Jeep Enthusiast Forums banner

Why are Taller Gears and Larger Tires Bad for Transmissions?

10K views 54 replies 19 participants last post by  Ironhead 
#1 ·
Why is running taller gears bad for your tranny when you get larger tires? When I had 3.73s and 33s I was told that my transmission was working harder and could wear out sooner which was one of the reasons why I upgraded to 4.88s. However, when I had the 3.73s I never used OD and my 3rd gear never got anywhere near full load (as confirmed with my scanguage), but now with my 4.88s and OD I'm constantly hitting full load - so is my transmission working even harder now?
 
#3 ·
The gears took strain off the engine and driveline especially under load (starts, towing, hill climbing, etc.), however the 4.88s are a little steep and cause you to tach out quicker than the 3.73s. You basically got a better crawl ratio, better low end grunt, and better mpg around town at a price of lower high end speed and hwy mpg.

I take it you think the strain is worse now that your rpms are above average on the stock tires and quite a bit higher than on the 33/3.73 combo? If you want to balance things out a bit go to a ratio in between or go larger tires, but do your research first!!!...wouldn't want for you to end up spending more money than necessary.

What rpm are you hitting in OD at 65?
 
#5 ·
I understand that the gears take the strain off of the driveline while under load - if my transmission downshifts to third, otherwise its at full load a lot of the time at highway speeds. My question is, "why do people say that you should regear to take strain off of your tranmission when you get larger tires?" I want to understand the reasoning behind that, not whether my choise of gear ratio was appropiate, or whether or not I should change it again.
 
#7 ·
Assuming the transmission has a lockup torque converter, it will be locked up in overdrive, but not in third. When it's not locked up, it slips. Slippage creates heat. Heat is hard on the transmission.

I'm sure it's also easier for you to take off from a stop now with less torque converter slippage and therefore less heat.
 
#9 ·
Assuming the transmission has a lockup torque converter, it will be locked up in overdrive, but not in third. When it's not locked up, it slips. Slippage creates heat. Heat is hard on the transmission.

I'm sure it's also easier for you to take off from a stop now with less torque converter slippage and therefore less heat.
Now were getting somewhere. So driving around all the time in third isnt a good idea then, and if I still had my original 3.73s gears then that would also be harder on the transmission when taking off, but how much wear and tear would that actually be? How many people have problems with their Transmission etc. from not regearing, and how long would it take to develop problems?
 
#8 ·
I think you heard someone talk about the mechanical force it takes to turn your 33" tires with 3.73's as supposed to say 4.10's, 4.56, etc......

Your driveline (Transmission, driveshaft, ujoints, etc.) are under much less mechanical stress when you have steeper gears installed. I know someone can explain this much better than I.
 
#13 ·
No, it requires the same amount of torque at the rear axle to accelerate at the same rate. That's why a vehicle with too high of an axle ratio accelerates more slowly. When geared too tall like when big tires are installed, less torque is available at the rear axle. The engine is straining because it is below its power band and producing less torque.
 
#14 ·
I respectfully disagree with you. It takes a given amount of torque at the wheel to accelerate a vehicle. A vehicle with 3:1 differential gears will require 25% more torque at the pinion than one with 4:1 gears in order to accelerate the vehicle at the same rate. The torque converter on an automatic transmission will allow the engine to reach maximum torque pretty quickly. But the taller gears will still be stressing the driveline more than the lower ones.
 
#15 ·
You're placing the cart before the horse. At the lower rpms the engine turns with the higher ratio gearing, the engine cannot put out that additional torque which is why it accelerates slower. If we could get engines to produce more torque by running higher ratios, dragsters and rock crawlers would be running 1:1 axle ratios.
 
