For me, a recent example of fuel savings by conservation through lower speeds is driving a 250 mile loop at 60mph that I normally do at 75mph or more. I saved at least $8 in fuel and managed over 30mpg with a 1991 Buick Par Avenue with 85,000 miles. The car just had a tire rotation and oil change.
Made for a huge difference but I admit it was brutally boring. You really need to adopt this lifestyle with conviction to maintain it. The proof is there in numbers, but the spirit must be on board.