhatsThat Path Profiler
I made one for my old commute:
Now I understand how i could get to work with 0.27L of gas but never got under 0.39L on the way back. (of course to get those values, every star in the Universe had to be aligned just right)
Best round trip ever was 0.28L+0.39L=0.66L which over the real distance of 17.4km makes 3.79L/100km or 62MPG.
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2016: 128.75L for 1875.00km => 6.87L/100km (34.3MPG US)
2017: 209.14L for 4244.00km => 4.93L/100km (47.7MPG US)