Seems pretty easy to quantify - it looks like there are ~37K motor vehicle deaths in a given year [0] and "Approximately 6.2 million patient transport ambulance trips occur annually." [1]
Ambulances would need to save their passengers' lives <1% of the time for them to "break even" so to speak.
Even the emergency trips, it's difficult to say whether the person would have died if they didn't have an ambulance.
Overall this would be pretty hard to quantify.