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!

Economy isn’t just about fuel economy. Less unsprung weight and highly inflated tyres reduce fuel consumption, but low profile tyres result in damaged wheel rims and lack of adhesion results in lack of control.

Alloy wheels with low profile tyres are lighter, but are also more prone to irreparable damage.

If you have the smallest steel rims that fit over your brakes (and the highest profile tyres to match manufacturer recommendations), you are less likely to have a puncture or damaged wheel, especially in potholed regions. (I live in Victoria, Australia).

If you have the correct tyres inflated to the correct pressures, they will last longer and provide the most optimal grip.

which is the most economical speed to drive.

That is not universally accurate. Even at the time, there were vehicles that were more efficient at higher speeds than 55mph. But those weren't American designs, of course so they were of course ignored in favor of the general majority. And those vehicles that were more efficient at higher speeds, got screwed for simplicity.

No... It was the most economical speed to drive for the average car at that time. Now it's 65 because that's what the federal test uses.