235/85's and 3.73 diff

[Dave Barbieri]

Since getting my truck running a year ago, I've slowly added/performed the mods in my sig. One of the first (unintentional) changes was installing a set of 238/85R16 tires. The originals were dry-rotted from sitting for over 4 years, and I happened to have a set of almost unused tires from a previous project truck. I lived with the speedo/odo discrepancy until this last winter, when I changed the jumpers in the DRAC and got everything back in line.

My next project is the differential. I plan on installing new bearings, new R&P and a No-Slip locker setup. My question is:
Is anyone running a 3.73 rear end and 235's on a three-quarter or one ton truck?
How's it run? What's your fuel mileage?

Although my GVW is 10K, the truck runs almost empty most of the time. It's my daily driver for commuting and I use it for heavy hauling maybe once every month. With the 235's, I'm running 2200rpm at 65. With the regear, I'll be around 2000. On my Ford F250 6.9 this was ideal. Not sure about the GM 6.5. TIA for any help/advice/opinions!

 

[ak diesel driver]

That's what I am running and no problems. I don't run alot of higher speeds (no interstates) so all I can say is your rpms sound about right. I didn't notice any changes in my mileage but I still have some issues I'm dealing with.

 

[Dave Barbieri]

So you're running the 235's and 3.73 gears? What kind of mileage are you currently getting? How about rpm's at 60? At 65? It seems like 1950 - 2000 rpm would be perfect for the 6.5 torque curve, but I'd kinda like to know for sure before I go to all the effort. Hate to spend all the time and $$$ and be disappointed.

 

[94K30]

Hello Dave, I am running 265/75/16's with 3.73;s. they are almost the same exact height as the 235/85's, and the RPMs sound about right, I am getting about 18 to 20 depending on hiway/sideroad mix

 

[midniteplowboyy]

My 94 has 3.73's with 265/75's(same height), I dont know the rpm(no tach), but it runs fine, better than it needs to considering its carry around an extra 5500lbs from stock form.

My 92 has 3.42's with a 5sp 245/75's, fuel mileage machine, pleasure to drive empty, it'll tow ok if you run 4th(actually a little lower geared than 4.10's in OD). I've put over 100k towing loaded 18-19k, plus the return trips 100k of 6k empty trailer weight. Trucks got over 350k now(retired from long distant towing since the D-max), oil pans never been off, factory IP/injectors, twice the best truck I've ever had.

My 93 had 4.10's, now has 3.21's with 305/40-22's(100lbs x 6 + adapters = heavy rotational mass)(basically the same height as 235/85's), truck pulls it fine, but its far from stock. Its good for interstate driving.

How fast do you drive, I think you would like 3.42's, overall my favorite gear. What type of driving do you do, how far what speed? When you tow, what do you tow/weight/how far?

 

[Dave Barbieri]

My 94 has 3.73's with 265/75's(same height), I dont know the rpm(no tach), but it runs fine, better than it needs to considering its carry around an extra 5500lbs from stock form.

How fast do you drive, I think you would like 3.42's, overall my favorite gear. What type of driving do you do, how far what speed? When you tow, what do you tow/weight/how far?

Thanks for the input! Mostly, I use the truck as a daily driver. My commute is approximately 30 miles each way. The first 25 are farm-to-market roads - 65mph, no stops. The last 5 miles is driving thru Tyler (stop & go) to get to our campus on the other side of town. Once every month, I head north to Oklahoma City (325 miles each way) to see mom & dad. The truck currently weighs 6500#. This summer, I'll add a lift gate (400#) and a 100 gallon transfer tank. With the tank empty, I'll top out just over 7,000#. With full tanks, I'm looking at over 7800#. Every three months or so, I go to Dallas (90 miles each way) to pick up used cooking oil for biodiesel. Six barrels = 2800#. Towing is very seldom. Mostly helping our daughter move. Three times in three years. ooof.

 

[midniteplowboyy]

I think you would like the 3.42's. The 3.73's would be alright for the FM roads and town, but sounds like you drive alot of interstate, go with the 3.42's.

Overton is down by Longview isn't it? I live in Dekalb, maybe we could meat up sometime in Mt Pleasant or Omaha and grab a bite to eat and I'll let you drive my 93 with the 3.21's.

 

[midniteplowboyy]

I checked my rpm's yesterday in OD(trans) and Direct(2sp),

1500=55mph
1600=60mph
1700=65mph
1800=70mph

,this is just off the tach and speedo, the tach is real close to right with my scanner, the speedo needs to be verified with a GPS but its close(maybe reading a couple mph to high). Maybe one of these mathematicians can verify.

 

[Matt Bachand]

I checked my rpm's yesterday in OD(trans) and Direct(2sp),

1500=55mph
1600=60mph
1700=65mph
1800=70mph

,this is just off the tach and speedo, the tach is real close to right with my scanner, the speedo needs to be verified with a GPS but its close(maybe reading a couple mph to high). Maybe one of these mathematicians can verify.

Thats some unbelievable low RPMS's!!!

 

[midniteplowboyy]

I checked my rpm's yesterday in OD(trans) and Direct(2sp),

1500=55mph
1600=60mph
1700=65mph
1800=70mph

,this is just off the tach and speedo, the tach is real close to right with my scanner, the speedo needs to be verified with a GPS but its close(maybe reading a couple mph to high). Maybe one of these mathematicians can verify.

Oops, forgot to say thats with my 3.21's and 305/40-22's on the 93.

Its nice to drive on the highway and I dont tow too heavy with it(no goose neck hitch), just a heavy duty 20' lowboy mostly.

I picked up some steel and lumber(aprox 5k) with the 93 today,

RPM's-1800-1900
MPH- 70-75
EGT's- 700-800
Boost psi- 7-12
Front W/temp- 200-205
Rear W/temp- 160-165
Fuel pressure- 11- 11.5
Eng/oil temp- 250-255
Trans temp- 210-215
Outside temp- 85-90

,this was on pretty flat interstate with little wind.

 

[ak diesel driver]

I checked today I was 2100 at 65

 

[Dave Barbieri]

I think you would like the 3.42's. The 3.73's would be alright for the FM roads and town, but sounds like you drive alot of interstate, go with the 3.42's.

Overton is down by Longview isn't it? I live in Dekalb, maybe we could meat up sometime in Mt Pleasant or Omaha and grab a bite to eat and I'll let you drive my 93 with the 3.21's.

Yup, Overton is South of Longview and about 30 miles east of Tyler (where I work). Thanks for the input and the offer. Actually, you're sorta on the way if I take the 'eastern route' (271) to OKC. Easy to drop by and say hi. I'd really like to see what you've done to your trucks - good ideas are always a good thing!

Since I already have a new Motive Gear R&P set and a new set of Timken bearings for the entire diff, I'm going to stick with the 3.73 idea. It's good to hear that this setup doesn't turn the truck into a slug. I understand the 235's give me roughly a 10% reduction, so that's going to put the final drive ratio equivalent to a 3.40. With the weight I'll be carrying, that's as low as I wanna go.

Kinda like the thought of adding a TrakLok No-Spin setup while I'm in there. ):h):h

 

[midniteplowboyy]

Oh, ok, I didn't know you already had the gears. Yea, go ahead and slap them puppies in there.

Hardest part to me on the 10.5 14bolt is if you use a crush sleeve, I'm partial to pinion shims instead(easier install and gives the pinion nut something solid to tighten against). Once you get the crush sleeve crushed/pinion bearing drag set, the rest is down hill, easy with a dial indicator and simple hand tools.