Ther is plenty of room in the front of the seat for a 40lb dog, but that may put a damper on available leg position options for you. With the D-rings that come stock there is not A suitable number to move the seat back...so you would need to add additional D-rings to do so. A dog of that size will fit behind the seat just comfortably, but there will not be an abundant amount of room i.e., roaming around, jumping from one side to the other...just enough to sit or lie down.