Originally Posted by worthywads
Against conventional wisdom, higher altitude can mean better mpg. Thinner air means lower air resistance, and thinner air means less power which requires less fuel.
Most of Florida has high humidity which, like high altitude, reduces the amount of available oxygen per cubic foot of air. The humid air is denser than high altitude air so there would be some extra aerodynamic drag. The barometric pressure sensor knows there's less air pressure at high altitude, there's no humidity sensor so adjustments rely on the O2 sampling. In the warm humid climate most people will run their A/C all the time, not so much in the mountains. When my Taco was in Miami it averaged 18MPG, in L.A. I'm getting 20+. If I didn't drive 80+MPH every time I get on the highway I'm sure it'd do better. In Miami it was my dad's truck so different driver, but he didn't hot-rod it or haul cargo. The only things he did that would negatively impact the mileage is running the A/C and running the tires at lower pressure for the softer ride. I run the tires high and the A/C only when it's stupid hot. We both ran the cheapest gas available and I probably average 5-10 MPH faster average driving than he did and have more hilly terrain. His traffic on average was probably only a little shittier than mine. His cold starts were usually 80+ degrees, mine much lower, and I've seen a big mileage difference in driving a cold engine vs. warm.
Living in Florida as well as mountains are two extremes compared to most of the country. I'm guessing that if someone took the time to analyze MPG differences across the country we'd find that driving in either extreme reduces fuel efficiency.