Diff gear ratio suggestions

[Dave Barbieri]

Last April, after spending a year tinkering and twiddling, I got my truck running. I bought it from the 2nd owner, after it had been sitting by the repair shop for over a year. (This was the first time it'd run in over 4 years.) The whole sorry saga is listed on the oilburners page under "Which pump? Which supplier?"

Well, it's been running for 6 months now, and I gotta tell ya that's one sweet truck! Comfortable, (fairly) quiet, plenty of power, and rock-solid reliable! I also have an 85 F250 with the 6.9, and it's really something to see the difference that 10 years makes in diesel technology. Can you say POWER WITH NO SMOKE!!

OK, as happy as I am with how it runs, I'd LOVE to get better than 13mpg. I drive pretty gently, and whenever possible, I lock up the TC and use the cruise. Even with very careful attention to how I drive, the best I've ever gotten is 14mpg. So, here's the question: Would swapping my R&P from 4.10 to 3.73 help any? Or would I just cause the engine to work harder, and consume even more fuel? You can see the mods in my sig. Here's the rest of the info:

Truck weighs 6450 empty. It's set up for a 5th wheel, but isn't currently used for towing. It's used primarily as a dailly driver - 30 miles each way on back country roads and some FM state roads. Very occasionally, I'll load it up with barrels of WVO - 1500# - 2000#.

Has anyone out there done this swap? If you have, I'd love to hear first hand knowledge of how it worked out.

 

[mfgguru]

You would see a marked improvement with the 3.73's. That, in my opinion, is the desired ratio for both economy and work. 3.42's are a bit tall for pulling. I have 3.73's in my truck and see 17-20 on the hwy and pull occasionally. I recently made a 200 mile trip with a 20 ft enclosed cargo trailer. No problem, 1800 RPM at 60 mph, even with the cruise on. Those are my thoughts.

 

[jbsaxman]

what size of tire are you running? That would make a difference. Too small and you're not going as far per RPM. Too big and you're using more fuel to go the same distance.

My rig is off on the speedo and the odo. It's got 245's on it right now. The VSSB is set for 265's. I am trying to decide if I am going to spend $1200 on 265's or go to 285's.

At any rate, if the life of your tires are about up, it might be more economical to get a slightly larger tire. Or do both.

 

[Dave Barbieri]

Looks like our wheelbases are pretty similar - you've got a crewcab short box and I've got an extended cab longbed. I bet our vehicle weights are kinda close, too. Although I've got two extra tires on the ground, you have the drag of a front drive axle. Right now, in OD with the TC clutch locked, I'm turning 2300rpm at 60mph. Dropping 500rpm would be sweet! Throw in 4 extra mpg's and I'd be pretty darn happy. And I gotta tell ya, a set of R&P gears is a LOT cheaper than a Gear Vendors OD setup!

 

[Dave Barbieri]

JB - I'm running 235/85R16's. They're a bit bigger & wider than the tires that were on it when I drug it home over a year ago. (I think those were 80's, not sure) I tracked my speedo/ODO using milemarkers on I20, and I'm right at 5% off. I can live with 3mph at freeway speeds. What I'd like to live WITHOUT is 13mpg!

How do you like your 3.73's? What kind of mpg are you getting? Do you do much towing or hauling? For probably the next year or so, I'll be suing mine as a daily driver as well as a heavy hauler. So the better milage I get, the less biodiesel I have to brew.

 

[Brooklyn Tow]

I've got 4.63's and would be happy with 4.10's......Then again i'm a 3500HD wrecker setup with 225 70 19.5. I'm also getting around 13 MPG. IMO the 4.63's are overkill and really not well suited for the 6.5TD Power Plant.
Just my 2 cents.

Totally understand where your comming from.

 

[just a number]

is that 13mpg combined, hwy,city. I had to ask because 13mpg seem awfully low. city I could see it but not hwy. I get about 15mpg combined with a 6.2 and 4.10s

 

[Dave Barbieri]

I've got 4.63's and would be happy with 4.10's......Then again i'm a 3500HD wrecker setup with 225 70 19.5. I'm also getting around 13 MPG. IMO the 4.63's are overkill and really not well suited for the 6.5TD Power Plant.
Just my 2 cents.

Totally understand where your comming from.

Well, when I pull these, you're welcome to them. Understand, it'll be sometime this Spring, but if ti's worth the price of shipping, they're yours. Out of curiousity, what's your engine rpm at 60?

