[suzieque]

I have a 1995 YJ with stock manual transmission, stock 4.0l, 3.07 differential gears and 33 inch tires.
The YJ seems fine driving around town, lots of giddy up and go. Level highway driving is fine too at 55mph In 4th gear, I never seem to use 5th gear.

Going on mild incline on highway slows it down and I can downshift to 3rd to get the rpm and speed back up. I’d like it to perform better on steep incline and every post seems to say that lower gears are needed, but i can already downshift so I’m not understanding why lower gears make a difference.

Can someone explain why regearing the differential fixes the problem that downshifting does not?

 

[RockWoRM]

By running 33” tires, you’re putting more stress on the clutch and engine than stock tires. There’s also loss of power because of this. If you were to, say, regear to 4.10/11, that would get you back into stock ranges and you’d likely get 5th back.
Then there is the whole issue of re-gearing a turdy 5… but thar be a whole nuther subject. 🙃

 

[JoonHoss]

In the most simple way, lower gearing applies the power quicker, and or takes less throttle input to do so.

You are definitely having to apply a lot of throttle input to get moving with 33s and 3.07s.

If you had a compound low 1st gear, you may not notice it as much, but you don't.

In essence, you are only using the first 3.5 gears in your trans, instead of all 5.

Lower gears will require less throttle input (power) to get the same results, leaving you with more power at the top end of each gear.

There's also the optimal power range for your engine (spoiler alert, you're not in it)

You absolutely want to regear

Hoss

 

[grandboost98]

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

You can use this site to enter the stock jeeps info vs what you have now and play around with the gear ratio to see what will bring you back into the stock power range

 

[ChiliPalmer]

Generally speaking 4.10 gears are used for 31's, 4.56 gears are used for 33", 4.88 for 35's, 5.13 for 37's, etc.

 

[Factoryhack]

I have a 1995 YJ with stock manual transmission, stock 4.0l, 3.07 differential gears and 33 inch tires.
The YJ seems fine driving around town, lots of giddy up and go. Level highway driving is fine too at 55mph In 4th gear, I never seem to use 5th gear.

Going on mild incline on highway slows it down and I can downshift to 3rd to get the rpm and speed back up. I’d like it to perform better on steep incline and every post seems to say that lower gears are needed, but i can already downshift so I’m not understanding why lower gears make a difference.

Can someone explain why regearing the differential fixes the problem that downshifting does not?

Counterpoint:

Down shifting to a lower gear and leaving your 3.07 differential alone is perfectly fine for normal street driving. A 3.07 ring gear is tall for 33's but not necessarily a big issue if you're just driving around town casually.

Where optimizing your gearing for 33 inch tires is much more important is offroading. Keep in mind most of the people in the forum are off-road people, need the gearing for low speed rock crawling, steep trails, etc., and will have strong opinions about the "correct" gearing for your tire diameter.

The answer really depends on how you use your Jeep.

You need to also consider the MPG penalty of a shorter 4.10+ gear vs. your 3.07. There's always an MPG trade-off and an expense when you go with short gears. Is it worth it?

Depends.

 

[fishadventure]

Generally speaking 4.10 gears are used for 31's, 4.56 gears are used for 33", 4.88 for 35's, 5.13 for 37's, etc.

You are a bit off there imho. 4.10s are excellent with 33” and 32” tires. 3.07 sucks a bit with 31s. Sure, highway might feel kinder and gentler with 4.56:1 but at 65-70 there’s pretty decent power and gas mileage with 4.10s. And you can buy a D30 and 8.8 already geared fairly inexpensively...

 

[JeeperDon]

It's all in the math. Your Jeep came with 3.07 gears and 28" tires (215/75r15). Chrysler chose that combination to make the motor happy. Those 33" tires are 18% larger, making the motor 18% less happy. That's not counting the greater mass of the larger tires, which are harder to accelerate than light weight 28" tires. The AX-15 trans is 1:1 in 4th gear and I believe 0.78 in 5th, which is 22% different. So as you found out, 4th is new new 5th.

Should you want 5th gear back, the gear ratio number to compensate should also be 18% larger, which is 3.62. The nearest ratio number available for a stock YJ D30 and D35 is 3.73.

 

[JoonHoss]

Counterpoint:

You need to also consider the MPG penalty of a shorter 4.10+ gear vs. your 3.07. There's always an MPG trade-off and an expense when you go with short gears. Is it worth it?

Depends.

