Community Forums

cyko · 04-03-2008, 11:24 AM

This may be a dumb question, but I want to ask it anyway. I have been driving at about 55 mph or so to conserve fuel. My engine rpm is about 1600 rpm in 6th gear(Allison 6-speed) at that speed. I have also been trying to shift down to 5th even on fairly gentle grades where the speed may start to sag, which puts my rpm at about 1800 or 1900. I think I have been getting significantly better mileage when doing this. My question is: at such slow speeds, would it be beneficial to shift down to 5th during all level driving, keeping the rpm closer (presumably) to the engine's optimum operating condition? I have always assumed that the overdrive position would be best during normal driving, but is that true? I do use the cruise control most of the time.

cyko · 04-03-2008, 11:24 AM

This may be a dumb question, but I want to ask it anyway. I have been driving at about 55 mph or so to conserve fuel. My engine rpm is about 1600 rpm in 6th gear(Allison 6-speed) at that speed. I have also been trying to shift down to 5th even on fairly gentle grades where the speed may start to sag, which puts my rpm at about 1800 or 1900. I think I have been getting significantly better mileage when doing this. My question is: at such slow speeds, would it be beneficial to shift down to 5th during all level driving, keeping the rpm closer (presumably) to the engine's optimum operating condition? I have always assumed that the overdrive position would be best during normal driving, but is that true? I do use the cruise control most of the time.

CC38EL · 04-03-2008, 12:20 PM

I hope I'm not starting mileage wars, but here are a few things I've learned.

Max fuel economy is where rpm and gear and max torque coincide. Translated, that means if your max torque on a power curve is between 1400 and 1600 rpm like I think it is on my Cummins ISC (2002 model) then the highest gear at that rpm will yield your best mpg. And the speed will be whatever the speed is for your transmission/rpm setup.

Cummins says on their website that rpms in the 1900-2100 range (for the new 2007 ISC) and geared to run at 65mph, produces the best fuel economy/performance. I'm not buying all of that because above 55mph, the aerodynamics of an RV start to really impact fuel economy. Cummins even states in other fuel economy Over the Road data that for every mph above 50, fuel economy (in trucks that run 'over the road') goes down .1mpg.

So after all this, I think its an experiment we all need to conduct to find out what rpm/gear our coaches like for the best fuel economy at some speed that keeps us happy.

My 'sweet' spot is 57mph in 6th gear at 1550 rpm. I get over 9.5mpg. My last measurement was with a moderate head wind and I still got 9.125mpg.

Happy experimenting.

chicken legs · 04-04-2008, 04:41 AM

I bought a scan guage for this very reason, I have the ablitiy to "trim" the MH for the road and weather conditions and get instant feed back on how well I'm doing for mileage. I think it's the perfect tool for finding the sweet spot in the mileage curve every time you drive. Wind, rain, road conditions, temperatures, altitude all factor in and change daily. On a recent 4K round trip to Fl I was able to adjust my speed to meet all the changing conditions and achive max mileage.

TandW · 04-04-2008, 06:14 AM

Drive the right lane @ 55 without engaging cruise control. Do not allow rpm to exceed torque max rating. Ease off the pedal allowing shift to next highest gear. Do not allow 2 simultaneous downshifts if following traffic allows you room to safely slow on a grade, but don't travel below posted minimum mph. Learn how to maintain converter lockup when increasing downhill speed on mild grades and the lane in front is open . When your right leg is tired, it's time to take a break. 10.5 mpg regularly!!

CC38EL · 04-04-2008, 07:02 AM

chicken legs.....

Do you use the scan tool to do the 'trimming' of your MH? Or do you use some other device to set the parameters and then use the scan tool to monitor the effects?

What kind of scan tool are you using? And if you're using some other 'trimming' tool, what is that one?

Gary RVRoamer · 04-04-2008, 07:44 AM

You want the highest gear that is not lugging and stays in that "sweet spot" range. And it is a range, though probably no more than 200 RPMs wide.
On my 370 ISL it works out to be at about 62-63 mph in 6th gear. Dropping under 60 mph results in a significant decline in mpg. I am able to monitor it on my Roadtek system that displays most ECM data, but there are many tools (Silverleaf is one of the best) that do this sort of thing and let you keep close tabs on it.

chicken legs · 04-04-2008, 09:33 AM

ua40j

http://www.scangauge.com/products/ here is the link to the product. I use the gauge to monitor my MPG as one of the displayed gauges so when I change speed or take her out of crusie or down shift I can get instant feed back on how the change affected the mileage.

CC38EL · 04-04-2008, 05:41 PM

thanks!

NeilV · 04-04-2008, 07:22 PM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by chicken legs:
ua40j

http://www.scangauge.com/products/ here is the link to the product. I use the gauge to monitor my MPG as one of the displayed gauges so when I change speed or take her out of crusie or down shift I can get instant feed back on how the change affected the mileage. </div></BLOCKQUOTE>

I use one and can get over 9 MPG on my coach if I pay attention.

I usually have it set to display Power Percentage, Throttle Position, MPG and Engine Temperature. On the F53 I know it is going to kick down a gear whenever the Power Percentage gets over 98% and I will be blowing a ton of fuel out the exhaust pipe.

It is a game of balancing things to maintain a reasonable speed while minimizing the ponies and fuel required to do so.

Tom N · 04-05-2008, 03:46 AM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by NeilV:
On the F53 I know it is going to kick down a gear whenever the Power Percentage gets over 98% and I will be blowing a ton of fuel out the exhaust pipe. </div></BLOCKQUOTE>

A state of the art, computer controlled, drive by wire, gas engine will not blow fuel out the exhaust pipe. The days of flooded engines have been over for sometime.

-Tom

NeilV · 04-05-2008, 06:43 AM

Tom,

It is a figure of speech. There are other things to call it when you are at 100% engine output and running at less than 1 MPG, computer or no computer, but I don't use those kinds of euphorisms.

bsinmich · 04-05-2008, 06:53 AM

With my WH I have found 61-62 to be more economical using Cruise. At 55 it is just where 5th. gear comes in and on any little rise it downshifts. At 61 it will maintain the speed up inclines without a downshift and it helps the mileage a little bit.