That's usually true but not always. For example, take a look at this stored hydraulic energy propulsion technology:
An excerpt from their web site:
==================
... The SHEP (stored hydraulic energy propulsion) system captures energy used during braking and recycles the energy back into the vehicle at the time it needs it most; when accelerating from a dead stop.
This is the time at which the engine is performing least efficiently, burning the most fuel, and emitting the most pollutants.
During the braking process, formerly wasted energy is captured in hydraulic tanks attached to the vehicle's chassis. When the vehicle accelerates from a dead stop, the computer instructs the pump to apply the stored energy to the drive shaft. The vehicle moves forward without requiring the engine to do so.
Once the energy stored in the hydraulic system has been used up, the computer instructs the normal engine function to take over.
The next time the brakes are applied, the hydraulic tanks capture the wasted energy once again, and the process is repeated when accelerating.
The SHEP system is a hydraulic drive system that recovers vehicle braking energy, and returns it to the vehicle when it needs it most- when the vehicle is at a stop...
==================
To me, this makes sense. Think about the following:
If a car weighs 1000 kg and accelerates to 60 mph (27 m/sec), the force required is about 365,000 joules of kinetic energy.
At 100% efficiency one gallon of gas contains the energy about
350 such acceleration cycles but of course no vehicle is anywhere near 100% efficient. With current technology, 10% efficiency is more like it.Stored hydraulic energy propulsion, if successful in recycling 70% of the 365,000 joules of energy referred to above would imply
255,500 joules saved per 0-60MPH vehicle acceleration cycle.A gallon of gas contains 132 million joules of energy, or the equivalent of the energy saved in 517 vehicle acceleration cycles.
132,000,000 / 255,500 = 517So how many 1000kg 0-60MPH vehicle acceleration cycles occur every day? Just in the US, there are over 200 million vehicles. Let's conservatively suppose that one quarter of them are used daily and experience two dozen (24) 0-60MPH acceleration cycles. That means:
(200M vehicles / 4) * (24 a.c.) = 1.2B acceleration cycles
1.2B accel. cycles / 517 = 2,321,083 gallons of gas saved per dayObviously you can slice and dice this many ways. We haven't factored in the capital cost of Stored hydraulic energy propulsion system either, and with gas under $2 gallon in most areas of the USA maybe it doesn't yet pay. Maybe it would with $5 gallon gas. Or maybe it would in a commercial application like buses or trucks. I dunno.
A variety of assumptions can be applied, but the main point is that there ARE things that can be done to improve vehicle efficiency that aren't yet being done and just because automobile manufacturer's aren't doing them yet doesn't necessarily mean they're not possible or not feasible. Usually these "miracle" gas mileage increasing things are gimmicks. But not always.
FH
P.S. I don't work for SHEP, own any of their stock or have any affiliation with them whatsoever.