So driving home tonight I spend a fair amount of time on a 50mph road.  Truck seems to run at about (never realized how goofy our tach markings are) 1800-1900rpm in 5th gear at 50, or drops to around 14-1500 in 6th.  Keep in mind I am empty (I know the towing answer) it’s pretty much flat the whole way (definitely use 5th to climb) just curious about the rest of the time.  Opinions?

Can't speak for everyone. But where I'm at there's plenty of hills and curved roads, especially heading off the island. Most areas are 40-45 but Usually set cruise at 50, depending on traffic in front of me. Leaving it in 5th, around 1800-2krpm.

Would say don't lug the engine, it's no good for it or the rest of the drivetrain so I personally don't cruise at 1600 or lower, Just downshift an keep the rpms up.

What size tires are you running? Then what gear ratio? 

 

3.55 is optimal for 31 inch and SMALLER tires.

4.10 is optimal for 32 to 37 tires.

Personally 245/75 R16 is great for daily driver on 3.55 gears. 66 MPH is exactly 2k RPM. 80 MPH is 2,400 RPM.

I am running a 235/80/17.  Debatable what I will go to when these wear out.  I have 3.55 gears 

Barrow a set of 245/70 R17 for the rear axle test drive that and see if you like it.

I will see about making that happen, in the meantime, your thoughts on the cruising RPM question?  If it makes more sense to run at the higher RPM I will need to work on getting a muffler to kill the drone that I think is leading me to believe/feel like the truck isn’t running as smooth in that rpm band.

On my 02 6 spd C&C I have 285/55r20 tires and turn about 2500 rpm @70mph on flat interstate... Its a single wheel currently also. 

1 hour ago, Stanley said:

On my 02 6 spd C&C I have 285/55r20 tires and turn about 2500 rpm @70mph on flat interstate... Its a single wheel currently also. 

What gear ratio?

14 hours ago, portlandareae28 said:

So driving home tonight I spend a fair amount of time on a 50mph road.  Truck seems to run at about (never realized how goofy our tach markings are) 1800-1900rpm in 5th gear at 50, or drops to around 14-1500 in 6th.  Keep in mind I am empty (I know the towing answer) it’s pretty much flat the whole way (definitely use 5th to climb) just curious about the rest of the time.  Opinions?

 

Personally for me it depends on what the boost is doing. if the engine can climb the baby hills on the lower rpm then your good. granted I have an automatic and I'm a dully i find this truck runs around at 1,400-1,600rpm fairly well.  Towing with my smarty I will downshift anytime i sense the engine starting to load up. And with my smarty that seems to be around 1,400rpm. Bit for me in O/D if your that low in rpm you should be in 3rd anyways.

Granted your a manual and these engines are not semi trucks, but I find that empty your fine to run down low as long as you have a programmer to get your timing and fueling. You will know if your lugging these motors around.

36 minutes ago, Mopar1973Man said:

What gear ratio?

I'm pretty sure I have 4.10s 

I havent counted teeth yet but, I'd love to go to 3.55s though. Shes pretty high up there. 

Mark the driveshaft and a tire. The driveshaft will turn either 3.5 turns or roughly 4 turns to one of the tire.

On 1/4/2019 at 11:42 AM, Mopar1973Man said:

Mark the driveshaft and a tire. The driveshaft will turn either 3.5 turns or roughly 4 turns to one of the tire.

 

That's 4.10's. 

 

My trucks from 265-285 tires in 6th going 70 is 2k ish rpms with 3.54 rears. 

I also have a 2001 3500 dually, 6-speed with 4.10 and have 235's on it. I rarely go over 1600 rpms, I'm in 6th by 40 mph around town, and get 18-20 mpg or 450-500 mi per tank. On the highway empty I hit 2k rpms at 72 mph. Keeping it at 65 I'll get 700 miles on the tank or close to 25 mpg. This is before the FASS I added 2 weeks ago, otherwise all stock.

On ‎1‎/‎3‎/‎2019 at 6:21 PM, portlandareae28 said:

So driving home tonight I spend a fair amount of time on a 50mph road.  Truck seems to run at about (never realized how goofy our tach markings are) 1800-1900rpm in 5th gear at 50, or drops to around 14-1500 in 6th.  Keep in mind I am empty (I know the towing answer) it’s pretty much flat the whole way (definitely use 5th to climb) just curious about the rest of the time.  Opinions?

 

If I am driving on a 50 mph road such as you have described, my truck will always stay in 6th gear, even when I am towing.  If I can easily accelerate (which I can), I see no reason to run a lower gear (with lots of engine noise) for such a light engine load. 

 

Remember, peak torque for a stock engine with the NV5600 is 505 lb/ft at 1600 rpm.  I think that would leave you with an over abundance of torque to motor down a flat road in sixth gear at 50 mph.

 

Just another opinion.

 

- John

 

1 hour ago, Tractorman said:

 

If I am driving on a 50 mph road such as you have described, my truck will always stay in 6th gear, even when I am towing.  If I can easily accelerate (which I can), I see no reason to run a lower gear (with lots of engine noise) for such a light engine load. 

 

Remember, peak torque for a stock engine with the NV5600 is 505 lb/ft at 1600 rpm.  I think that would leave you with an over abundance of torque to motor down a flat road in sixth gear at 50 mph.

 

Just another opinion.

 

- John

 

That's pretty much my answer as well...as mentioned best mpg would be at peak torque of the mighty Cummins (1600ish).  Cruising it within a couple hundred rpms of peak torque like you are doing is absolutely no problem at all.  I'm auto trans, 3.55's and fairly modified but I always cruise at similar rpms as you...even a little lower.

For me on 33's:

35 mph=1000-1100 rpm

50 mph=1200-1300 rpm

70 mph=1750 rpm

For me on 35's:

35 mph=950-1000 rpm

50 mph=1100-1150 rpm

70 mph=1625 rpm

Been doing it that way for a really long time.  Oh and each hash/line on the tach is 125 rpms...a little bit goofy I agree. 

 

 

Since top gear is basically the same for all transmissions.

 

66 MPH - 3.55 gears - 2,000 RPM - 245/75 R16

80 MPH - 3.55 gears - 2,400 RPM - 245/75 R16

10 hours ago, Nutter said:

I also have a 2001 3500 dually, 6-speed with 4.10 and have 235's on it.  On the highway empty I hit 2k rpms at 72 mph. 

 

I am thinking with those numbers, you have 3.55's.

According to my paperwork, in 2001, the 1st year of the HO (245 hp; VIN 7) could only be equipped with a 6-speed and 4.10. I've never physically inspected or measured, but that's what the paperwork says. it's also 2WD.

There is a label in the glove box with the vin number and some build info.  One piece of info is the differential size.  It's faded but I can still read mine.  

I have an '01 6spd dually, I will hit the freeway to go home tonight and see what my 6th gear speeds look like.  I was running 55mph in 5th this morning at 2050-2100rpm.  using the spreadsheet built on this website, that puts me at 3.55 gears with my 235/80/17 tires.

13 hours ago, NIsaacs said:

 

I am thinking with those numbers, you have 3.55's.

Yea, my 02 6spd at 70 is 2500 with 285's. I'd say he has 3.55s... but I could be wrong.