I have slowed down my driving some as I've aged, but I still am one of the faster cars on the road. I actually believe that, provided you keep your car in control, being one of the faster cars is a safety advantage. (Yes, you read that right.) That's because you are seeing everything coming up, and you are unlikely to get clipped from behind. My parents actually witnessed a fatal accident last year where a guy was clipped from behind on I-15, hit the wall, and his vehicle flipped.
If someone is driving recklessly, it's better to see it up ahead than have it surprise you from behind.
Anyway, my two best-ever driving times were:
1) Reno to Las Vegas, 450 miles, 2008. I made it in 5 hours, 20 minutes. Average speed = 84.4 MPH
2) Las Vegas to Buena Park, CA, 270 miles, 2000. I made it in 3 hours, 10 minutes. Average speed = 85.3 MPH
Finishing with an average speed in the mid-80s is tough because you will inevitably have to slow down at some points due to traffic, presence of cops, construction, small towns where speed limit is much lower, surface streets, etc.
My highest speed ever driven was 120, though I didn't maintain that speed. I did it on that long flat stretch between the bottom of the hill right before Primm, going northbound on I-15. It was during the day and I could see there were no cops around, and there were no other cars around, either, so it was the safest time and place to do this. I sped up to 120 just to say I drove 120 once, then slowed back down.
I have never maintained a speed above 100 because it's considered reckless driving (a misdemeanor) in most states.
There are tiny stretches in Texas with 85 MPH speed limits. There are much larger areas in other states (Idaho, Utah, Wyoming) with 80 MPH speed limits. I have never driven in these areas, at least not since the speed limit went up that high. The highest speed limit where I've driven have been some of the 75 areas in Nevada, such as between Las Vegas and Laughlin.