Diesel routes does works in Google maps algorithm. I have tested it personally in the UK. I don't know how good or bad this algorithm works currently for other countries. This is something new in it.
On exactly same trip between point A and B I went several times, when started using google diesel method I noticed slightly better fuel economy and as well slightly different route. Fuel was taken always from the same fuel station.
If you use petrol and diesel cars, then diesel is more eco on motorways rather than petrol, while petrol is always more economical in cities when a lot stop&go rather than diesel. That's the key factor for diesel routes, I think. Definitely it was putting me on slightly different routes.
Spoiler: screenshot
It does work for me, that's why I started to looking for map solution (hnr file) for iGO if exist. I can see short/fast/eco routes hnr, I hope it's just matter of time and it will be introduced hnr map file like Diesel, EV, petrol.
There is not much to do in maps apart of map route updates so I think that companies are looking for the way to make their product more attractive. A lot new options that's rubbish but this diesel thing is really working for me.
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