Air resistance increases as velocity square increases. The power consumed to overcome that air resistance increases as the speed cube. Rolling resistance is the dominant force below approximately 40 mph. Above that, every mph costs you miles.
Go as slow as traffic and your schedule will allow you. Drive below 60-65, as the air becomes exponentially denser, in the aerodynamic sense, the faster we drive. To be precise, the most efficient speed is the minimum speed of your car at its highest gear, as it provides the best speed ratio per RPM. This is usually 45 to 55 miles per hour.
I've always heard that you use more fuel if you go faster, although I would say the opposite. Here's a Better Assumption from People Who Aren't Cars. Fuel consumption, as far as I know, is not a linear function of speed. The faster you go, the worse the mpg you'll get.
Therefore, a slower speed will burn less gas traveling the same distance. Lossless, more and more work is needed to increase the speed of the car as it accelerates. Efficiency is usually subject to the law of diminishing returns. Which means that the more you have of something, the less efficient it becomes.
This is intuitive if you imagine running instead of walking. If you run three miles you're going to use a lot more energy than walking. As you start walking and start to increase your speed, there's a point where you'll need to change the way you move to go faster (you'll need to lift both feet off the ground at the same time). I hope that was relatively useful, even if it weren't in terms of cars.
Try to make a slow stop at the traffic lights. If you can keep it moving, the 1-2 mph you're going to travel will reduce your gasoline consumption when you take off. Every time you step on the accelerator, you pump more fuel. If you drive too slowly, your fuel economy will be bad just because your engine uses some fuel to keep you idling, even if none of that energy will accelerate it or combat aerodynamic drag.
It relates to engine efficiency at a given speed, gearing, transmission efficiency, transmission harmonics, rolling resistance, aerodynamics, and how they all interact. I've heard something along the lines that for every 5 miles over 55 you drive, you lose a little percent in gas efficiency. Now the airflow over the vehicle starts to produce noticeable drag, which slows down the car, so you have to use more gasoline to overcome it. For example, your car will most likely consume less gasoline with its engine running at 1800 RPM compared to 2300 RPM, and it's possible to drive at a handful of road speeds other than those RPM.
Now you don't need to waste gasoline and endanger your life and the life around you by driving recklessly. When I started driving, I drove slow because I thought it would save gas, then I started driving faster when a friend told me that you use the same amount of gasoline, whether you drive slow or fast (as long as it's the same distance), you would only drive fast for a shorter time and drive slow for longer a lot of time, but in the end it burns through the same amount of gas. In addition, since the title says that cars are slower in traffic, you will have to brake for other cars and traffic lights and so on, thus wasting energy. To answer your original question, the most efficient way to approach that hill is to increase a little speed before the hill (ideally through momentum) and just accelerate it on the way up (stay in your maximum gear).
So I would say that the slower car would use more gasoline, but maybe I'm missing a variable or maybe the engines use different gases per revolution in different gears. When you start hitting the road's minimum speed limit, switch to a proper gear and only give it the amount of gasoline needed to keep it at the minimum speed limit until you crest the hill. Assuming the same type of gasoline, car, efficiency and RPM, let's say the fastest car is in sixth gear and the slowest in third gear. You want to drive on average at a speed where the engine is very efficient and the car doesn't waste energy pushing through the air and the road.
Instead, use the left lane to pass slower traffic when necessary to maintain the same speed, especially in light traffic. When I say that it is inefficient, I mean that it only converts about 25-35% of power into gasoline and converts it into propulsion. Please tell your friend that not only is he wasting gasoline, but the poor wanderers trapped behind him are wasting gasoline. .
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