RPM Concern

[mjkruid]

I have a 2016 27' Class C motor home on an F550 chassis with 6.7L diesel. The rear axel ratio is 4.88. My question is. I run 2500 rpm at 70 mph. On long interstate runs am I harming the engine by running 2500 to 3000 rpm all day? It seem to me that I am revving the engine very high. I am considering a change to 4.30 gears. Its an expensive change but I plan to own this unit for many years. Any thoughts are appreciated!

 

[snicklas]

Most diesel engines are designed to run on the governor, all day, every day. What is the "red line" on the 6.7? I've not driven one, but I know my 6.0 is 4000+ for the red line. Running 3000 and under shouldn't hurt anything, except fuel economy. 2500 at 70 isn't too bad, as my 6.0 runs 2000@70.

 

[mjkruid]

Redline on the 6.7 is 4000 also. Just seems to me like it is screaming. The motor home is only 27'. Its basically the same as a large truck camper on an f550. I'm seriously considering the change to 4.30 gears.

 

[snicklas]

Honestly, at that RPM, the only thing being hurt is your fuel economy/wallet.

I would also do some calculations and see just how much changing gears is going to effect you cruising RPM/fuel economy vs the expense of swapping gears. It could take a long time to break even on fuel saving to pay for a gear swap. Another thing to keep in mind is changing gear ratios also changes you pulling/climbing power. If you tow or are in the mountains that's something to keep in mind. Something that you were able t walk up before, now might need a downshift and have the revs even higher to get up.

My personal thought is if the Coach Manufactures spec'ed the 4.88 there is probably a reason, especially if there was an additional cost to get it from Ford equipped that way. No manufacturer/upfitter is going to spend a dime on something that is unnecessary. So if a 550 comes standard with the 4.30, then they paid for the 4.88's. Just trying to get some feedback, because I would hate to see you spend the money, and come back at say it was the worst thing you ever did.

 

[mjkruid]

I've done the calculations and from what I can find I would come down from 2500 rpm to around 2200 rpm at 70 mph. I am thinking an improvement of 2 mpg. With todays fuel prices it would take between 75 and 100k miles to pay. (I do plan to keep it that long) My biggest concern was weather I was hurting anything by running 2500 to 3000 rpm. I am used to a Ram w/cummins that I have run for work, Typically around 2000 rpm at 75 mph. But the red line on the cummins is 3000.
Thanks for the feedback! If i'm not hurting anything I will probably stay as is!

 

[bcvickers]

You're not hurting the engine per se, running it at 2500-3k rpms but it does have a big effect on your miles per gallon and some would argue that it will wear the internal parts faster because you're putting more rotations on everything, over a shorter period of time. I don't know if the mpg difference would be 2mpg but it has to be significant. With the 6 speed transmission, that has two over drive gears, a drop in rear end gearing wouldn't hurt you too bad. Of course it's easy for me to say since it's not my money but it almost seems worth it.

Also, most diesel engines that are setup to be run under constant load for its entire life is not running at it's redline but rather in the sweet spot of the rpm range where torque and fuel consumption are optimal. The red line on over the road vehicles is meant to be the absolute max rpm the engine ever see's for a very short period of time.

 

[mjkruid]

You hit the nail on the head with my exact concern. Wearing the internal parts faster over a shorter period of time.
I would say that 80% of its miles will be spent between 2500 and 3000 rpm. We make three or four trips a year that are over 2000 mile runs.
I am at 27k miles now and I don't ever recall shifting down out of overdrive weather in the mountains or pulling my boat.
Thanks for the input, I am still leaning towards the change.

 

[kenter55]

I have a 2016 27' Class C motor home on an F550 chassis with 6.7L diesel. The rear axel ratio is 4.88. My question is. I run 2500 rpm at 70 mph. On long interstate runs am I harming the engine by running 2500 to 3000 rpm all day? It seem to me that I am revving the engine very high. I am considering a change to 4.30 gears. Its an expensive change but I plan to own this unit for many years. Any thoughts are appreciated!

 

[kenter55]

No way would I change it, if it’s running 2500 most of the time; 3000- maybe. At a lower RPM, your turbo is going to spin faster also. As was stated, pulling a load, up hill, in thinner air, your going to need those gears. PLUS, there are many times unforeseen and unintended consequences of doing any job, not to mention mistakes: If it ain’t broke- don’t fix it!

 

[bcvickers]

At a lower RPM, your turbo is going to spin faster also.

Wait, what?

As was stated, pulling a load, up hill, in thinner air, your going to need those gears. PLUS, there are many times unforeseen and unintended consequences of doing any job, not to mention mistakes: If it ain’t broke- don’t fix it!

The 6 speed has plenty of gears just for this reason. Additionally; are you making an argument that the only good truck is a 100% stock truck because I think we can all agree that is definitely not the case!