The formula is: gallons per mile/people in car A Suburban getting 20 MPG with 3 people is using 0.016667 gallons per person per mile, ((1/20)/3)=0.016666667 A Suburban getting 20 MPG with 6 people is using 0.0083333333 gallons per person per mile, ((1/20)/3)=0.0083333333 A 15 passenger van getting 15 MPG with 15 people is using 0.004444444444 gallons per person per mile), ((1/15)/15)=0.004444444444 A Prius getting 50 MPG with 1 person is using 0.016667 gallons per person per mile, ((1/50)/1)=0.02 A Prius getting 50 MPG with 4 people is using 0.005 gallons per person per mile, ((1/50)/4)=0.005 So as a way to efficiently move people, a Prius getting 50MPG carrying 4 people is less efficient than a 15 passenger van getting 15 MPG with 15 people on board. With 5 people, the Prius is slightly more efficient than the van (0.004 vs 0.004444) Aaron Z