This is a popular question in safety classes. Drivers want to know how far they can show themselves driving on their logs, without it being questioned. The short answer is, you are supposed to log it as you drove it. In other words, if you left Carlisle, PA at 8:45am, and arrived in Columbus, OH at 3:00pm, then that’s what you put on your log. If you log it as you do it, you don’t have to worry about miles driven, etc.
However, to answer the question, the DOT will generally allow you to log 5 mph below the speed limits in whatever states you’ve driven in. For example, if you are driving in states where the limit is 65, you can have 60 x however many hours you’ve driven, without arousing too much suspicion. Many drivers try to cheat an extra hour here and there by driving 12 hours, but logging 10.5 or 11. This is common practice, but just remember, if you are involved in a big wreck, the police and the district attorney are going to go over your logs with a fine tooth comb.