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Rear end gear ratio change

Yahoo Message Number: 92588

Couldn't find anything on the subject with a search so here goes:

My 2000 Intrigue 36, non-slide weighs in at about 26k pounds and is equipped with a 350 Cummins. That equates to about 75 pounds per HP. That is very good power since one HP per hundred pounds is acceptable for a diesel pusher. So, with so much power and so little weight, why the 4:88 to 1 rear end gears having the engine turn 2100 at 70 MPH?

I contacted Cummins to get the engine power curve and although they wouldn't reveal the details, they told me that peek HP is developed at 2000 RPM and peek torque is developed at 1400 RPM and that the engine should be running at around 1600 RPM at highway speed in order to maintain vehicle speed over the terrain. So why would CC set up a chassis to be operating at peek HP and above, at highway speed? Who knows?

After some calculations 4:11 to 1 seemed to be just right. I had the rear end gears changed from the 4:88 to 4:11. That put put the RPM at 1625 @ 65 MPH - perfect. I drove the coach from Melbourne to Tampa to see how it performed and it was awesome. It felt right!

Now for the speedometer problem: The speedometer is a pulse driven unit that needs to be reprogrammed for the new gear ratio. That didn't work on my coach as it has a speedometer with no programming dip switches. At this time I'm still waiting for CC tech to figure it all out, but in the mean time I installed a GPS driven speedometer that has several nice functions, including two trip odometers, altimeter, and others. Replacing the speedo is very easy as it's accessible from the top of the dash.

I did take the coach to Cummins to reprogram the ECM for the new gears so that the engine and transmission would be in sync at the lower RPM vs MPH. That turned out fine and it drives and shifts properly.

Stay tuned for fuel mileage improvement and mountain performance report when I return from trip to Tucson and back (to FL) over the holidays.

So far, I am very happy with the gear change!


Re: Rear end gear ratio change

Reply #1 –
Yahoo Message Number: 92592
Your rpm's were higher than mine - I show right at 1600 rpm when I am going 65 and mine doesn't get up to 2100 at cruising speed unless I drop down a gear. Did you ever check to make sure your transmission was shifting into the highest gear? Mine does that at around 60 mph and that drops the rpm's significantly. I have no idea what my gear ratio is but it is possible that I have a 4:11 from the factory. What fuel mileage were you getting with the 4:88 gear?


Ray 2000 Intrigue 11040

Re: Rear end gear ratio change

Reply #3 –
Yahoo Message Number: 92992
With the new rear end ratio 4:11 to 1 and freshly programmed ECM, an odometer reading of 667 and full tank of fuel (115 gallons), towing about 5k pounds, I drove as close to 65 MPH thru New Orleans, Houston and San Antonio on to Kerrville. That's a total of 660 miles and the fuel gauge reads about 3/8 tank remaining. With the old 4:88 rear end driving the same speeds, I'd have been looking to refill the tank by now as mileage with that gear set was maxed at 7.5 towing a 5k pound vehicle.
We filled up in Ft. Stockton! Fuel mileage 8.89 MPG running as close to 65 as possible all the time. The bonus is this: an extended range of about 200 miles - much quieter in the cabin - and most of all the torque up hills is awesome. At 65 the engine is turning just above 1600 rpm and the fan remains at a quiet idle speed. At 65 mph with an ambient temperature of 60 degrees, the radiator fan did not come on except when steep hill-climbing.
Heading up significant inclines the speed dropped a couple of MPH until RPM got to 1500 rpm (62 - 63 mph or so) where it stays and climbs the hill. Only once did the transmission downshift (climbing out of Las Cruces heading west), when speed dropped to 55 MPH in 5th gear, but stayed there all the way up. When the engine is turning 1500 rpm the torque will keep the coach from losing speed. This engine has so much torque at 1500 rpm! I can't understand why CC originally geared it to run so fast (with 4:88 gears) and out of the high torque range.
It seems the selection of 4:11 gears was spot-on for getting the coach geared properly to run properly and economically at highway speeds yet maintain very acceptable hill-climbing ability. Only one time did she downshift to 5th gear climbing, but then shifted back into 6th at 58 mph to regain the cruise control set speed of 65 mph.
Was it worth the expense? In my opinion, absolutely! Here's the cost: Changing the gears, replacing the speedometer and programming the ECM cost me about $4500. The mileage improvement is about 18%. At $4 a gallon that's a cost savings of about $.10 a mile. You would have to drive 45000 miles to recover the cost in fuel savings alone, but the other improvements have to be a consideration.

