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Switch out 1989 tranny for a 6-speed?

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Joel Ashley:
I'm certainly happy with our almost over-powered 36' Monterey with the 400HP C9 and Allison 6, towing our 4300 lb Explorer.  On my last trip from Portland to Hagerman, ID, about 500+ miles of mostly I-84, I did something I don't usually do.  I determined to try for maximum fuel efficiency, and I know that the cruise control system has a much steadier foot than I do, so I put it to work.  From the start I set the cruise at 57mph, just above truck speed in the right lane, and tried to stay there.  Where I had to pass, I took my time and tried to do it without changing the cruise control or stomping on the throttle.  Where the speed "limit" went up, I tried to cruise at the posted truck speed.

Only rarely did I override the cruise, say up a steep grade somewhere or to pass without holding up left lane speed-a-holics, or to engage the exhaust brake down a long hill.  Normally we would stop in La Grande or Baker City to refuel and eat, and I normally have 1/2 tank at that half-way mark.  I was flabbergasted as we reached Pendleton and then Meacham, and my guage and Alladin indicated I was no where near down to a half tank.  In fact, I didn't hit that point until around the air base at Mountain Home, east of Boise.   The Idaho section required a bump up in speed to be safe, due to the higher posted limits.  Nevertheless, it was an awakening when I finally filled the tank in Twin Falls, and calculated 9.6 mpg.  Keep in mind that this CAT had only 8000 miles on it at the time.  Imagine the numbers when it gets broken in.

In dozens of identical trips over the decades, in cars and our old gas motor home, it took at least 9 hours, which included rest stops, refueling, and meal stops.  I was so comfortable on this trip, that I drove the entire way non-stop.  8 hours.  My take is that at the moderate speeds I never got stressed, and could even actually enjoy the country as we went by.  I've always found that hard to do at 65-70 mph.  The comfortable ride probably helped, as did no work for my right foot.  I could have switched driving with the Missus, but was never tired until a half hour after we got past Boise's more stressful freeway section and darkness fell.

The cruise control is probably the number one contributor to the good mileage - set it and pretty much forget it, except the occasional exhaust brake or manual shift on hills or to pass.  Minimize the amount of time your foot is on the throttle or brake.  Secondly, be patient and enjoy the trip, even if its one you've taken dozens of times.  Stay at or just above the posted truck speed;  65 to 75 mph may be normal in Texas, Wyoming, or Montana, etc., but if you're serious about minimizing fuel use, for yourself and for the country's sake, then make the conscious, deliberate decision to slow down.  Our most enjoyable trips are on good state and county "backroads" at 40-45 mph.  Anything over 60 on a freeway with all the nuts is hardly enjoyable.  Or fuel efficient.

-Joel

Sorry Ted- we kinda got off-topic. :-/

Edward Buker:
Don, I have not done the pass on I70 west of Denver in this coach and that pass would probably be a challenge to hold speed on. It would be interesting to see what speed the coach would hold. From my recollection I would guess that 62-65MPH would be a very tall order. I have done the passes on I90 out of Seattle going to Cour d'Alene. Also the climbs on I5 going from Oregon into CA heading toward Sacramento. Those did not pose a speed problem. There have been other passes, just hard to recall where. A couple of other things to note. My coach is lightly loaded with water and waste when I travel, I have a light tow, a Honda Accord, my coach is shorter than the non tag 2000 and 2001 versions which saves some weight, and I run 115PSI cold pressure in all tires which have reduced rolling resistance by design.  

This coach does not shift into high gear until I reach 60MPH and I have tried traveling at 62 vs 65 MPH and have not observed any discernable mileage difference. There is probably a tenth or two MPG difference but I have not consistently seen it. I primarily use the economy mode on the transmission selector and the cruise control for travel. I have considered a slower speed (57MPH or so) for improved mileage but thought that traveling in 5th gear would be counterproductive. It would seem that the gearing has me traveling above 60MPH just to maintain top gear.

I am impressed with the Cat C12 and the Allison 4000, seems like a marriage made in RV heaven for this coach.
Later Ed

Joel Ashley:
Thanks for the reminder, Ed.  I forgot to mention I try to stay in Economy Mode most of the time also.  I've wondered about the 5th to 6th shift myself, and wished that point was closer to 55 mph.  There was some discussion in 2007 about Allison reprogramming (lowering) the shift point, but I don't think they did.  That might satiate concerned owners like me, but their tests probably indicate that the benefit, if any, isn't as substantial as you and I might think. As I recall the shift point is around 63?   The trip I referred to in my previous post was 85% in 5th gear netting over 9 mpg.  Dare I make the next one at 65 in 6th just to see?  I dunno;  maybe at night, but otherwise I like to smell roses  8).

-Joel  

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