There are TWO TOTALLY DIFFERENT "LET THE TRANSMISSION CHOOSE
THE CORRECT GEAR" MODES/PROGRAMS in the Allison ECU which is the
"electronic brain" controlling shifting and other functions.
In ECONOMY MODE, the transmission will not downshift even at WOT
(Wide Open Throttle) until the engine pulls down to peak torque
RPM in some application and 200 RPM lower that "regular mode" in
others. And similarly, on up-shift, economy mode will shift into
a higher gear EARLIER-- with the difference only at or near WOT.
In REGULAR MODE, the transmission WILL downshift much earlier
(to maintain higher engine RPM).
ONLY at higher throttle positions is there any difference, so on
flat ground you will NOT notice any difference (except
accelerating from a stop IF you are at or close to wide open
It can make a BIG difference in rolling hills. If you are in
rolling hills and regular mode (particularly with the cruise
control on), it is common for the transmission to shift down to
5th on the uphill and back to 6th on the downhill. Repeat this
process hundreds of times. In economy mode, you will stay in 6th
gear unless the hill is so steep or so long that the engine can
not pull it without dropping below peak torque RPM. If you can
pull a hill in a higher gear (lower engine RPM) AND the engine
does not overheat, THAT IS WHAT CATERPILLAR AND CUMMINS
RECOMMENDATION FOR THE MOST ECONOMICAL WAY TO CLIMB A HILL with
a modern turbo, after-cooled diesel engine.
If you know you will need a lower gear because of the steepness
of the grade and/or are engine temperature is rising higher than
thermostatically controlled temperature, if driving in economy
mode, use the down arrow to drop a gear (this is what I do) or
switch out of economy mode. Be sure to switch back into economy
mode when past the steep section, or agree to pump extra fuel in
IF your engine begins to overheat, your HP/weight ratio is low
OR if it irritates you to loose a few mph on a hill in the name
of saving fuel, in the hills, by all means drive in regular
It confuses me to hear people advocate driving in economy mode
only on flat ground, as there is not 1% difference in shift
RPM's between regular and economy mode on flat ground, excepting
accelerating from a stop if you use WOT.
Every time you start the coach, the transmission is in regular
mode. This is the default setting. IF you push the mode button,
it goes to "economy mode" AND the light illuminates.
There is no "absolute" on how much difference in fuel economy
driving in economy mode will have. On flat ground where you will
be in 6th gear irrespective of what mode you are in, there will
be ZERO difference. The MOST difference in mileage will be in
rolling hills, where in regular mode, particularly if on cruise
control you will start up a hill in 6th gear, go to WOT in 6th
gear, downshift of 5th gear still at WOT (WHERE IT IS USING A
LOT MORE FUEL). After the hill is crested, the transmission will
up-shift to 6th, then likely coast a little in 6th gear (unless
you are driving with the exhaust brake on-- if you are it then
applies the exhaust brake AND downshifts TOWARD the pre-select
gear which is generally either 2nd or 4th).And so on
A modern turbo inter-cooled diesel is much more efficient at low
RPM high throttle settings. This is NOT my opinion. It is stated
in just those words in the Cat Owner's Manual and likely the
Cummins manual as well.
Note: In either mode, you are free (and welcome) to use the up
and down arrows to PRO-ACTIVELY choose the correct gear. You can
not screw anything up-- even if you down-arrow to 1st gear at 70
mph, the transmission understands that you meant "please
downshift to the next lower gear as soon as the engine RPM will
not exceed the pre-set amount. Then downshift again when
By the same token, you can shift between regular and economy
mode as often as you want with the transmission in any gear when
you make the change.
OPINION: I drive in economy mode 99% of the time, including in
REAL mountains, but use the up and down arrows to choose the
proper gear. I use regular mode ONLY when I am willing to say,
"I am willing to throw a lot of fuel away to gain a little
performance." When passing on 2 lane roads, THIS IS the case.