The number of miles your husband can legally drive in a single day will vary according to where he is running. If he runs Ohio the posted speed is 55 mph. If he is running out West, posted speeds are 75 in some states. Most of the larger carrier's have their trucks governed. That can impact how fast he is allowed to run. I can run from 500-700 miles or more in a single day, depending on where I am driving. I used to run Tennessee to California in 3 1/2 days. That is 2,289 miles door to door. California has a posted speed of 55 mph for big trucks. It is several hundred miles from the California state line to where I dropped my load. In between, most states have a posted speed of 70 mph. I think using 50 mph for an average is probably good unless he is running primarily in the lower speed states, such as Ohio or California. For those states, you may want to lower the average to around 45 mph to be safe. He should be able to average 50 mph or more in most states. If he can do more, then he is ahead of the game. I find it good to have a target stopping point for each day. I have traveled some area's so often, that I favorite stopping places. I always try to time my stops accordingly.