5th gear at 62mph

[Letsgoagain]

Evening.
Looking to slow down a little and get better mileage. The sweet spot on our motorhome rpm wise is 1750 which ends up being around 67mph. I plan on dropping my speed 5mph but wondering if the resulting drop in rpm will lug the engine. Would driving in 5th gear be the answer or will the gear drop eat up any mileage gains from the slower speed?
Anyone else do this. Like to hear from you and your results.
Thanks.

[AFChap]

I drive in the 63-65 mph range, in 6th gear most of the time with no undue lugging. But I HAVE noticed that in hilly terrain it works much better if I push the speed up to apx 67mpm ...that causes the automatic downshift to 5th at higher rpm than when I am running at lower speeds. At 63-65 the downshift comes almost too late to help, and due to lugging down a further downshift is frequently required.

[Norm4015]

Can you explain the "Sweet Spot?" Surely that is not where you get the best miles per gallon. I have owned two Cummins engines - a 315 hp ISC and a 370 hp ISL. I have found the best fuel mileage to be 50 mph which is the speed just before the transmission drops down to 5th gear.

Your computerized engine and transmission will not let the engine lug. Lugging an engine is considered running WOT (wide open throttle) with the engine operating below peak torque. I don't know the torque curve of your engine but it should be somewhere between 1300 and 1400 rpm. If you put the pedal to the metal at 1300-1400 rpm the transmission will downshift and not allow the engine to lug.

[Monacoach]

I cruise at 62 but I like to keep it in 6th gear with economy mode on. I think even in 5th you will save on fuel.

[Elkhartjim]

Quote:

Originally Posted by Letsgoagain Evening. Looking to slow down a little and get better mileage. The sweet spot on our motorhome rpm wise is 1750 which ends up being around 67mph. I plan on dropping my speed 5mph but wondering if the resulting drop in rpm will lug the engine. Would driving in 5th gear be the answer or will the gear drop eat up any mileage gains from the slower speed? Anyone else do this. Like to hear from you and your results. Thanks.

After attending Camp Freightliner in Gaffney we headed to Savannah, GA. I kicked my rpm's up to 1850 (68 mph) and did a much better job climbing the hills. I got 9.8 mpg with a Cummins 325ISB hauling 28,782 pounds. I had been driving at 1800 rpms (63 mph). Mike Cody suggested bumping it up 50 rpm's. I mainly noticed the transmission not down shifting as much.

I know that doesn't answer you question but it was my experience. Btw, on this 2700 mile trip with head winds, cross winds, etc, I averaged 9.7mpg.

[Dirtbuster]

Did you seriously take " Mike Cody " for his word? Just joking , Mike does his job very well!

[Letsgoagain]

Quote:

Originally Posted by Norm4015 Can you explain the "Sweet Spot?" Surely that is not where you get the best miles per gallon. I have owned two Cummins engines - a 315 hp ISC and a 370 hp ISL. I have found the best fuel mileage to be 50 mph which is the speed just before the transmission drops down to 5th gear. Your computerized engine and transmission will not let the engine lug. Lugging an engine is considered running WOT (wide open throttle) with the engine operating below peak torque. I don't know the torque curve of your engine but it should be somewhere between 1300 and 1400 rpm. If you put the pedal to the metal at 1300-1400 rpm the transmission will downshift and not allow the engine to lug.

Perhaps my definition of lugging is wrong. Our motorhome shifts into 6th gear at 55mph. At that speed the rpm's are below 1500rpm's and the motorhome is very sluggish. I thought that was lugging. Not enough rpm's at a given speed.
I have never tested out the different speeds to see where I get the best mpg but I will say when I describe the sweet spot I mean that is where the motorhome in 6th gear seems the most responsive and the mileage does indicate better on the vsMpc. I guess the responsive is a subjective thing but after 52000 miles of driving our motorhome I would say for me it is true.

[Mr_D]

Quote:

Originally Posted by Letsgoagain Perhaps my definition of lugging is wrong. Our motorhome shifts into 6th gear at 55mph. At that speed the rpm's are below 1500rpm's and the motorhome is very sluggish. I thought that was lugging. Not enough rpm's at a given speed. I have never tested out the different speeds to see where I get the best mpg but I will say when I describe the sweet spot I mean that is where the motorhome in 6th gear seems the most responsive and the mileage does indicate better on the vsMpc. I guess the responsive is a subjective thing but after 52000 miles of driving our motorhome I would say for me it is true.

The ISC torque peak is 1400 rpm, some say best MPG is 200 rpm above torque peak.

[azloafer]

I lost the "average MPG" readout from the information bar in the dash. This happened once before and someone gave the procedure for getting it back, but I didn't write it down and now I can't remember the steps to take. Can someone post the procedure if they know it? Coach info below. Thanks, Joe

[wa8yxm]

Will dropping the RPM's lug the engine down? Depends.

Some engines like to rev around 3K RPM, some 2K. Some more, Some less, and without knowing YOUR engine, which I do not, this is all I can say.

Now, I drive a Gasser, 8.1L vortec V8.. Seems to like 2K RPM, GM's are "Low reving" engines I'm told.. Do not know if this helps.

[Catch and Release]

Quote:

Originally Posted by azloafer I lost the "average MPG" readout from the information bar in the dash. This happened once before and someone gave the procedure for getting it back, but I didn't write it down and now I can't remember the steps to take. Can someone post the procedure if they know it? Coach info below. Thanks, Joe

Call the freightliner 800#. They are great.

[Elkhartjim]

Quote:

Originally Posted by azloafer I lost the "average MPG" readout from the information bar in the dash. This happened once before and someone gave the procedure for getting it back, but I didn't write it down and now I can't remember the steps to take. Can someone post the procedure if they know it? Coach info below. Thanks, Joe

Sorry the thread was hijacked and I'm party to it. Try this link to the Freightliner manual. Go to Chapter 2. It gives complete instructions for navigating the LBCU screen.

http://freightlinerchassis.com/docum...4711000000.pdf