Gas Mileage Tests

How much gas does my car burn when the engine is idling?
Does my vehicle get better gas mileage when I go faster?
Does my vehicle get better gas mileage when I have the windows down?
Does my vehicle get better gas mileage when I have the air conditioning on?
Does my truck get better gas mileage when the tailgate is down?

Over the past 9 months, I've been running experiments on my vehicles to measure the fuel-usage in different situations. This involved a lot of daily trips to the gas station using the exact same gas pump and several hours of my time to ensure variables such as tire pressure and wind/weather were consistent. Each of these situations was tested a minimum of 6 times, shift points and acceleration were consistent, the same GPS device was used in both vehicles, the route never changed and highway speeds were set/maintained with cruise control. Here is what I've found:
Vehicles tested:
4.6L V8 1996 Thunderbird LX 4spd Automatic
4.3L V6 1998 4x4 F-150 short bed 5spd Manual
Engine idling in the Thunderbird cost me 0.46 miles worth of fuel every 60 seconds.
Engine idling in the F-150 cost me 0.38 miles worth of fuel every 60 seconds.
Driving at 45mph on the highway (in Overdrive) in the Thunderbird achieved 23.42mpg.
Driving at 55mph on the highway (in Overdrive) in the Thunderbird achieved 22.17mpg.
Driving at 65mph on the highway (in Overdrive) in the Thunderbird achieved 20.38mpg.
Driving at 45mph on the highway (in 5th gear) in the F-150 achieved 17.91mpg.
Driving at 55mph on the highway (in 5th gear) in the F-150 achieved 16.81mpg.
Driving at 65mph on the highway (in 5th gear) in the F-150 achieved 14.58mpg.
Driving with the windows down in the Thunderbird achieved better fuel mileage at speeds lower than 46mph compared to driving with the A/C on. Driving with the windows down in the Thunderbird at 60mph lowered the fuel mileage by 3.31mpg compared to having the A/C on. (Over 15%!!!)
Driving with the windows down in the F-150 achieved better fuel mileage at speeds lower than 53mph compared to driving with the A/C on. Driving with the windows down in the F-150 at 60mph lowered the fuel mileage by 1.36mpg compared to having the A/C on.
Driving the truck with the tailgate in normal position at 65mph achieved 14.58mpg
Driving the truck with the tailgate down at 65mph achieved 14.12mpg
(I tested this 7 times in multiple weather conditions, each time the tailgate was down I achieved slightly less fuel economy. I always thought the tailgate-up increased the drag!!!)
Driving to work and back with 180lbs of sand in the trunk of the Thunderbird =19.97mpg average. Driving to work and back with no sand in the trunk of the Thunderbird = 20.62mpg average.
Driving to work and back with 180lbs of sand in the bed of the F-150 = 14.97mpg. Driving to work and back with no sand in the bed of the F-150 = 15.31mpg.
Conclusions:
-Drive slower on the highway to save gas. (Physics says the faster you drive the harder the wind pushes back and more drag you create, this is clearly evident.)
-Drive with the windows up and A/C on if driving faster than about 45mph. (Likely also related to the exponentially-increasing drag at higher speeds.)
-Get as much weight as you can out of your vehicle to save fuel. (Not necessarily true in slick conditions where traction may be an issue and thus fuel is used to spin tires without moving forward.)
-If you drive a truck keep your tailgate up. (This came as a surprise to me, I'm assuming it has to do with the way the Ford engineers designed the truck? Seems like the tailgate down would be more aerodynamic and therefore more fuel efficient... but aerodynamics are complicated!)

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