Gearing question

[Mikes91]

I posed this question to Brian at BTS, so we'll see if he has a chance to respond to my e-mail.

I'm thinking about changing my truck's rear end gear ratio back to 4.10, or maybe somewhere between 3.73 and 4.10, from 3.55. Truck bogs down on hills when loaded, and really saps when accelerating a load after it shifts into 3rd IIRC.

I typically pull in overdrive at 68-73mph. Is there a way to figure out what rear end gear ratio I need to put the engine at its best-towing RPM at that speed? I'll need to account for my tires, too--they're slightly larger than stock.

Once I figure this out, I'll also need to figure out what speedo gear is needed...


Thank you,

Mike

 

[Mr_Roboto]

Have you tried simply running in 3rd (direct) when you are loaded?

Regearing will only change the RPM in O/D. In other gears, the transmission will simply shift sooner.

 

[Mikes91]

Thank you for the note. I certainly don't need this thing shifting into 3rd earlier when accelerating the trailer...

When I run out of OD at highway speeds, I'm turning more RPMs than the truck needs to turn. I could slow down, but why?? I'd rather trade a little top speed and turn OD a little faster. I never pull fast enough in OD to get the engine RPMs into the sweet spot. That'd be plumb crazy... about 80-85 with 10,000 pounds behind me. Nope. Sweet spot would be 68-70mph in OD.

I wish I could regear the whole transmission. Sounds like and issue that Gear Vendors or U.S. Gear unit could resolve... <not gonna happen...$$$>

 

[The Warden]

I've read that OD with 4.10's gets you about the same RPM's as direct-drive (3rd on an E4DOA) with 3.54's. That may be off a little bit, but probably not enough to matter (again, this is based on what others have said, not from direct experience).

So, I would suggest against swapping to 4.10's. 3.73's might work, or just tow in 3rd. If you really want to swap to 4.10's, you might consider swapping the entire axle...there are people around with 4.10 axles who want to swap to 3.54's.

 

[MIDNIGHT RIDER]

Major surgery like this is not something I would under-take and then head out on a cross country trek.

With a diesel, 3:55 are a better choice for road driving.

If you were dragging big loads of cows out of country roads to the local yard, then I would say 4:10 or even lower.

 

[Mr_Roboto]

Here's the thing, say you swap to 4.11's. The only real difference will be lower gearing from a stop in first gear, and higher RPM's for the same road speed in overdrive. Other than that, the trans will still shift at more or less the same RPM, just at a lower road speed.

Something you might want to look at is increasing the transmission shift points for towing. I believe the problem that you are running into is when the trans makes the 2 - 3 shift, it drops the engine down below its torque peak range. You can adjust the FIPL sensor / TPS to increase the shift point somewhat. There are also aftermarket programmers that do this better (Bauminator).

I used to drive semi's OTR. The company I drove for had cabover tractors with fairly small turbodiesels. If you got the engine out of its torque peak it was all over. The shift must be made so that when the RPM's drop in the next gear, the engine is within it's peak torque band.

I'm not saying that re-gearing doesn't have its place. It's a good choice for a truck that is heavily loaded most of the time. However speeding up the driveshaft with an overdrive, then slowing it back down with different gearing is wasteful.

 

[Mikes91]

I've read that OD with 4.10's gets you about the same RPM's as direct-drive (3rd on an E4DOA) with 3.54's. That may be off a little bit, but probably not enough to matter (again, this is based on what others have said, not from direct experience).

So, I would suggest against swapping to 4.10's. 3.73's might work, or just tow in 3rd. If you really want to swap to 4.10's, you might consider swapping the entire axle...there are people around with 4.10 axles who want to swap to 3.54's.

That's just the info I needed. I don't want to go as high as something that's like running in 3rd while in OD. I need something just a hair below that. 3.73s might be about right...

OR... Mr. Roboto's advice is quite valid... I'll have to see what RPMs I'm turning when she shifts into 3rd under load. Perhaps the OkieGringo solution is the correct path to take... and just swap back to stock tire size OR consider 3.73s for the road.

Thank you,
Mike

 

[HiWayMan]

Thank you for the note. I certainly don't need this thing shifting into 3rd earlier when accelerating the trailer...

When I run out of OD at highway speeds, I'm turning more RPMs than the truck needs to turn. I could slow down, but why?? I'd rather trade a little top speed and turn OD a little faster. I never pull fast enough in OD to get the engine RPMs into the sweet spot. That'd be plumb crazy... about 80-85 with 10,000 pounds behind me. Nope. Sweet spot would be 68-70mph in OD.

I wish I could regear the whole transmission. Sounds like and issue that Gear Vendors or U.S. Gear unit could resolve... <not gonna happen...$$$>

Mike, don't know if this site will help you but it's interesting none the less. https://home.comcast.net/~jcmiley/index.html
Once you're on the site click on "Gear Calculator".. Also check out the 4bt he has in an early bronco...




Al

 

[bikepilot]

I recently went from 4.10's to 3.55's and I would have to dispute the idea that OD with 4.10's = 3rd with 3.55's. The difference is not that drastic, that is, 3rd with 3.55's is much lower than OD with 4.10's.

 

[NapaBavarian]

a .76 od with 4.10 final drive is equal to a 1:1 top gear with 3.12 final drive.

0.76 x 4.10 = 3.12

.76 is what is used on the ZF5, so check what your transmissions top gear is and substitute that if it differs.

 

[tuckerd1]

Mike is running a E40D. That gives 4.10 in 3rd and 2.91 in OD. 3.55s should give 3.55 in 3rd and 2.52 in OD.

 

[NapaBavarian]

Mike is running a E40D. That gives 4.10 in 3rd and 2.91 in OD. 3.55s should give 3.55 in 3rd and 2.52 in OD.

Youch! I can see where that might present its self as a bit of a problem with a 10k load

 

[Mikes91]

tuckerd1,

Thank you for your note. That extra 0.39 difference between 4.10 and my 3.55 sounds like it is substantial judging from NafaBavarian's reaction. What is your experience towing with the 4.10s on your truck?

I'm reluctant to go all the way to 4.10 due to mileage concerns... but at the same time don't want to swap to 3.73 if that'll be a waste of money--i.e., no notable performance gain while towing.

Can anyone comment on what parts are required to swap back to 4.10 or 3.73? Are there other common gear ratios I could consider? I have no idea if it is as simple as a gear... or what...


Thank you,
Mike

 

[tuckerd1]

Mike,
I have no problem with the 4.10. I run ~2000rpm at 60mph in OD. Fuel mileage is ~17 empty and 9.5-10 towing 12K. The problem I have with my setup is the gear ratio difference built into the E40D. Too big of a jump for efficient towing. I'm going to either a USGear or a GV unit eventually for some gear splitting capabilities while towing.

I'm seriously considering 19.5" wheels w/245 tires and a USG underdrive unit. That throws more kinks into the ratio calculations when you do that.

BTW a 3.73 gear would give you 3.73 in 3rd and 2.65 in OD.

 

[pafixitman]

Mike,
I got weighed last year and run about 12k complete - almost 4 grand on each axle. I got 10.5 mpg towing at 2,000 rpm @ 60 mph. I agree w/ Tuckerd. The 3/4 jump is big. I might line Okie's pocket and get the Baumann control (he owns them, right? ) so I can control the downshifts better.

I have a Factory Tech valve body, but that did not change the ratios.