4:56…. Or … 4:88? That’s the Question

[Dantheman-2003]

So the build is coming along nicely. Lots of hard honest work and lessons learned along the way. Exactly what I’d hoped for when I took this 4x4 project on. Thanks to this forum and other resources, I’ve had allot of support along the way. That said, I’m to the crossroads of re-gearing the rear differential. Currently running 35x12.5x20 wheels on a 6” lift. I’ve read all kinds of threads and heard the arguments from all sides and points of view. Kind of goes back to the main question, what is the purpose of the vehicle and what am I doing with it? Mostly will just be a fun rig for my son and I drive drive around town, run up to the lake etc. I may pull a small trailer or boat now and again. We live in the hills of Northern California, so not flat land by any means.

So the question is, what gears? Right now running stock G80 3.73’s. And honestly, it doesn’t lag all that bad. Not as much as I thought it would. Now I’m leaning towards 4:56 gears because I’d like to drive it down to work now and again, freeway miles about 50 each way. Maybe an occasional hunt trip down in the valley. I’ve heard some say go 4:88 while others say that is overkill.

Welcoming your thoughts and much appreciated!!

Dan

 

[swathdiver]

So the build is coming along nicely. Lots of hard honest work and lessons learned along the way. Exactly what I’d hoped for when I took this 4x4 project on. Thanks to this forum and other resources, I’ve had allot of support along the way. That said, I’m to the crossroads of re-gearing the rear differential. Currently running 35x12.5x20 wheels on a 6” lift. I’ve read all kinds of threads and heard the arguments from all sides and points of view. Kind of goes back to the main question, what is the purpose of the vehicle and what am I doing with it? Mostly will just be a fun rig for my son and I drive drive around town, run up to the lake etc. I may pull a small trailer or boat now and again. We live in the hills of Northern California, so not flat land by any means.

So the question is, what gears? Right now running stock G80 3.73’s. And honestly, it doesn’t lag all that bad. Not as much as I thought it would. Now I’m leaning towards 4:56 gears because I’d like to drive it down to work now and again, freeway miles about 50 each way. Maybe an occasional hunt trip down in the valley. I’ve heard some say go 4:88 while others say that is overkill.

Welcoming your thoughts and much appreciated!!

Dan

Hi Dan,

When your truck was stock, if it had 265-70-17 tires, the rpms at 70 mph with 3.73 gears were about 1,992 rpms.

Now, they should be about 1,799 rpms with the 35s. Your current RPMs are like having 3.42 gears with a stock tire, 1,826 rpms.

A 4.10 gear will deliver 1,979 rpms at 70 mph on 35s.

A 4.33 gear will deliver 2,090 rpms at 70 mph on 35s.

A 4.56 gear will deliver 2,199 rpms at 70 mph on 35s.

A 4.88 gear will deliver 2,353 rpms at 70 mph on 35s.

This assumes a 4L60 series transmission too.

 

[drakon543]

if your running a 3.73 now and haven't had any issues why change it. 4.88 will be a pretty significant change and may even make normal dd annoying as it will possibly want to jump from a stop. the usual goal when regearing for tires is to bring it closer back to stock rpms so 4.11s would do that for you. 4.56 would give you more power for hills and that boat vs prior to the 35s when it was stock. if your not trying to have it live off road or pull large loads constantly 4.56 is the biggest i would go.

 

[NickTransmissions]

if your running a 3.73 now and haven't had any issues why change it. 4.88 will be a pretty significant change and may even make normal dd annoying as it will possibly want to jump from a stop. the usual goal when regearing for tires is to bring it closer back to stock rpms so 4.11s would do that for you. 4.56 would give you more power for hills and that boat vs prior to the 35s when it was stock. if your not trying to have it live off road or pull large loads constantly 4.56 is the biggest i would go.

35 inch tires changes (reduces) the mechancial leverage the vehicle's final drive has relative to engine rpm and corresponding vehicle speed. As a result, the engine isnt producing adequate rpm at any given speed compared to when the tires were stock diameter.

Over time, this deficiency stresses the transmission, particularly the 3-4 clutch pack as there is less line preasure rise generated inside the transmission to clamp/hold that clutch pack so it begins to slip before too long.

4.56 gears would restore the relationship established by the factory however in my humble opinion, 4.88 would be ideal.

Whever someone calls wanting to do a trans rebuild and i learn they have a truck or SUV, the first question I ask is "is the truck lifted and if so, what's the tire size" if they tell me its like 33" or larger, and they havent upgraded the gears, I tell them that I cant give any warranty if they elect to move forward with the rebuild and advise they have their gear ratios updated before overhauling the trans.

 

[S33k3r]

Hope that helps as well.

