37" tires with 4.10 for those who do have

[Reloaderguy]

Here is the spacer with bolts: http://www.nitro-gear.com/GM-9-25-AAM-9 ... rg925a.htm

[Reloaderguy]

With 37s, there is only a 100 RPM difference between 4.88 and 5.13 at 70mph. 5.13 with 40s is roughly the same as 4.10 with 33s.

[OffroadTreks]

With 37s, there is only a 100 RPM difference between 4.88 and 5.13 at 70mph. 5.13 with 40s is roughly the same as 4.10 with 33s.

Did you make your final decision yet? The extra UMPH with 5.13's and 37's before going to 40's is kind of appealing.

But, overall, here is what I got from an online calculator

4.10 stock & 33's
2713.45 RPM @ 65mph

4.88 & 37's
2880.52 RPM @ 65mph

5:13 & 37's
3028.09 RPM @ 65mph

Now add 40's and

5.13 & 40's
2800.98 RPM @65mph

----

5.13's seem like a perfect fit for 40's. And maybe... maybe a FUN fit for 37's over 4.88 if you don't mind the higher RPM's

Anyways, that was just a calc on Summit, so take that accuracy with a large grain of salt.

[Reloaderguy]

I got a little different numbers but they're still close. Yeah, 5.13s it is. The RPM difference is more critical for diesel applications.

[Colibri]

If you’re going 40’s you’d be far better served by 5.38’s

[Reloaderguy]

God damn it!

[OffroadTreks]

Can you even get 5.38's for our gen though?

[Reloaderguy]

Yes. I'm still going to use 5.13. I'm sure 5.38 would be better in party mode but 5.13 has OE gear sets available and I'm going to error on highway streetability.

[EasyDoesIt]

With 37s, there is only a 100 RPM difference between 4.88 and 5.13 at 70mph. 5.13 with 40s is roughly the same as 4.10 with 33s.

Did you make your final decision yet? The extra UMPH with 5.13's and 37's before going to 40's is kind of appealing.

But, overall, here is what I got from an online calculator

4.10 stock & 33's
2713.45 RPM @ 65mph

4.88 & 37's
2880.52 RPM @ 65mph

5:13 & 37's
3028.09 RPM @ 65mph

Now add 40's and

5.13 & 40's
2800.98 RPM @65mph

----

5.13's seem like a perfect fit for 40's. And maybe... maybe a FUN fit for 37's over 4.88 if you don't mind the higher RPM's

Anyways, that was just a calc on Summit, so take that accuracy with a large grain of salt.

I'm not sure what transmission you used in your calculations but those RPMs cant be right.

With stock gearing and stock tires in my 2018, my RPMs at 65mph were 1800.
With stock gearing and 37's tires, my RPMs at 65mph (via gps) are 1500.

[Reloaderguy]

http://www.grimmjeeper.com/gears.html

4.10 w/ 33

Road RPMs at given speed in miles per hour
Gear
10

35

65
1 1348 4719 8764
2 764 2674 4966
3 589 2060 3826
4 417 1461 2713
5 338 1183 2198
6 259 906 1682

5.13 w/37

Gear
10

35

65
1 1505 5267 9781
2 853 2984 5541
3 657 2299 4270
4 466 1631 3028
5 377 1321 2453
6 289 1011 1877

[Colibri]

Tire size to gear ratio calculators are very useful tools, BUT, they fail to take weight into consideration.

My tire/wheel combo comes in at about 140-150 pounds with 37x13.50r17 toyo mt’s and Hutchinson’s.

I have 5.13’s a high stall converter and a built transmission, I still sometimes wish I’d gone deeper with the gearing. It’s pretty damn quick for such a big and heavy gasser and it tows reasonably well up to about 8-10k but you can really feel the weight and rolling resistance of the bigger rubber. 40’s will be worse. 37 is 94 pounds and the 40’s are 104 pounds. My wheels are heavier than most but either way that’s a shit ton of rubber to push around.