#16 ·
You're still not getting it. Let's say it takes 1000lbs-ft. of torque (just a round number, not based on any real-world figures) to go up a given hill. With a 4:1 rear gear ratio, it will take 250lbs-ft. of torque at the pinion to produce the required 1000lbs-ft. at the wheel and move the Jeep. With a 2:1 rear gear ratio, it will take 500lbs-ft. of torque at the pinion to produce that same 1000lbs-ft. at the axle. Now how can you say the transmission and driveline are less stressed in the second example? :confused:
 
#17 ·
I understand what you're saying perfectly well and I have a good understanding of torque and its traits. First, you don't accelerate as fast after doing something to cause the ratio to be too high, like installing bigger tires. To increase the torque, you'd have to be acclerating as fast as you were before which with the too-high ratio, would be difficult. To generate the higher level of torque through the transmission, you'd have to be accelerating much harder than usual to overcome the gearing mismatch. And with the higher gearing (mismatch) you could never generate as much torque as you can with the proper gear ratio.
 
#20 ·
So your saying that even though the engine is working harder to move the bigger tires (which Im sure we all agree on), the trans still recognizes its shift points from the engine and still operates accordingly? I would think that the trans would become "confused" for lack of a better word and shift erratically, thus causing more internal heat and friction than normal. Am I totally off here?
 
#24 ·
Bottom line:

Taller gears (stock type gears) load both your engine and transmission more.

Deeper gears load on the engine and transmission less.

If it did not work this way, what would be the point of a longer lever to move heavy objects.

Taller gears require more throttle input to cause the same vehicle acceleration as deeper gears. This extra throttle input will have a direct heating affect in the transmission due to the fact torque converters are not 100% efficient.

Why is towing hard on transmissions? Because of many reasons really, but high throttle input through the transmission while accelerating slowly builds heat - similar to having the wrong gears.
 
#26 ·
tall gears and large tires ride the stall on the auto and causes excessive heat which slowly bakes em..

this is the reason for the bad rep of the gm th700r even running with stock tires and 3.42 gears and people towing with em in OD. regearing and a cooler solves the problem by making the OD useable without riding the stall or sliping the lockup clutch.

stalls are typically 1800-2200rpms for 6cyl's. so if yer running 1800 or under at 65 you are riding the stall and creating excessive heat every time ya accellerate and while pulling a slight grade the lock up drops off without downshifting.
basicly if you're geared at 1800rpm @65 and the stall is 2200 your really riding the stall and much better off for ecconomy to regear you cruise rpm above the stall rpm. that will also allow the convertor to effectively lock up and run 1-1 (same as a manual tranny's clutch/no torque multiplication) instead of searching for gears or locking and unlocking constantly. the 42rle does lock up in 3rd with the od on.

you can find your stall rpm by holding the brake in the driveway, the rpm ya can get it up to without moving is the stall rpm

I've been building autos for a little over 20 years, trannies that are properly geared don't get too hot as they do with tall gears. when I have rigs of any make come in with large tires and stock gears, I will not gaurantee it past the driveway. I've had way to many come back toasted within 3 months no matter how beefed it went out. diesels are especially bad whitout a very low stall convertor.
 
#48 · (Edited)
thats gotta be the worst advice possible for this thread. the guy is trying to learn. not give up and throw money at it. Jerry and one or two others have a very good lesson going here and input like that isnt even appropriate.

OP, take the lever example stated earlier. take a long lever to try to move a rock and itll move. easy concept right? but in the case of gears and axles the levers are your tires and your gears. your tires being a long lever and youre given the short end. the gears are like a lever that moves another lever helping you out.

Assuming the transmission has a lockup torque converter, it will be locked up in overdrive, but not in third...
that simply isnt true. with OD off the torque converter will lock up in 3rd not allowing any slip. lockup wont engage in 1st or 2nd though. but your theory is sound.
 
#31 · (Edited)
I gotta chime in on this one.....Your engine has a power band which accurs within a certain rpm.....it also has a definate amount of power it can develop.This is your limitation when using any engine.You must match the OVERALL gearing to the engine rpm and power available.

Let me make a overall statement:Torque is how much work the engine will do and HP is how fast it will do it.The engineers that choose which gearing to put in your jeep looked at a few things before choosing which gear ratios to use.

First the engines output T&HP,next the vehicles weight and and HOW FAST IT NEEDS ACCELERATE!With only so much T&HP available everything else must match the engine.It is impossible to create more T&HP at the wheels than the engine can make.

