Well, I saw sompun differ't da other day....a Prius that wasn't going for mpg but just going. On a four lane section of highway I was moving 2 or 3 mph slower than the speed limit in the right hand lane. A smaller truck pulling a trailer passed me doing ~ 5mph over the speed limit in the left lane. Tailgating him was a Prius. Once they both were passed me, the Prius snapped into my lane & accelerated past the already speeding truck & trailer by at least 15mph.... maybe a bit more. He past out of sight over a hill ahead of me. After topping the hill, I had a clear view for a half a mile ahead. A state trooper had pulled the Prius over & was giving him a speeding ticket.
My wife was riding with a friend saw a Prius pulled over. She thought it was her friend at first since they were going at a good clip. The Prius is actually relatively high on the speeder list.
I actually have been saved by driving my 2012 Toyota Hybrid a little more reasonably within the speed limit to see what kind of MPG I could get. I passed several new Marietta GA motorcycle cops with radar standing in the strangest places. Clever spots guaranteed to capture anyone who is a little over the limit to begin with before dropping down a hill.
That's why I have the radar detector installed! I however find it pointless to speed with the Prius. If I wanted speed & tickets I would have gotten a Challenger!
Is it worthwhile to buy a Prius if driving like that? I get about 60mpg average year-round with mine, but rented a Camry while in Nevada. I drove 430 miles at about 70mpg from Las Vegas to Reno (yawwnnn!), and got 41 mpg. I don't think my Prius would do that well at that speed- and don't intend to find out.
I routinely drive 70mph to 85mph on the interstate with the fast-lane flow of traffic. This nets 45mpg average. When I drive intown or on 50mph state highways, I get 50mpg easily even with winter tires on the car. When I drive to the capitol that stretch of interstate is 75mph speedlimit and average speeds in the left lane are between 90mph and 100mph. The Prius keeps up with them no problem and gets closer to 42mpg. This section has a "speed monitored by overhead aircraft" sign... OK...
I routinely got 45 mpg or above on my '04 when traveling at highway speeds (65 -- 75 mph). The last trip I made on that vehicle was at 70 mph with the A/C on for a 115 mile trip, getting a calculated 48 mpg. And the opposite of miles per gallon the OP's example must be minutes per mile per gallon. They seemed to doing ok on that until they got pulled over.