while using less fuel and reducing noise. Also,
when slowing down for reduced speed zones,
remain in your running gear and reduce engine
rpm to stay within the speed limit. Avoid down-
shifting until you are ready to return to highway
cruising speed.
3.
For highway cruising, and for best fuel economy,
run the engine at 80 to 90 percent of rated rpm
to maintain highway speed. Engines with 1800
rpm ratings are exceptions and will provide fuel
economy when run at their rated speed. Proper
gear selection should permit cruising in the
economy range with no appreciable sacrifice in
desired highway speed.
It is okay to operate below rated rpm at full
throttle if you are satisfied with the way the ve-
hicle performs. However, there are times when
hilly terrain, high winds, or other conditions make
it impractical to operate without reserve power.
Such conditions are better met if the vehicle is
operated in a lower gear with reserve power
available for changes in terrain, wind, etc.
4.
The proper use of gears will shorten time on hills
and minimize the amount of shifting. When start-
ing up a hill, gradually depress the throttle pedal
all the way and keep it there as the vehicle
moves up the grade. If the engine continues to
maintain a satisfactory road speed, remain in
that gear for the entire grade.
If the hill causes a steady decline in engine rpm,
downshift as required until the engine can main-
tain a stable uphill speed. Make full use of each
gear before going to a lower gear. By remaining
in a gear until arriving at the speed of the next
lower gear, the vehicle will top the grade in the
best possible time on less fuel and fewer shifts.
5.
Because of their constant torque over a wide
speed range, Detroit Diesel engines can be oper-
ated at full throttle at lower rpm than other en-
gines. This offers benefits in fuel economy and
engine life. And more than likely, it will be pos-
sible to top most grades without downshifting.
When the vehicle starts into a grade, allow the
engine to lug down to maximum torque rpm be-
fore downshifting. Downshift, if required, at maxi-
mum torque rpm also or at the predetermined
road speed for the next lower gear.
Do not be afraid to lug the engine down. It has
more than enough torque at low rpm to keep a
loaded vehicle moving against a grade and it
won’t harm the engine.
6.
The driver who is not familiar with the vehicle’s
shift points can greatly improve driving skill by
learning them for all gears. By knowing rather
than guessing where the shift points are, it is
possible to avoid overspeeding the engine by
downshifting too soon or missing the full use of a
gear by downshifting too late. The shift points of
any vehicle can be determined by a simple road-
test method. Run the vehicle and determine the
maximum road speed possible in every gear at
the engine governed full-load speed setting.
The top road speed possible in a gear would be
the shift point for that gear. The results should be
recorded in the proper order of shifting and dis-
played inside the cab.
CAUTION
Do not allow the engine to exceed its governed
speed, or serious engine damage could result.
7.
To slow the vehicle on downgrades and curves
(using the engine), shift to a lower gear and
allow the vehicle to decelerate in that gear. The
engine provides maximum braking effect when
running at the top end of the operating range,
but it must not be allowed to exceed its full-load
rated rpm. Continue to downshift as further re-
duction in vehicle speed is required. If the ve-
hicle is above the allowable maximum speed of a
lower gear, use the service brakes to slow the
vehicle to an acceptable speed where the trans-
mission may be downshifted safely. Again, the
importance of knowing the shift points is demon-
strated.
IMPORTANT: The engine governor has no con-
trol over engine rpm when the engine is being
pushed by a loaded vehicle down a grade. Use
service brakes and gears in combination on
long grades to keep the vehicle speed under
control and the engine rpm below full-load rated
governed speed.
8.
Essential information regarding the operation and
care of Allison automatic transmissions is con-
tained in the Allison
Driver’s Handbook.
Applying
the knowledge presented will not only make driv-
ing easier, but will give the maximum benefits
from an Allison-equipped vehicle.
Engines and Clutches
7.14
