I had reason to drive about 70mls (each way) on the M6 the other day, through Birmingham (aaaargh!). Air temp was around 20C, dry, little wind, more or less ideal.
The traffic flowed reasonably OK but speeds were low, typically between 40 and 50mph without any significant stops, and I always try to hang back a bit to smooth out the speed. This again is actually probably near ideal conditions for fuel economy.
I was deliberately trying to maximise economy, so accelerating very gently where possible.
My car is a 2018 SE CVT with just over 3k miles on it (yes, I know........). OK, these are the trip meter readings, possibly a bit optimistic, but even so .....
Trip north, average 81.4mpg.
return trip south, 76.2mpg.
A magazine did a Land's End/JoG run in a manual Jazz and reported 94mpg at an average speed of about 40mph. I can believe it is possible. I usually see numbers just above 60mpg around here, but I don't do commuting or traffic if possible. I'm very impressed with what this Jazz is capable of returning, conditions permitting.