Taller gearing

[JDunmyer]

2001, NV5600, (new re-manned Jasper after a total failure at 125K miles). 2 WD, 4:10 axle ratio

The gearing is just about ideal for towing, it's turning 2000 RPM at 60 MPH, works great. However, I'd like to drive it solo a bit more, and it's turning way too fast at 70+ MPH for my tastes. Seems like alternatives are: Gear Vendor overdrive or change the axle ratio. I think the choices originally were 3.54 or 4:10, is that correct? I understand that the later trucks used an American Axle rear with a 3.73 ratio. Would it be feasable to change the entire axle, getting disk brakes in the bargain?

Or should I just live with the poor mileage and high RPM at cruising speed?

 

[ROADWARRIOR]

I'm not sure how much your mileage would actually improve with the 3.54s. I've heard varying results. I believe if your don't run over 2200 RPM, you're still in the good economy range. I ordered my truck with 4.10s and have shared your same thoughts many times over the last 2 decades. I love my 4.10s for towing in the hills and backroads of where I live, but would like 3.54s for the few time I get out on 65mph roads.

 

[JDunmyer]

I had a '94 with auto transmission and made one long trip solo, cruise set at about 65 MPH and it got just shy of 22 MPG. The '01 does 17-18 or thereabouts. Both trucks do/did about the same towing the same trailer, right at 14 MPG if I keep it just shy of 60 MPH.

 

[ROADWARRIOR]

I had a '94 with auto transmission and made one long trip solo, cruise set at about 65 MPH and it got just shy of 22 MPG. The '01 does 17-18 or thereabouts. Both trucks do/did about the same towing the same trailer, right at 14 MPG if I keep it just shy of 60 MPH.

That might not be all gearing though, I believe the older 12 valves got better mileage in general.
With my '99, I got 23.5mpg on one long trip with 265/75r16 tires and my 4.10s, cruise set at 60 mph. Best ever for me.

 

[Cummins12V98]

Throw on taller tires.

 

[bcbender]

I'd like to drive it solo a bit more, and it's turning way too fast at 70+ MPH for my tastes. Seems like alternatives are: Gear Vendor overdrive or change the axle ratio.

I tussled with that question for 10y, but towing heavy (the reason I had the truck) it was perfect and I have had way less drivetrain issues than my buddie has with his 05 (3:73) truck... I switched to the factory tire size (~32") to 275/70/18s (~34") which gained me about 5mph on the top end so I was then gear limited to just north of 90. (plenty fast) but was towing around 2400 at 75. IMHO.. the over / under drive unit or even gear ratio swap will never gain you enough fuel economy difference to offset the cost of the mod.

 

[GAmes]

I don't see a cost benefit if you are only trying for better mileage. You might get one or two mpg better, emphasis on might. How long would it take to recoup the cost of a gear change? BTW, I have 3.5 gears and the best I've ever done is 22 mpg, as an experiment, at 57 mph from LA to central TX. Traveling at 62-65 mph gives me 19-20 mpg.

You mentioned the newer trucks with 3.73s but overlooked the fact that their stock tires are taller than the stockers we have. The difference in tire size compares to the 3.5s I have.

 

[JDunmyer]

Actually, my main concern was more to lower the RPM at a cruising speed of 65-75 MPH than to improve the mileage. The truck won't be my daily driver, just that I'd like to use it for a few trips.

Full disclosure: my daily driver is a Chevy Bolt EV, I have no other car, just the truck. Several times/year (8 or 10) I drive longer distances than what a single charge will take me and it would be quite inconvenient to charge the Bolt, even if a fast charger was available. At present, I borrow my son's Nissan Murano for those trips.

Thanks much for the commiseration and advice!!

 

[GAmes]

If mileage isn't that much of concern it would be hard, in my mind, to justify the expense of a gear change. The engine is redlined at 3000. It won't hurt it a bit to operate at 2300 or 2400 rpm for extended periods. At first it will drive you crazy but it doesn't take long to adjust. I know, I lost 5th gear in my NV4500 in ND and drove it in 4th gear back to TX at freeway speeds.

 

[JDunmyer]

It just seems like the Cummins is happiest at 2000 RPM or less, and sounds very "busy" at 2400. Years ago, I rode Honda motorcycles and thought nothing of seeing the tach at 5000 RPM for long periods of cruising. But those were really meant to crank up.

As an aside, I recently found out that a woman I went to HS with has a daughter who worked in IT at Cummins for several years, and her husband was an engineer there.

 

[bcbender]

It just seems like the Cummins is happiest at 2000 RPM or less, and sounds very "busy" at 2400.

Actually, I had consistently lower EGT at higher RPM and not much difference in fuel economy running at 24k GCVW. In fact I would run in direct at speeds lower than 55 for that reason.

 

[ROADWARRIOR]

It just seems like the Cummins is happiest at 2000 RPM or less, and sounds very "busy" at 2400. Years ago, I rode Honda motorcycles and thought nothing of seeing the tach at 5000 RPM for long periods of cruising. But those were really meant to crank up.

As an aside, I recently found out that a woman I went to HS with has a daughter who worked in IT at Cummins for several years, and her husband was an engineer there.

I spoke with an engineer at Cummins a long time ago and he told me my ISB would happily run all day long at 2700 RPM

 

[JDunmyer]

Thanks for the comments about running at the higher RPM, I guess I'll forget gearing changes.

'preceiate it!!

 

[geardaddy]

Thanks for the comments about running at the higher RPM, I guess I'll forget gearing changes.

'preceiate it!!

i have had a 94-94-96 97-98 and 2 2001,s. all 3.54 gears and manual trans..2003 and 2005 drw with 3.73 and 285-75-17 tires--6 speed, towed 45ft living quarters race trailer all summer..very good mileage up to 74 mph then it would drop about 3-5 mpg..pulled very good..cold air intake was a must..

 

[Cummins12V98]

Thanks for the comments about running at the higher RPM, I guess I'll forget gearing changes.

'preceiate it!!

Good call.

 

[9DodgeFan]

Thanks for the comments about running at the higher RPM, I guess I'll forget gearing changes.

'preceiate it!!

Sounds like you already made a decision,
but figured I'd chime in. I've heard you can get other gear ratios such as 3.73 from aftermarket companies. A quick internet search I just did found several.

 

[JDunmyer]

I'm thinking that 3.73 would be a nice compromise, but it probably isn't worth the trouble. Dunno who I'd get to change the gears (that I'd trust) and I'm no longer up to doing it myself. I've done the setup on a couple of Dana axles, but not enough times, nor recently enough to be real confident of my ability.

 

[SnoKing]

Second gen autos did best towing with 4:10's. The manuals did better with 3:54's and they have less of an overdrive ratio, and as you know the engine is busy at interstate highway speeds.

 

[bcbender]

I ran the NV5600/4:10 for 10y towing 24k and never had any engine issues.. EGTs always low. FE was fine,,, but I wasnt that worried about chasing MPG. IMHO if you're going to use it for towing / hauling stay with 4:10s. If your just running around town and DD it, then maybe but still the cost to R&R the gears but since its 2wd its 1/2 the cost

 

[Ozymandias]

Throw bigger tires on it, way cheaper then a gear change and reversible at the next occasion.
Would also look way better then the stock wieners.