Changing gear ratios ONLY changes the TIME it takes to apply the power to the road!


If it takes 1 min to move your jeep at full T&HP up a hill with 1:1 gear ratio it will take 2 mins with a 2:1 ratio.This is how a loaded tractor trailer can get 80k lbs up a hill.It might take 5 mins but the work gets done.

The best you can ever do is find a gear ratio that puts the engine in its design T&HP band and matches the speed you want to accelerate and drive.As the size of the tire increases the effective gear ratio goes down.EVERTHING STILL TURNS AT THE SAME RATIOS but the vehicle MOVES FARTHER DOWN THE ROAD at the same rpm SO..........it takes longer to move and we need more T&HP to do it in the same time.This works as long as we are NOT at MAX t&HP available,we can just push harder on the accelerator pedal.BUT if we lower the internal gear ratio to match the OVERALL ratio we had before we increased tire size we can accelerate close to same rate keeping the engine in its best power band.


;)
 
#35 ·
If it takes 1 min to move your jeep at full T&HP up a hill with 1:1 gear ratio it will take 2 mins with a 2:1 ratio.This is how a loaded tractor trailer can get 80k lbs up a hill.It might take 5 mins but the work gets done.

;)
a semi is a bad comparison to a jeep, tho it is an excellent example of proper gearing...

2100 lbft. of torque, 37" tall tires with 3.08 gears... sounds unreal right? some regional long haul trucks run 3.31's that are in mountaineous regions and run in a low speed limit.

heres why it works with tall gears... my truck for example is a 550 c-16/18spd

theres a 300 rpm drop between each and every gear.... it keeps the engine in it peak torque range no matter what speed ya run. from .5mph to 100mph.
the peak torque number we are dealing with here is 2200 lbft. @1200rpm, 550hp @2100

not the 200 lbft. @2100 of a 4.0ltr. jeep

O yeah... there's not many lifted jeeps with big tires that I can't pass pulling a hill with the said 80,000# load from the apparent lack of proper gearing.. so.... who's the one taking 5 mins to pull the hill?

only in a diesel...

 
#32 ·
Now i will address the tire=trans question,the added T&HP we are adding to move the larger tires at the same acceletation and speed must be transfered from the engine through the transmission so of COURSE it will run hotter just as the engine and gears will run hotter moving more T&HP.

;)
 
#34 ·
I gotta add one more thing,no matter if you regear or not the larger the tire the more T&HP is will take to move.Regearing will only move the engine into a better power band,you will still need to use more power to move the larger tires in the same amount of time as you did the smaller tires.Thats why you ALWAYS get better fuel mileage with smaller tires.
 
#36 ·
Just goes to prove what i stated in the 1st place....the engine is what determines how a vehicle is geared overall.If you put the jeep engine in the tractor tralier it would need completely different gearing and move very slow.
 
#38 ·
very good,

which is the reason 5.38's, 33's tires and a auto with a OD ratio of .69 is a good combo...
 
#39 ·
another example is the foreign jobs.. like the honda passport and isuzu rodeo, trooper, they each have power plants with similiar power and torque as the jeep 4.0, yet they put 4.56 gears in em with 27" tires and a .75 OD ratio... dang an you'd think they don't know what they're doing when all us are sittin here saying 2200rpm is ok or to high...

hmm.
 
#41 · (Edited)
I don't claim to know the answer for sure, but I think it's a combination of things.

When regeared it would take less torque for engine and tranny to turn the driveline, which in turn means less TC slippage at startup, and you can now use OD on the freeway (no TC slippage in OD). Also, the engine can now run at its best, most efficient, and most powerful rpms during taking off from a dead stop and also on the freeway, thus saving some strain/lugging on the engine and saving some gas too.

I think the above is correct. Won't swear to it.

Lastly, if you have the auto tranny (like I do), its a sucky tranny IMO. My biggest complaints with it are the OD is too high. There is too much gap between 3rd and 4th, which makes it really easy to lug down in OD. Also, it shifts to OD at to low of an rpm. Lastly, the OD might be just to high geared period, though I get by using the OD button to manually shift from 3rd to 4th.

