60 mph and being able to get 6.2 m/kWh? No way. It must be a very short highway or slipstreaming.
Then, all other speeds must be below 40 mph.
I did similar challenge just yesterday.
No faster than 40 mph, no heat or anything. It felt so awful. Yet, I got 4.7.
Nice, considering other days 3.5 or less. Sure, MI cold days do not help.
I am DONE with this.
I can have my own challenge - 120/20. I will have blast from driving it. Hard take offs and let it coast.
I did not buy this car to move like a turtle. I have 200 HP and over 250 lb*ft and I want to use them.
Tires? Oh well... Electrons? Oh well...
I am glad, though, to know that my car is capable of getting over 6 miles to kWh. Or in my route - almost 5.
Good luck on 20/20 adventures.
One thing I wanted to add here.
The other day I wanted to see how the car rolls. It coast in N beautifully.
Having said that - for the challenge, or just for the best results - let it coast in D. Not L. Avoid any regen at all. Regen by default is 70% efficient as driving, so if one can predict where the coasting will take them, that is the best way to get to higher numbers.
Funny, but that also applies, to much greater extent - in ICE cars. Not the acceleration matters as the stopping - give it a good accel and then let it coast to stop. It will be better than gentle accel and grind the brakes.