Most cars have their most fuel efficient speed at around 80-100 km/h, but most main roads are 100-130 km/h in terms of maximum speed. Shouldn’t we change cars?
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In general, you would want the car’s engine and transmission to be designed in such a way that it’s fuel-efficient at ALL speeds. Cars drive in cities at low speeds, and on the expressways at high speeds, and not everyone is going on vacation and driving on the expressway at 120 km/h the entire time. In fact, the most typical scenario is driving city speeds going to/from work every day, and on the highways, being stuck in the traffic slowdowns as you commute to/from work.
As far as the engine efficiency, it is a bell curve, and if the top of the bell is at 80-100, you have pretty good efficiency a little bit higher than that at 100-130, and a little bit lower than that 50-80 km/h (city speeds). You pretty much want to put the bell curve in the middle of your range of speeds, rather than at one end, so that the car is fuel-efficient at a good range of speeds.
>Most cars have their most fuel efficient speed at around 80-100 km/h, but most main roads are 100-130 km/h in terms of maximum speed. Shouldn’t we change cars?
A maximum speed is just that – the **maximum**. Anyone wishing to be fuel-efficient is free to drive at a lower speed.
As to the engine part, no doubt an engineer will chime in and give a more proficient answer than I ever could as to how car efficiency speeds are a mere byproduct of physics and design, not a goal post that is actively modified or striven towards.
The biggest problem is physics. Air resistance increases with the square of your speed; if you increase your speed from 50kph to 100kph it requires 4x as much force to move your car down the road. At high speeds (above about 120kph) it really isn’t possible to be super fuel efficient as you have to burn more fuel to overcome the air resistance.
That’s also why the most efficient vehicles (Teslas, Prius, etc.) have a teardrop-like shape; it minimizes the effects of air resistance.
During the fuel crisis in the late 70s the US limited speed limits nationwide to 55mph (89kph) to reduce fuel consumption.
Air resistance is approximately proportional to the square of the speed. That means the faster you go, air resistance increases very quickly. The amount of work required to push the air out of the way at high speeds means the most efficient speed can never be very fast.
So if its less work to go slow, why isn’t it most fuel efficient to drive very slow? The motor wastes energy just making itself run and powering the various other systems of the car. So if you drive slow, you spend more time out on the road.
So you want to find the sweet spot where you aren’t driving for too long, or driving too fast.
The most efficient speed for your car is whatever speed you can sustain at lowest rpm while adding just enough gas that you don’t slow down. There are many “efficient” speeds depending on the situation. The most efficient low speed is basically 1st gear crawl, and most efficient high speed is whatever you can sustain with least gas oedal use on highest gear. Efficient driving also means slow acceleration.
Different cars have different speed where they are efficient.
Speed limits are set according to geometry, safety standards, road risks analysis, road conditions and road type. They have nothing to do with efficiency.
Of you want to be efficient, you use big machines with big motor to move loads of people; Like a bus. Or you use a small vehicle that is light with small engine that runs at basically at one speed and the loads of gears that achieve correct speed/torque levels at the best point on the efficiency curve.
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