Originally Posted by Auto Enthusiast

I often do cross-country roadtrips as a summer vacation. Normally I find cheaper one-way car rentals going west to east, but most recently I found the best deal going east to west. When I did my fuel forecasting, I remembered from the past several years that flat terrain averages ~35 mpg, ~33 mpg for sections at 70-75 mph, plus 10% with a tail wind, and ~26 mpg when driving in the mountains. This time heading west, I started with the baseline 35 mpg, and subtracted 10% from what I expected would be a head wind. So far, so good. I should still have enough gas each day to reach either my hotel for the night or the tour starting spot in the middle of the day.

After leaving Gettysburg on my way to Pittsburgh one evening, I found myself stopping in disbelief at a rest area on I-76 somewhere in central PA. I thought I had enough fuel to make it to my destination, but my forecast was 150 miles wrong. That's when I realized what had happened. I had forgot about the Appalachian Mountains. Since I normally head east on these trips, I had neglected to put the 26 mpg at the beginning of the forecast. Had I done it the right way, I would have allotted the time to make the stop. Luckily I had pro-actively pushed my Pittsburgh tour to the next morning when I saw the traffic zoo around Gettysburg.