Hence your questions about how did the regear really help. Additional reasons why I think you wonder if it helps is that it only helps in some situations, but not all.

My answer is that it did help you a lot for taking off in 1st gear, and it may have helped you somewhat for cruising on level freeway a bit faster. For cruising uphill on the freeway it didn't help you a bit and probably hurt you (IMO) because now 3rd is to low for freeway hills and 4th is still to high, if your auto tranny is like mine.

I keep the OD turned off until I reach 2600 to 2700 rpms in 3rd (on level freeway), and then I manually upshift it to 4th by pushing the OD button.

Someday I'd like to install a tranny kit that makes it not upshift to 4th until at 2650 rpms in 3rd. Also, I'd like it to wind up the rpms a little higher in the other gears, before upshifting. That would cure a lot of problems that regearing helped, but regearing alone cannot cure. In the meanwhile, until I can afford a tranny kit, I'll continue manually upshifting from 3rd to 4th by use of OD button.

Anything I do can only hope to improve the situation. It will never be fully cured because the OD is to high geared for the 4L engine, even with stock tires, and moreso with my 33" large tires.

Would the high OD be nice with a 4.7L stroker, 33" tires, and 4.88 gears? Maybe. I think it might then be nice.
 
#42 ·
I don't claim to know the answer for sure, but I think it's a combination of things.

When regeared it would take less torque for engine and tranny to turn the driveline, which in turn means less TC slippage at startup, and you can now use OD on the freeway (no TC slippage in OD). Also, the engine can now run at its best, most efficient, and most powerful rpms during taking off from a dead stop and also on the freeway, thus saving some strain/lugging on the engine and saving some gas too.

I think the above is correct. Won't swear to it.

Lastly, if you have the auto tranny (like I do), its a sucky tranny IMO. My biggest complaints with it are the OD is too high. There is too much gap between 3rd and 4th, which makes it really easy to lug down in OD. Also, it shifts to OD at to low of an rpm. Lastly, the OD might be just to high geared period, though I get by using the OD button to manually shift from 3rd to 4th.

Hence your questions about how did the regear really help. Additional reasons why I think you wonder if it helps is that it only helps in some situations, but not all.

My answer is that it did help you a lot for taking off in 1st gear, and it may have helped you somewhat for cruising on level freeway a bit faster. For cruising uphill on the freeway it didn't help you a bit and probably hurt you (IMO) because now 3rd is to low for freeway hills and 4th is still to high, if your auto tranny is like mine.

I keep the OD turned off until I reach 2600 to 2700 rpms in 3rd (on level freeway), and then I manually upshift it to 4th by pushing the OD button.

Someday I'd like to install a tranny kit that makes it not upshift to 4th until at 2650 rpms in 3rd. Also, I'd like it to wind up the rpms a little higher in the other gears, before upshifting. That would cure a lot of problems that regearing helped, but regearing alone cannot cure. In the meanwhile, until I can afford a tranny kit, I'll continue manually upshifting from 3rd to 4th by use of OD button.

Anything I do can only hope to improve the situation. It will never be fully cured because the OD is to high geared for the 4L engine, even with stock tires, and moreso with my 33" large tires.

Would the high OD be nice with a 4.7L stroker, 33" tires, and 4.88 gears? Maybe. I think it might then be nice.
I was wondering about TC slippage in OD - I'm glad that you mentioned that. It would also be cool if I could adjust the shift point for OD, but I have no idea how that would be done. Your also right about the RPMs being to high for the passing gear and too low for the OD, but if I were to get my engine stroked or drop in a V8, then that would be sweet.
 
#53 ·
So I put 31 inch tall tires on my 96 grand cherokee 4.0 auto trans. seemed to make the tranny want to be in overdrive more than normal. Within two months, the tranny went. Metal shavings in the pan. New reman. trans from jasper, less than 60,000. I'm going back to stock tires and am sold on the fact that jeeps have horrible trannies!.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top