Some states have 80mph stretches, but it's not the norm. Also, I think inner-city driving is a lot more time consuming than the long, high speed stretches between urban centers. If you only consider reducing speed on highways, then you have to cover very large differences before you encounter an additional hour off round-trip commute time.
At 50mph, you'd need to drive about 90 miles to add an hour to your total round trip, compared to driving 70mph. At 55mph, you'd need to drive about 130 miles, to add an hour to your total round trip, compared to driving 70mph.
Unfortunately, driving in cities is inelastic when it comes to fuel efficiency unless you hop on a bus. For highway driving, 55mph is annoying, but it's not unbearable, and you're making a real and measurable difference. For reference, 55mph is the standard in California for vehicles with more than two axles.
In the mountain west 80mph is the norm, and even then it is more of a suggestion, flow of traffic is closer to 90mph. Some states are posted 70mph on single-lane county roads. Many practical logistics of living in those regions become infeasible at 50mph because people have schedules and finite amounts of time for travel. You are being unrealistic about the implications.
When you live in the parts of the country that have 80mph limits, and I have, it is normal to spend 3-4 hours driving in a day. Things are very spread out and the distances are far, which is why the speed limits are so high. I've lived in towns where basic things like shopping were a 75 mile drive each way. Casually increasing those drive times by 75% is not a small thing and, frankly, no one would comply. You'll end up in exactly the situation you already have -- people in urban areas will drive slowly and people in more sparsely populated regions will drive as fast as is reasonably safe.
The fact that these higher limits are largely in remote areas also has the implication that not that many people are driving that fast anyway, so it is unlikely to have much impact anyway. It would rightly generate considerable resentment with no meaningful impact on carbon emissions.
All that aside, my car hits peak efficiency at around 70mph, as did my prior car. You can monitor this in many cars and I do. Drivetrains are optimized for typical highway speeds. It isn't the 1970s.
I guess what I've read about fuel economy is outdated. My main point is that: we shouldn't drive faster than our vehicle's limit of optimal fuel economy, since after that point fuel economy drops sharply.
All that said, I don't know what to say about driving 3-4 hours a day. After working 8-10 hours, and driving for 3-4 hours, there's not a lot day left before you have to start all over. Driving a total of 150 miles for groceries blows my mind; that's basically the width of the top half of Louisiana.
At 50mph, you'd need to drive about 90 miles to add an hour to your total round trip, compared to driving 70mph. At 55mph, you'd need to drive about 130 miles, to add an hour to your total round trip, compared to driving 70mph.
Unfortunately, driving in cities is inelastic when it comes to fuel efficiency unless you hop on a bus. For highway driving, 55mph is annoying, but it's not unbearable, and you're making a real and measurable difference. For reference, 55mph is the standard in California for vehicles with more than two axles.