Just sayin

[OffroadTreks]

Tire size to gear ratio calculators are very useful tools, BUT, they fail to take weight into consideration.

Just sayin

Agreed. As a guide only, because they all seem to vary.

[KevinABQ]

My tire/wheel combo comes in at about 140-150 pounds with 37x13.50r17 toyo mt’s and Hutchinson’s.

What model Hutchinson work best?

[Colibri]

Mine is a third gen. It’s the 5.28” backspace model. Not sure what would be optimal for a radius arm truck just off the top of my head though. Really great wheels.

[Reloaderguy]

If you’re going 40’s you’d be far better served by 5.38’s

Having now driven the truck with 5.13's and 37" tires I think you are probably correct. The 66RFE trans gearing is tall enough that 4.88 should be the factory gearing, not 4.10. 5.13's with 37's feels pretty good and will probably still be OK with 40's but 5.38's will never be too deep. The only downside is there aren't any OE gear sets available in 5.38.

[adeluca73]

If you’re going 40’s you’d be far better served by 5.38’s

feels pretty good and will probably still be OK, but 5.38” will never be too deep.

That’s what she said ...

But seriously, I have a ½” worth of gear envy. This is sort of analogous to the 2.5” vs 3.0” front King debate. When do you hit the point of diminishing returns? Not sure, I know 2WAGs blew up his rear end with 5.13’s, who knows why, shit gear, bad install, lash errors, metallurgy failure, but I do know there’s less margin as you scale up tire size and scale up the ratio (down the gear size). 4.88’s were my comfort zone, pumped to see your 40’s crawling on those 5.13’s. Maybe I’ll get the itch again..,

[Reloaderguy]

If you’re going 40’s you’d be far better served by 5.38’s

feels pretty good and will probably still be OK, but 5.38” will never be too deep.

That’s what she said ...

But seriously, I have a ½” worth of gear envy. This is sort of analogous to the 2.5” vs 3.0” front King debate. When do you hit the point of diminishing returns? Not sure, I know 2WAGs blew up his rear end with 5.13’s, who knows why, shit gear, bad install, lash errors, metallurgy failure, but I do know there’s less margin as you scale up tire size and scale up the ratio (down the gear size). 4.88’s were my comfort zone, pumped to see your 40’s crawling on those 5.13’s. Maybe I’ll get the itch again..,

Lee broke his front gears pulling someone in reverse. I don't think you can go too low gears with the larger tires. 5.38's are probably too low for 33" tires.

[OffroadTreks]

I'm sold. I just gotta go to the secret Maple Base to get help with my gears and get a rear bumper built.

[Reloaderguy]

2k RPM's at 70MPH seems perfect. I should have gone 5.38. It's not going to be a bust but 5.13 feels so good with 37's. Older trucks with the 545 is a whole different story.

[adeluca73]

feels pretty good and will probably still be OK, but 5.38” will never be too deep.

That’s what she said ...

But seriously, I have a ½” worth of gear envy. This is sort of analogous to the 2.5” vs 3.0” front King debate. When do you hit the point of diminishing returns? Not sure, I know 2WAGs blew up his rear end with 5.13’s, who knows why, shit gear, bad install, lash errors, metallurgy failure, but I do know there’s less margin as you scale up tire size and scale up the ratio (down the gear size). 4.88’s were my comfort zone, pumped to see your 40’s crawling on those 5.13’s. Maybe I’ll get the itch again..,

Lee broke his front gears pulling someone in reverse. I don't think you can go too low gears with the larger tires. 5.38's are probably too low for 33" tires.

Yep...as you’re upping the tire size, you gotta grow the gears to turn them, so the effective ratio by RPM at 65mph with 5.13 & 40’s, is about the same as 4:88 & 37/4:56 & 35/4.11 & 33 , about 2800RPM +-~5%.

