most cars are more efficient at 55 to 60 MPH than at 40 MPH
I think the studies show 50 mph is the sweet spot for the same distance on highways. But you are right it varies by both vehicle and driving style. But the difference between 50 and the current 70-80 mph people drive on freeways is substantial for any vehicle.
Certainly true for highways, yes. There's also next to no good alternative to individual cars for highway driving, though. Amtrak is very, very limited, Greyhound is slow *and* very limited, airplanes are grossly fuel inefficient and are generally limited to relatively long distances, and that's about it.
The other part of the equation is how many miles someone drives. Lower speeds mean people drive fewer miles because the real cost of a trip is the time it takes. If you drive 8 hours at average 50 mph you only go 400 miles. You drive at average 60 miles per hour you go 480. If you use 5 gallons to go 100 miles then you use an extra gallon of gas.
Not sure why you think people would drive fewer miles. Most people in cars are trying to get to a specific place, not driving just to drive. Would people plan shorter trips? Maybe for some small percentage of leisure driving, but leisure driving is already a small percentage of driving, so that's a small reduction in fuel use for a small percentage of a small percentage of trips. That's hardly worth the negative impact on everything else.