Highway Engine RPM

[Jim W]

Ok, I have a slightly older 3/4 2008 Dodge 6.7L truck with 4X4, 3:73 gears and 33" tires towing a 11,000 LB 5th wheel. I have put about 40,000 miles on this truck pulling this 5er out west and back east. The mileage is tracked at every fill-up with the trailer and without the trailer. I usually tow in 6th gear around 1600RPM's and in the mountains I am in 5th gear doing 55MPH at 2100RPM's up I70 in CO. Why to keep the EGT's down and boost around 30PSI.

My fuel mileage when towing averages between 10.5 and 12 depending on terrain and head winds. Without the trailer attached I have seen as high as 19.5 on the highway and usually around 14.5 in town.

I am just saying you should have asked first about regearing for your truck.

 

[petersonj]

I usually tow in 6th gear around 1600RPM's and in the mountains I am in 5th gear doing 55MPH at 2100RPM's up I70 in CO

I could be wrong (I was wrong once - maybe twice), but it seems like 2,100 engine rpm would be 4th gear (direct drive) with a 3.73 differential and 33" tires.

- John

 

[AH64ID]

I could be wrong (I was wrong once - maybe twice), but it seems like 2,100 engine rpm would be 4th gear (direct drive) with a 3.73 differential and 33" tires.

- John

I calculate the same. ~1600 would be 5th.

 

[turretplug]

So far all I’m getting is that you have an unrealistic expectation of your truck. Dropping 2 gears for grade in the PNW is normal with stock sized tires and 35’s won’t help that. Dropping 2 gears puts you in direct drive, and the appropriate gear for the grade when towing as overdrive reduces engine torque to the wheels from engine output torque. TH mode doesn’t change gear ratios, just shift patterns and even that’s designed to run lower gear more than higher gears. Some more recent model years can hold 5th, but the power output and gear difference adds up.

35’s just aren’t that much bigger than stock. 4.56’s is just a bit much, so you either got some really bad intel or advice. 4.56’s for 35’s is something that’s done in a Jeep with a small gas engine. Once you’re out of first gear the towing advantage to gearing starts to diminish. Gears are great for slow speed use, towing or off-road, but going that low for highway and interstate speed towing isn’t helping economy but I’ll bet it’s responsive.

The truck had 3.42’s from the factory, not 3.73’s. All 13-18 SRW’s had 3.42’s.

Without a tuner you don’t have to be concerned with EGT’s, let it run. It won’t hurt itself.

You have two very unlucky friends, the grid heater bolt failing is very very rare and easy to prevent with the jiggle test.

Drive it and enjoy it, because you’re where you’re at and even if you regeared to the proper gears for your tire size the mileage gains wouldn’t likely ever pay for the regear.

I didn’t have unrealistic expectations, I just learned with my past Gen II that I initially left a lot on the table and once I made some modest upgrades I got a ton more performance and efficiency that definitely made the investment worth it. I had a lift and large aggressive tires but was able to exceed what stock performance and efficiency that didn’t affect reliability. I just started hauling heavier than what that truck was capable of as I acquired equipment and livestock. I knew from the very beginning when I put big aggressive tires on with that I was on a fools errand but if there was knowledge out there I wanted to make sure I found it. The performance I gained from the induction upgrades and gears is noticeable but it has set me back in efficiency. Even knowing that a half a billion dollars or more has been spent engineering my truck I can’t stop listening to the “good idea” fairy. I spend a lot of my days sitting in a cockpit watching the world go by from FL350, affords me way to much time to “research” and think.

 

[AH64ID]

I didn’t have unrealistic expectations, I just learned with my past Gen II that I initially left a lot on the table and once I made some modest upgrades I got a ton more performance and efficiency that definitely made the investment worth it. I had a lift and large aggressive tires but was able to exceed what stock performance and efficiency that didn’t affect reliability. I just started hauling heavier than what that truck was capable of as I acquired equipment and livestock. I knew from the very beginning when I put big aggressive tires on with that I was on a fools errand but if there was knowledge out there I wanted to make sure I found it. The performance I gained from the induction upgrades and gears is noticeable but it has set me back in efficiency. Even knowing that a half a billion dollars or more has been spent engineering my truck I can’t stop listening to the “good idea” fairy. I spend a lot of my days sitting in a cockpit watching the world go by from FL350, affords me way to much time to “research” and think.

There are performance gains to be had from gearing, and as you've noticed you gone far enough to effect overall performance.

There wasn't a lot, if anything, to gain from the intake mods. The OEM intake was already a CAI with lots more airflow capability than needed for stock power, or even 150+ hp over stock. Hopefully the banks stuff doesn't give you CEL's, it's hit and miss on stock trucks.

Your 4th gen isn't even breaking a sweat with the trailer you've got behind it, it's quite capable with a blind eye to EGT's.

