Actually, the VX's automatic tranny is not a problem like you might think, because it runs full-lockup in all but first gear, so the power loss due to an oil coupling in the drivetrain is just not there. The fact is that the VX is a very heavy vehicle (for its size), with full-time 4WD, a big frontal area, a high-drag body, and an engine that gets a lot of torque out of a gallon of gas. You're not going to make a 25 mpg gas-sipper out of that.
I've gotten as low as 13.9 mpg with my Yakima cargo box in the winter and as high as 18.2 in the summer with no box. (The box costs me about 1 mpg whatever the season) Winter air is denser, but colder, which I think is what reduces mileage in the winter on ALL vehicles.
Speed and the amount of stop-and-go is the big difference from what I can tell. I push 80 mph on the highway, and I can tell the engine needs a lot more pedal to keep the pace. When I run long distances on back roads between 45 and 65 mph, I get the best mileage (a hair over 18 mpg). It drops at least 1 mpg when I'm running on the highway at 80. On the other hand, my 20-mile drive to work at 40-50 mph on back roads with several stop signs and lights makes my mileage drop to about 16.5 or so. Same speed range, but more stopping-and-going make the difference.
Another thing I noticed on my old Prelude is that a change in tires made a big difference in the mileage. With the original tires (Bridgestone all-season sport tires) I got as much as 26 mpg highway. When I swithed to Avon three-season performance tires, it dropped to about 23.5 mpg. The mileage you get in the stock car can change with almost any mod. All the little factors play together to determine your mileage. Change any of them, and you may change your mileage.
Locale and types of gas make a huge difference, too. The gas you get in one city will not be the same formulation that you get in the next - even if it's the same grade and the same company!! In fact, the landscape of gas formulations is wildly complex, and it's a principle cause of the generally high price of gas. Refineries have to make literally dozens of different formulations of the same octane grade to supply each and every county with the type of gas they think will be best for their environment. Car & Driver did a great article on this a few yeas ago. It's a ridiculous situation, really.
If you want cheaper gas, tell your state congressman to reduce the number of formulations that localities have to pick from.
Ok, time to get off the soap box and get to work...
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