Gear Ratio and Tire Size Chart

 

[Trey Hardy]

So the build is coming along nicely. Lots of hard honest work and lessons learned along the way. Exactly what I’d hoped for when I took this 4x4 project on. Thanks to this forum and other resources, I’ve had allot of support along the way. That said, I’m to the crossroads of re-gearing the rear differential. Currently running 35x12.5x20 wheels on a 6” lift. I’ve read all kinds of threads and heard the arguments from all sides and points of view. Kind of goes back to the main question, what is the purpose of the vehicle and what am I doing with it? Mostly will just be a fun rig for my son and I drive drive around town, run up to the lake etc. I may pull a small trailer or boat now and again. We live in the hills of Northern California, so not flat land by any means.

So the question is, what gears? Right now running stock G80 3.73’s. And honestly, it doesn’t lag all that bad. Not as much as I thought it would. Now I’m leaning towards 4:56 gears because I’d like to drive it down to work now and again, freeway miles about 50 each way. Maybe an occasional hunt trip down in the valley. I’ve heard some say go 4:88 while others say that is overkill.

Welcoming your thoughts and much appreciated!!

Dan

4.56 all day. Especially if you actually want to drive it around town some if it was just an off-road truck only then 4.88 would be ok but still overkill. My buddy ran 4.56 in everything he had and he could blow 37” mud grapplers off 1-2nd gear chirping 3rd.
He also had a built transmission…
4.56 will make that bus pull like a ***** ape especially if you get a custom tune to dial everything in

 

[swathdiver]

35 inch tires changes (reduces) the mechancial leverage the vehicle's final drive has relative to engine rpm and corresponding vehicle speed. As a result, the engine isnt producing adequate rpm at any given speed compared to when the tires were stock diameter.

Over time, this deficiency stresses the transmission, particularly the 3-4 clutch pack as there is less line preasure rise generated inside the transmission to clamp/hold that clutch pack so it begins to slip before too long.

4.56 gears would restore the relationship established by the factory however in my humble opinion, 4.88 would be ideal.

Whever someone calls wanting to do a trans rebuild and i learn they have a truck or SUV, the first question I ask is "is the truck lifted and if so, what's the tire size" if they tell me its like 33" or larger, and they havent upgraded the gears, I tell them that I cant give any warranty if they elect to move forward with the rebuild and advise they have their gear ratios updated before overhauling the trans.

Nick, why do you like 350 rpms over the original 2000 when it was on stock tires? Is it to compensate for the additional rotational weight/mass of the 35s?

 

[NickTransmissions]

Nick, why do you like 350 rpms over the original 2000 when it was on stock tires?

Better acceleration at lower rpms (and corresponding better fuel economy) and, of course, better line pressure rise inside the transmission for applied elements, particularly the direct and overdrive clutch packs (for all transmissions). Lower RPMs are fine for HWY fuel economy considerations as long as you're not putting the trans at risk of having insufficient line rise due to insufficient mechanical leverage in the final drive...I've seen quite a few transmissions over the years out of lifted trucks where the owners had no idea why the trans started having issues/failing once they put the lift kit/big tires on the vehicles.

 

[swathdiver]

Better acceleration at lower rpms (and corresponding better fuel economy) and, of course, better line pressure rise inside the transmission for applied elements, particularly the direct and overdrive clutch packs (for all transmissions). Lower RPMs are fine for HWY fuel economy considerations as long as you're not putting the trans at risk of having insufficient line rise due to insufficient mechanical leverage in the final drive...I've seen quite a few transmissions over the years out of lifted trucks where the owners had no idea why the trans started having issues/failing once they put the lift kit/big tires on the vehicles.

So I reckon you would caution against restoring the rpms as built, akin to 4.10 gears in this example? Not enough pressure still?

 

[NickTransmissions]

So I reckon you would caution against restoring the rpms as built, akin to 4.10 gears in this example? Not enough pressure still?

I used to have a '76 K5 blazer w/373 gears, TH350 and 35" tires and it was an absolute dog. Zero acceleration, took forever to get moving (had a 350 engine). I installed 4.56 gears in the rear (removed the front drive shaft prior to) as I wanted to see what sort of difference it would make before doing the front diff. It made a subtle difference, acceleration improved but not dramatically. I then installed 4.88s and it made a much more notable difference, woke that little 350 up and it made for a much more enjoyable driving experience, especially on the trails. I then put 4.88s in the front diff. I sold that thing as fuel mileage wasn't measured in miles per gallon, it was measured in gallons per mile and was a rust bucket. But the gears made a big difference.

As long as the vehicle is in the 'green' with the selected gear ratio per those gear ratio charts, it will be fine but for 35s in a 5-6k lb vehicle, I'd go 4.56 at minimum.