Did you fill the tank at any time prior to the trips?
Did the boat smaell og gas when you first boarded or only after running for a while?
Did you smell for gas fumes in the bilge before you started the motor?
Have you pulled the deck plates over the tank and looked for seepage or gas odors?
Was the helm covered when the boat sat?
What side is the fill on? Have you sniffed up close to the gas fill?
Did you smell for fumes in the cabin?
During the test drive did you mention that you smell gas fumes? If so, was the seller on the boat?
If so, did you ask the seller about the smell?
If so, did the seller make a comment?
Was anyone else on the test run who was witness to any seller statements?
The question about filling the tank is important since filling the tank could contribute to leaks, not only in the tank, but seepage in the fuel hose or vent hose.
Smelling gas fumes at 40 MPH is unusual and can indicate that there are gas fumes in the forward bilge. The forward bilge by its nature often does not drain completely aft unless the bow is angled up at a steeper angle than the hull normally sits.
Of course, you could have a leaky tank. That can be a pretty big job to check.
What I would do is not fill the tank more than half way for now. I would mix up some boat soap and water and rinse/flush the bilges. Leave the deck plates off ( assuming dry weather) for a day or more after flushing the bilge to see if you smell gas both before going out and after.
It is important to try to determine if the odor is worse on full tanks as opposed to partially filled tanks. Based on what you find, we can make addition suggestions.
Be careful. Gas fumes in the bilge are dangerous.