I think that’s a myth to an extent. Every vehicle has an optimal speed based on aerodynamics, transmission gearing and some other factors. For instance at 65 my truck gets 17mpg on the interstate in VT, at 75 it gets 19 and at 77 it gets 21. I don’t know why, I think it has to do with carrying speed on the small ups and downs we have, so the cruise control doesn’t downshift as much on little hills, but it’s a thing in some vehicles. It also generally rides smoother at 77 than 70, my tires have a small imbalance as the are all terrains and it’s impercievable at 60, but becomes a tiny vibration at 65-73 and then goes away.

And to your point about speed and time it’s dependent on commute. If you are driving 3 miles to work the person doing 60 gets there in 3 minutes and the person doing 45 gets there in 4. That is a small difference working out over a year to 8 hours of extra time in the car for the slower driver, and over a 40 year career 13 days lost. If you extend that commute to 25 miles which is fairly typical in VT it’s 17 minutes a day 71 hours a year 118 days in the 40 year career. That’s 1/3 of a year in a finite life span being wasted just driving slower than is needed. That’s .4% of one’s life they waste.

When you add in all the other non-work driving we all do, that person who commutes 50 miles a day round trip probably drives 350 miles a week. It’s 100hrs a year, if you do that for 55 years which puts age say 20-75 typical driving years, it’s 230 days lost just sitting in the car on the way to the destination. And it grows the more you drive.

I just did out the math, and since age 20 I have driven 410,000~ miles in 15 years. While speeds are not constant I do 10-12 over pretty much all the time, so while this isn’t perfect math it’s close as benefit of that is greater at slow speed in town than at highway speed and a lot of the driving is in towns. If I had gone 15mph slower on those miles I would have at age 35 spent 93 more days in my car than I have.

If you can’t figure out what you would do with 93 days of free time returned to you just by pressing the gas pedal a little more then idk what to say. But it’s not “small” amounts of time when you look at it for a lifetime. With the amount I drive it will be an entire year of my life avoided in the car.

Additionally I think if you were to ask those that know me they would say I am punctual to a fault, very anal about being on time, I just in my time of driving have really nailed down the time it takes to do things in different conditions. Sometimes I get somewhere early, but that’s just more time at the destination (fun if it’s something I want to do, and I can finish sooner if it’s something unfun like work), but I’m never late.