In school we learned ” what you save in energy, you have to increase the way.” By that rule you should use the exact same amount of energy (fuel) for the same distance no matter what speed. I’ve asked a few people, but no-one could give me a good answer.
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Combustion engine cars are tuned for peak efficiency at around 80kph (50mph). Internal combustion engines actually have the best fuel in to power out ratio at around 70% load believe it or not. So if you drive too slowly and never load up that engine it is counter productive. Cars with smaller engines get better mileage as the engine spends more time ‘heavily loaded’.
Step up to the bigger engine and you aren’t loading it enough for peak efficiency. But if you are often driving very fast, climbing mountain highways or towing that bigger engine isn’t being overloaded as often so it may be a better choice to you.
Above around 80kph (highly dependent on aero), drag takes over and load increases exponentially.
Electric cars ARE more efficient at slow speed.
Hybrid cars allow that engine to be downsized so it can spend more time in that peak efficient loading, coasting or not running at all. The electric motor takes up the slack.
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