Should I hold it in?

My boyfriend (of 8yrs) and I are driving home from a ski trip. I saw a pregnacy test in the last gas station and couldn't resist (my period was due today so...) well it was positive! Now I don't know if I should tell him in the car ride 9hrs, or wait to take another test I almost can't believe it. Any advise or ideas on how to announce it? Thanks!