Let's see. If we assume 3000 miles at 64mph, that's 46.875 hours. 3000 miles at 70mph is 42.857 hours, a difference of 4 hours. Of course, you won't be able to average those speeds anyway, considering mountain traffic, towns, gas stops, etc., but the time difference isn't that great considering the distance.
As a former OTR truck driver, I can tell you that the top speed of the truck is not what shortens the time. Staying in the seat shortens your time on the road. When you stop for the night, you actually sleep instead of staying up ratchet jawing all night in the truck stop, then waking up later in the morning and having to hammer on it to make up for lost time. And of course if your well rested, your performance on the road is much better and everyone is safer.