Our tripometer doesn't work!

Sunday morning, we left bright and early and headed out for church. I had my directions from Mapquest in my hand and we were off! Everytime we would turn onto a new road, I would tell him how long we were supposed to be on that road so we knew when to start looking for the next one. He would tell me the mileage on the tripometer and I would calculate what the mileage would say at our next turn. Well, after about 4 turns, we kept noticing that the numbers weren't matching up. They were completely different than what I said they were supposed to be. Then I realized, he was reading me the numbers on the Miles Per Gallon screen!!