All else about a car being equal which transmission type would return the
best fuel mileage? Manual, auto or CVT? You may have noticed
when looking at car fuel mileage figures that manuals always have a better
fuel mileage than automatics for the same car. This is due to the fact that
a manual transmission system maintains an entirely mechanical link between
the engine, gearing and wheels via cogs, a metal chain and a clutch. The
biggest cog of the gearset is first gear and the smallest cog is the highest
gear. In a manual the job of the clutch is to disconnect the engine from
the cogs allowing the vehicle to be stopped or gear selection to be made
without having to stop the engine from running. The amount of torque applied
by the engine can be varied by altering the clutch input. This mechanical
coupling of the engine and gears in manuals gives high fuel mileage.
Automatics differ in that they do not maintain a continuous mechanical linkage since they employ a fluid filled torque converter to connect the engine and gearset. A torque converter is a hydraulic fluid coupling. Automatic transmission has worse fuel mileage than manual mainly because of pumping losses in the torque converter and hydraulic actuators. On top of this the hydraulic control system draws power from the engine causing it to work slightly harder. In a manual your leg replaces an auto's control system when you press the clutch pedal! The fuel mileage of automatics can be improved by the inclusion of an economy setting which causes the transmission system to upshift at lower engine revolutions and also sometimes amends the throttle mapping so that its less responsive to heavy footed input.
CVT stands for continuously variable transmission and its becoming more
widespread among new cars. It maintains a direct mechanical link between
engine and wheels however it requires no clutch and no gear changing. In
fact, true CVT doesn't even have any gears. So how does it work? CVT has
a single cone shaped gear and when the car is traveling at low speeds the
larger end of the cone is used by the engine. At high speeds the smaller
end of the cone is used. Because cogs are replaced by a cone you have an
unlimited number of gear ratio's. With true CVT the gearing ratio constantly
changes as you drive. However many CVT cars have a number of defined gearing
points on the cone which when used with gear selectors give the feel of
a clutchless manual.
Because of the direct link between engine and wheels
CVT fuel mileage can exceed an automatic. CVT also has a distinct advantage
over manual transmission. In a manual car the engine speed (revs) for a
specific gear is determined by how fast the car is traveling. This is not
the case for true CVT cars who can constantly vary their gear ratio to keep
the engine revs at their most efficient level for maximum fuel economy.
Continuously variable transmission is not a new invention. It has appeared
in cars as far back as the 1950's however due to the materials available
at the time its use was limited to engines of less than 100 horsepower.
CVT technology and materials dramatically improved around 10 years ago.
Tougher CVT systems appeared allowing their use with more powerful vehicles.
The use of continuously variable transmission became more widespread and
today manufacturers using CVT systems include Nissan, Honda, Audi and Subaru.
Of all the transmission systems available, CVT has the capability to return
the
highest fuel mileage, followed by manual gearboxes,
and finally auto's have the lowest potential to return good fuel mileage.