Weather is probably the main concern. The almanac shows Summer relatively wet. The Spring and Fall months are drier on average. A quick call to the Weather Office in Billings confirms the upcoming Fall week dry except for light showers in the North-east corner of the state.Accomodations are easy to find with a map and tourbook, especially off season, so we didn't bother making reservations. A Triple-A card nets $10 off the regular rates at most stops. Our general plan is to drive Eastern and Western loops of Montana. Taking a day or two to reach the state and two or three days to explore should fit well with our 5 to 6 day schedule.
A discreet call to the Highway Patrol office in Billings (406 245-6193) finds that there are in practice speed limits, but that they are open to interpretation based on conditions of traffic, weather, terrain, car and driver. As a practical matter, 85-90 MPH are no problem under good conditions but higher speeds or inappropriate speeds for conditions can result in a citation. A strict nighttime limit of 65 or 55 applies; an abundance of deer makes this very sensible.
Our rule is to be courteous to other drivers and to go a speed reasonable and prudent for conditions.