Here’s the results from my test from a 58 mile mostly 70mph journey I do regularly:
Normal driving at 70mph using acceleration - 1 hour , 50mpg £10.10 cost
Maximum speed 55-60mph and only gentle acceleration - 1 hour 10 minutes, 62mpg £8.10 cost
So, an extra 12mpg and a saving of £2.
However, as I drive in the busy times I won’t be doing it again as £2 is worth it for safety.
Apart from lorries you are the slowest thing. Even though vans should only be doing 60 they actually do 80 and you are swarmed.
I even had someone in a Yaris hybrid flash me for doing 61 in a 70 limit before they zoomed off into the distance at 80!
Although I did see one suv only doing 45-50mph.