When I purchased my coach the dealer didn't do a good job with the PDI and I didn't do a good job asking questions. One thing I was not aware of until the other day was the function of the mode button on the Allison transmission selector. After learning the button functions as an economy mode I used it on my return trip from Michigan to Georgia and averaged 0.5 MPG higher than on my trip up.
Here is a good write up I found online about the mode button.
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.
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 throttle).
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 the tank.
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 mode.
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 6-5-6-5-6-5-6-5.......
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 safe....."
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.