 

[Dave Barbieri]

is that 13mpg combined, hwy,city. I had to ask because 13mpg seem awfully low. city I could see it but not hwy. I get about 15mpg combined with a 6.2 and 4.10s

It's pretty much what I get tank after tank, whether I'm commuting or on the highway. Seems pretty consistent.

What's your rpm at 60? Reason I ask, 2300rpm at 60 seems kinda high. I'm wondering if one of the PO's didn't install some aftermarket gearset. 4.56, maybe? I'll take a closer look Monday at work.

 

[midniteplowboyy]

How fast do you normally drive? I've had all the gear ratio's put in the 2500-3500 6.5's and one they didn't.

 

[Dave Barbieri]

How fast do you normally drive? I've had all the gear ratio's put in the 2500-3500 6.5's and one they didn't.

On the open road, my truck and I are most comfortable at 65mph. Throw in the 5% error (68-69 real mph) and we're still under the radar. On the back roads, it's usually around 50mph or so. No jackrabbit stuff, fairly steady even acceleration, and I try to keep things pretty steady as I drive. Good for fuel consumtion and good for equipment. I'll save the racing and power displays for the younger guys with more $$$.

 

[docdray]

14 city, 18 hwy @ 55mph. I considered changing to 373 but I pull 5500 to 8000 regularly and the extra load on tranny and engine is not worth it as well as several people recommeded strongly keeping the 410 for the 3500 6.5td. 265/75/16 dropped rpm @ 65 from 2500 to 2375.

 

[Turbine Doc]

DD 5500-8000" is nothing for 6.5 with 3:73, & a well maintained 4L80, that is range of the factoy rating for mine 6600# as a K1500 before I modded it up to handle 18,000# occasionally with 3:73 in OD on my 4L80

 

[Dave Barbieri]

DD - Thanks for the input on rpm/speed. 2500 at 65mph is exactly what I see, so it looks like I've still got the stock 4.10 R&P.

Tim - 18K is more than I ever plan on towing. Most of what I'll be doing involves loading equipment/stuff into the back and carting it around. Although I can carry 3500# and still be within my GVW (barely!), I doubt I'll ever see half that. I plan on installing a PowerTrax locker when I regear - sorta take care of everything all at once. I stumbled on your article on this unit on the other Place. Great writeup; your first-hand account of living with one day-to-day helped make the choice.

 

[midniteplowboyy]

3.73's or 3.42's would work for what your doing.

This is what I suggest for cruising speeds in OD,
70mph-3.21's
65mph-3.42's
60mph-3.73's
55mph-4.10's
keeping the engine in its sweet spot.
For highway towing I like the 4.10's, then the 3.42's. For back road cruising I like the 3.73's. For highway cruising I love my 3.21's.

For best milage, if you drive 65mph, I'd put the factory size tire back on and some 3.42's.

Where is Overton? I'm up by Texarkana.

 

[davo727]

Hey midniteplowboyy: I didnt even know they made 3.21 sets for a 14 bolt full floater, So I just looked it up on randys ring and pinion. U got a high speed cruiser there!

 

[midniteplowboyy]

Hey midniteplowboyy: I didnt even know they made 3.21 sets for a 14 bolt full floater, So I just looked it up on randys ring and pinion. U got a high speed cruiser there!

I didn't know they made them either, I ran across them when I was looking for some 3.42's for that truck. Yea she's a pleasure to drive compared to the 4.10's, nice and quiet now.

 

[Veg_Out]

I would travel far and wide to trade my 410's for a set of 3.42's.

I would just flat love a set on my Suburban. Then, change the stiffness of the suspension, and this thing might just be the cat's meow on the highway.

 

[Turbine Doc]

DD -
Tim - 18K is more than I ever plan on towing. .

Was more than I had planned, divorce between in laws forced the hand, I had been keeping my tractors at his place 160 miles from mine, he tried to make them part of the dispute, to get me to leverage her for concessions he wanted, anyway I had need to move them no money for big truck so I had to make mine stouter. Would not pull this everyday as configured, but couple of times I need to move it during the year I'm okay. Power plant is better than stock early D-Max now.

As for Power Trax, go with the no-slip locker IMO better than the Lock-Rite I ran on my C1500 gasser for years which is much harsher in turns, the no-slip has made all of my tows put to it; including a few of the 18K ones, no issues, but I also have a PML 3qt extra capacity deep dover over the rear.

 

[Turbine Doc]

I would travel far and wide to trade my 410's for a set of 3.42's.

I would just flat love a set on my Suburban. Then, change the stiffness of the suspension, and this thing might just be the cat's meow on the highway.

Ive seen 24 on mine running 70 that configuration, hoping to get some more mpg with the Mitsubishi/ATT on it.