I tested my car as a hypermiler over thousands of miles on dead flat roads empty of other vehicles, my daily commute was 78miles. The most economical speed was 30mph in 5th gear with cruise control on. For most people that’s not a practical speed for any kind of long journey,but I was getting over 100miles per uk gallon in the summer (hot weather helps with fuel economy, as does not having headlights or ac on). I got through so many audiobooks driving so slowly!