[Reloaderguy]

That was my original train of thought. 5.13 w/ 40's is roughly equivalent to 4.88 w/37's. However, that makes the assumption that 4.88 w/ 37's is optimal. Now that I have hands on experience with 5.13 w/37's I would probably not go with 4.88 w/ 37's. I doubt I would yank out the 4.88 for 5.13 though.

[Bill2014]

If you only do work in the forward direction, it reduces to a question of optimization...
If backing something heavy up a winding hill in 4Lo is in the picture, I would think twice about the "weaker" gear set.
I've been in 4Lo reverse with the wheels taking turns breaking loose - stock gears - hoping I don't hear a snap and have things come to a grinding halt.

If it wasn't for the strength concern, I may have regeared even with the 33's. (for the way I use the PW)

On the other hand, the Power Wagons aren't high torque diesels so maybe it doesn't matter.

[Reloaderguy]

If you only do work in the forward direction, it reduces to a question of optimization...
If backing something heavy up a winding hill in 4Lo is in the picture, I would think twice about the "weaker" gear set.
I've been in 4Lo reverse with the wheels taking turns breaking loose - stock gears - hoping I don't hear a snap and have things come to a grinding halt.

If it wasn't for the strength concern, I may have regeared even with the 33's. (for the way I use the PW)

On the other hand, the Power Wagons aren't high torque diesels so maybe it doesn't matter.

With 37's you're more likely to hear your transmission snap.

[olyelr]

If you only do work in the forward direction, it reduces to a question of optimization...
If backing something heavy up a winding hill in 4Lo is in the picture, I would think twice about the "weaker" gear set.
I've been in 4Lo reverse with the wheels taking turns breaking loose - stock gears - hoping I don't hear a snap and have things come to a grinding halt.

If it wasn't for the strength concern, I may have regeared even with the 33's. (for the way I use the PW)

On the other hand, the Power Wagons aren't high torque diesels so maybe it doesn't matter.

I am on stock size tires as I think you are, and even with those I would like 4.56's, if not 4.88's. The trans gears are just so high.

When the jump to 37's comes I think I will go to 5.13's.

[adeluca73]

If you only do work in the forward direction, it reduces to a question of optimization...
If backing something heavy up a winding hill in 4Lo is in the picture, I would think twice about the "weaker" gear set.
I've been in 4Lo reverse with the wheels taking turns breaking loose - stock gears - hoping I don't hear a snap and have things come to a grinding halt.

If it wasn't for the strength concern, I may have regeared even with the 33's. (for the way I use the PW)

On the other hand, the Power Wagons aren't high torque diesels so maybe it doesn't matter.

I am on stock size tires as I think you are, and even with those I would like 4.56's, if not 4.88's. The trans gears are just so high.

When the jump to 37's comes I think I will go to 5.13's.

Funny—all this talk of gearing and tires, at one point 4.88’s & 37’s were like the holy grail step-up for the Best-Gen trucks, put us on par or above with the ‘13 and older crowd who mostly gravitate to 35’s and keep their 4.56’s. Now the move is to 39/40” tires and 5.13’s—and I’ve been secretly discussing with my inner engineer the virtue of abandoning the 37” experience and moving down to lowly 35’s ( HUSH YOUR MOUTH) when these Nittos are done

The unsprung weight difference, added driveline stress, and steep increase in rolling moment of inertia haunts my nerd alter-ego, but the cool kid in me wants 5.13’s and 40’s (STILL YOUR MIND, DONT LISTEN TO THOSE MOAB HERETICS AND GYPSIES, THEY’ll PILFER YOUR TREASURE AND PLUNDER YOUR TRUCK) so i war with my innersef, but i think 35’s and 4.88’s look pretty great for all around use....maybe a bit higher RPM at 75mph than I’d like, but I’ll take that over the fear of constant brake fade, and NO, I’M NOT INSTALLING HYDRO ASSIST STEERING, OR A BIGGER MASTER CYLINDER AND HIGH CAPACITY STEEL BRAIDED LINES—GET THEE BEHIND ME SATAN