It's a Mad Mad Mad Mad World (1963) plot summary
Three cars and a moving van witness a man drive over a cliff in a car. When they pull over to see what happened, the man tells them about $250,000 he secretly buried "...under a big W". They agree to not let the police know and all start racing towards the location of the money. They don't realize they are under police surveillance the whole time they are enroute and do everything but kill one another in trying to get to the money first.
Submitted by Al Rooney