The law as it appears in 392.6 of the Code of Federal Regulations answers the question “How many miles a day can I drive?”: the answer is “Drivers are required to drive at posted speed limits ( and to remember that these limits are for clear conditions, dry pavement, and you are expected to drive more slowly in bad weather or when pavement is wet or snow-covered.)
Language of the law is “Runs of more than 500 miles a day are considered incapable of being made in compliance with the speed limits and hours of service limitations”.
In short, the USDOT will not question logs which show that you have driven 500 miles per day; they WILL challenge those which show that you have driven more than 500 miles per day, and the USDOT officer will ask you to demonstrate how it was possible to do so lawfully.
Similarly, if a daily log shows an average speed of over 60 miles per hour, you may be asked to prove that you have driven lawfully – as to average 60 mph you must have driven 70 – 75 mph to balance lower speeds of 35- 45 mph in areas of heavy traffic.
Take reporting of your daily activity seriously; log as you go; avoid the challenge and citations that are the result of an inaccurate or hastily-assembled log book.