When this trip is over, I'll calculate the overall MPG and post it.

Footnotes:

  • Originally, I wanted to install a 2-speed rear end, but it was too difficult. It turns out, that wasn't necessary to accomplish the goal.

  • Cummins ECM programming computer would not accept a 4:11 ratio, but did allow for 4:10 gear programming, so that's what was used. (Insignificant difference!) They updated all the ECM software which I highly recommend as a drivetrain upgrade in-and-of itself.
  • Driving in economy MODE (red diode lit) is now the norm as up and down shift between 5th to 6th occurs at 58 mph in economy MODE. Since engine RPM is relatively low between 60 and 65 mph, the transmission will unnecessarily downshift when accelerating below 65 mph in STANDARD mode (rather than using the available engine torque).

  • Speeds of 75 MPH plus are possible if desired! (Make sure your tires are rated for those speeds.)
  • Overall, the coach is totally improved in performance, economy, quietness, pleasure of driving, and fewer fuel stops.

  • At 65 mph the coach feels comfortable now instead of straining. If anyone reading this has ever driven a Prevost conversion you'll understand this comment: This CC drives, feels, and is as quiet as the two Prevost conversions I previously owned. 1996 Marathon XLV and 1999 Vantare H3-45. I will now update the interior. Sandy and I love this coach and since it fits in my Man Cave - it's as perfect as they get.
Feel free to email me with any questions.

Re: Rear end gear ratio change

Reply #4 –
Yahoo Message Number: 92993
I was getting a max of 7.5 under ideal conditions. I.e.: flat road, not towing. The gears were 4.88 and now are 4.11 - problem solved.

Re: Rear end gear ratio change

Reply #6 –
Yahoo Message Number: 92995
That's the size of tire that caused many of us numerous problems in the late 90's. In order to handle the weight of a 40' Intrigue or Allure the factory had to replace the tires with 275 R 80. I also did the same thing with my 36'. If your coach has one slide the left front tire is over loaded dynamically. In order to handle the weight the R70 tire needs to be inflated to 130 lbs. That's above the rating of the wheels. So the only solution is to go to the 275 R80 which will reduce the RPM. You could probably gotten by with a 4.44/4.56 rear end. You also need to check if you have a coach with the two ride height valve s on the front and one in the rear. If So, They need to be swapped. I got a registered letter and an official recall notice while I was at the 1999 Home Coming rally and had the work on the ride height valves performed at the factory and had the Toyo tires replaced with Continentals. 275 R 80s. Of course now that I'm running 315 R 80 in place of the 12R which are the same diameter on my Intrigue the 4.88 rear gives almost the same performance you have with you 4.11. A 4.56 would be closer; but, for 6 to 7% difference I'll stick with what I have now. But As I weigh over 40,000 lbs down the road, I need the higher gear ratio. If I had the 505 Cat the 3.91 rear end would have been perfect with the 4000 transmission. TWI. Originally 98 Allure 30255, now 2004 Intrigue 11731
 

Re: Rear end gear ratio change

Reply #7 –
Yahoo Message Number: 93017
Thanks for the info! I took all known facts into consideration before making the decision for a 4;11 gear-set.
My coach has no slides and weighs in at about 27k loaded. With a 350 Cummins that equates to nearly 1 hp per 77 pounds. That is more than adequate!
The tires are Michelin which carry the weight will less pressure (I run 90 front, 85 rear) than Toyo or Brigestone and were a good choice considering the weight and weight distribution. Each tire position was weighed prior to the tire choice. Weights are L/F 4700, R/F 4500, L/F 8650, R/R 8600, with a driver in position during the weighing and full fuel and fresh water.
I doubt a reduction in RPM in excess of 16% could have been accomplished by tire size increase without creating other problems. Additionally, the original gear-set was noisy on pull. So, changing the gears was the most obvious route to accomplish the goal since all other specifications supported a positive result..