Just out of curiosity what are you seeing for EGT's and how are you monitoring them?

 

[turretplug]

Ok, I have a slightly older 3/4 2008 Dodge 6.7L truck with 4X4, 3:73 gears and 33" tires towing a 11,000 LB 5th wheel. I have put about 40,000 miles on this truck pulling this 5er out west and back east. The mileage is tracked at every fill-up with the trailer and without the trailer. I usually tow in 6th gear around 1600RPM's and in the mountains I am in 5th gear doing 55MPH at 2100RPM's up I70 in CO. Why to keep the EGT's down and boost around 30PSI.

My fuel mileage when towing averages between 10.5 and 12 depending on terrain and head winds. Without the trailer attached I have seen as high as 19.5 on the highway and usually around 14.5 in town.

I am just saying you should have asked first about regearing for your truck.

Thanks for the data. I am just a little less than what you are reporting for towing numbers but only have a couple thousand miles with my new set up. I had about 25k on the previous setup which had a bit better mileage but when I compared it to similar trucks of friends that were still stock or had smaller gear changes I liked the performance increases. The pros I consulted on the gears were the number one gear shop in western Washington. Every repair shop owner (4) I knew unequivocally recommended them. I will just keep my towing speed to 60 instead of 65 and I am sure my mileage will thank me. Probably going to go from 35 to 37 inch tires when these wear out and go with a more all terrain tread pattern to reduce rolling resistance a bit. The 6% circumference difference will also make the rpm’s drop a bit at cruise.

 

[turretplug]

There are performance gains to be had from gearing, and as you've noticed you gone far enough to effect overall performance.

There wasn't a lot, if anything, to gain from the intake mods. The OEM intake was already a CAI with lots more airflow capability than needed for stock power, or even 150+ hp over stock. Hopefully the banks stuff doesn't give you CEL's, it's hit and miss on stock trucks.

Your 4th gen isn't even breaking a sweat with the trailer you've got behind it, it's quite capable with a blind eye to EGT's.

Just out of curiosity what are you seeing for EGT's and how are you monitoring them?

 

[turretplug]

I also tow 17k (gooseneck with an excavator or ten round bales). The EGTs are monitored using the Banks probe with wiring harness and which displays on the Banks DataMonster I have installed. I will probably go to 37’s when I need to change tires, drop the tread to something a little more to all terrain vs mud terrain like I run now. The 6% circumference change will help to slightly lower my cruise rpm’s and the slightly less aggressive tread pattern will lessen the rolling resistance.
I know Banks is in the business of selling kit but I also think Gayle Banks knows a lot about optimizing air flow. Clearly I am a fan boy so don’t have an exactly unbiased opinion. You also can’t argue that the red enameled parts look pretty choice under the hood. Half my truck ownership is the work I do with it but the other half is having a cool looking truck. And I live in an area where probably 70% of the houses have a diesel truck, so we all care about our trucks out here. Thanks for the quality discussion.

 

[AH64ID]

What are you seeing for EGT’s? With how these motors are timed I’m guessing 1300-1400° when pushing it hard?

There is an OEM post turbo probe you can monitor thru the OBDII port.

lol, I feel the opposite about banks red. It usually tells me someone easily falls for over-marketed and over-priced hype.

4.56's and 37's will put you at about 1700 at 65 (2700 in 4th for hard pulls), and just under 2100 at 80.

 

[Jim W]

I calculate the same. ~1600 would be 5th.

I could be wrong, and I am sometimes but I just went out and drove my truck. Per the tach the engine RPMs in 5th gear at 1600 RPMs was per GPS reading from my Garmin were 51MPH. 55 MPH according to the GPS the engine RPMs were 1900. My speedo is slightly off in MPH, so I go by the GPS unit which is an RV unit.

 

[AH64ID]

I could be wrong, and I am sometimes but I just went out and drove my truck. Per the tach the engine RPMs in 5th gear at 1600 RPMs was per GPS reading from my Garmin were 51MPH. 55 MPH according to the GPS the engine RPMs were 1900. My speedo is slightly off in MPH, so I go by the GPS unit which is an RV unit.

Those numbers don’t correlate to each other.

51 mph at 1600 in 5th works with a tire that has a rolling diameter of ~32.7”. Taller than your average 33, and what my 33.7” 285/69R20’s roll at.

That same tire would give you 60 mph at 1900 rpms, not 55.


Going back to your original post. Using the ~32.7” tire from 51 @ 1600 in 55th.

I usually tow in 6th gear around 1600RPM's

This would be ~66mph

in the mountains I am in 5th gear doing 55MPH at 2100RPM's up I70 in CO.

2100 in 5th is 67 mph
55 in 5th is 1730 rpms.