ratios lower than about 0.7. Increasing the first gear ratio to about 4.0, however, provided better
standing start performance. The Nissan production five-speed transmission uses a 3.85 first gear
ratio and a 0.69 overdrive ratio for a 5.56 ratio spread. At constant performance, Nissan showed
fuel economy gains in the 3 percent range.^160 Mercedes, the only other manufacturer to have
introduced a five-speed automatic, confirmed that the fuel economy benefit over a four-speed
automatic was in the 2 to 3 percent range. Ford estimates that their planned five-speed automatic
would provide a 2.5 percent fuel economy benefit at current performance levels, but could have
much smaller benefits at other levels.A 2.5 percent fuel economy benefit appears representative of a five-speed automatic over a
four-speed automatic. With either a six-speed or seven-speed transmission, complexity and weight
increases appear to offset fuel efficiency benefits.A continuously variable transmission (CVT) offers an infinite choice of gear ratios between
fixed limits, allowing optimization of engine operating conditions to maximize fuel economy.
Currently, Subaru is the only manufacturer that has offered a CVT in a small car in the United
States. Although there are several designs being tested, the CVT that is in production features
two conical pulleys driven by metal belts. The position of the belts on the conical pulleys
determines the gear ratio between input and output shafts. Under steady-state conditions, the
metal belt system can be less efficient than a conventional system, but the fuel used over a
complete driving cycle is decreased because of the optimized speed/load conditions for the engine.
Nissan and Ford have developed CVTs using rollers under radial loads that may be more efficient
than metal belt designs.
Shift performance of the CVT should be equal to, or somewhat better than, conventional
automatic transmissions, with its main benefit the absence of shift shock associated with discrete
gear changes. However, a CVT can produce unexpected changes in engine speed--that is engine
speed dropping while the vehicle speed is increasing--which may deter consumer acceptance.
Moreover, attaining acceptable startup vehicle performance could require the use of a lockup
torque converter or a conventional planetary gear set, or both, which would add to cost and
complexity. Nevertheless, developments in the metal belt system coupled with weight reduction of
future cars are expected to enhance the availability of the CVT for use in all classes of cars and
trucks in the 2005 time frame.
During the early 1980s, CVTs were expected to provide substantial fuel economy benefits over
three-speed automatic transmissions. Researchers from Ford^161 showed that an Escort with a
CVT of 82 percent efficiency would have a fuel economy 14 percent higher than the fuel economy
with a three-speed automatic; at a CVT efficiency of 91 percent, the fuel economy benefit was
computed to be 27 percent (91 percent was considered to be an upper limit of potential effic-
iency). Similarly, Gates Corporation installed a CVT in a Plymouth Horizon and found a fuel
economy improvement of 15.5 percent over a conventional three-speed automatic with lockup, at
almost identical performance levels.^162 Design compromises for drivability, however, as well as
improvements to the base (three speed) automatic since the time these papers were published
160 Ibid
161 R.R. Radtke et al., "Optimization of a CVT with Emission Constraints," SAE paper 810107, 1981.
162 Steig R., and S. Worley, "A Rubber Belt CVT for Front Wheel Drive Cars," SAE paper 820746, June 1982.