The problem is turning it into actual real world meaningful gains on a larger scale and without other potential unintended side effects.
Dropping all the speed limits indiscriminately by 10 MPH for instance probably won't have the effect it might look like on paper.
Agreed. If you look at studies, depending on the car, there isnt a great difference between 60mph and 70mph for the mpg once you have got up to speed. However once you break the speed limit and go 80mph is massively more fuel than going from 60-70mph.
So perhaps the answer is not to drop speed limits but just get people to stick to them or enforce them.