Thats true ish, but in actuality, keeping your motor in the 'correct' rpm range can provide better mileage than lugging it along in the incorrect gear, and you have to side step the clutch and use more gas to get it rolling. applying your logic people should run 4 speeds instead of 5 speeds with overdrive to get better mileage

Also, Your assumptions about down shifting for 5th gear totally ignores the issue in the first four gears

Hoss

 

[ChiliPalmer]

I concur with both accounts and agree that 3.73 are mathematically the closest option; however, when adding other things - the aforementioned weight of the tires, off-road gear, camping gear, tools/spares, etc. and regardless if you're on stock tires or bigger tires, you should probably be over-geared.

An example is my current DD on 30's (1998 XJ - my 5th XJ) I will put 4.10's in it because I have a battery, fridge, all the tools I need, a temporary water solution (15 gallons) and an RTT. I will need the 4.10's on 30's. If I ever build a trailer (I already have the axle) I will move to 32's and 4.56's for the same reason - I want/need to be over-geared.

In the end it is up to what the owner wants/needs based on his/her setup.

 

[Factoryhack]

Thats true ish, but in actuality, keeping your motor in the 'correct' rpm range can provide better mileage than lugging it along in the incorrect gear, and you have to side step the clutch and use more gas to get it rolling. applying your logic people should run 4 speeds instead of 5 speeds with overdrive to get better mileage

Also, Your assumptions about down shifting for 5th gear totally ignores the issue in the first four gears

Hoss

Well, sure. While I understand and agree with what you're saying in general terms, my point was more along the lines of "Is it worth the time and expense to change out two ring gears for this guy's intended usage?"

Would it be worth it? Dunno exactly, but it seems like he's just cruising around and not necessarily looking to spend the funds to have a more ideal factory intended RPM range with 33" tires, (which would probably be 3.73's), when he can just use a lower gear at a given MPH, while also acknowledging 5th is basically unusable with a 33" tire and a 3.07.

Heck, It'd likely be quicker / easier to just go back to a stockish 28" tire, pick up the correct RPM ranges, and improve his MPG's by having a lighter wheel/tire combo, no?

Anyway, I do get that there is a sweet spot for final drive ratio with larger diameter tires. You have to compensate for percentage of the diameter increase and the increased weight of the wheel/tire, which will somewhat trash your MPG's.

Of course, MPG's shouldn't be a big priority if you're using a Wrangler for your daily driver, right?

 

[JoonHoss]

Full disclosure; I owned 3 DIFFERENT YJs with the 4 cyl/5 speed combo. Two had 32s, one 33s. Both had the stock 4.10 gears, and all were underpowered due the the oversize tires and stock 4 cyl gearing. 2 of the 3 were also DD for me (and my wife)

I know full well all of the driving/drivability issues related to having an under-geared YJ.

Yes, in theory you just lose use of 5th gear on anything other than level ground with no head wind. In actuality, the You lose much more performance, mileage, and enjoyment across the entire spectrum, in every gear, and at every speed.

Sure, you can remedy that by going back to smaller tires, or you can remedy that by keeping the larger, cooler looking tires and simply change the gears to the correct range for the application.

I think another important note is that 3.07 gears are sub par even in stock form. So it's not like he's a little off the ideal gear, he was off to begin with, and then exacerbated it by going up in tire size. Realistically, he could've regeared to 3.73s on the stock tires and it would have been beneficial and enjoyable, and he should probably go to 4.10/4.56s now...

Hoss

 

[Factoryhack]

Full disclosure; I owned 3 DIFFERENT YJs with the 4 cyl/5 speed combo. Two had 32s, one 33s. Both had the stock 4.10 gears, and all were underpowered due the the oversize tires and stock 4 cyl gearing. 2 of the 3 were also DD for me (and my wife)

I know full well all of the driving/drivability issues related to having an under-geared YJ.

Yes, in theory you just lose use of 5th gear on anything other than level ground with no head wind. In actuality, the You lose much more performance, mileage, and enjoyment across the entire spectrum, in every gear, and at every speed.

Sure, you can remedy that by going back to smaller tires, or you can remedy that by keeping the larger, cooler looking tires and simply change the gears to the correct range for the application.

I think another important note is that 3.07 gears are sub par even in stock form. So it's not like he's a little off the ideal gear, he was off to begin with, and then exacerbated it by going up in tire size. Realistically, he could've regeared to 3.73s on the stock tires and it would have been beneficial and enjoyable, and he should probably go to 4.10/4.56s now...

Hoss

As an aside, I’m finding this is a really good forum compared to others I’ve been involved in. Being barely three weeks in, it seems there’s a lot of good wisdom floating around sans the usual snark or insults.

I suspected Jeep people would be genuine and helpful. I wasn’t wrong.

 

[Goodoldjeep]

It's all in the math. Your Jeep came with 3.07 gears and 28" tires (215/75r15). Chrysler chose that combination to make the motor happy. Those 33" tires are 18% larger, making the motor 18% less happy. That's not counting the greater mass of the larger tires, which are harder to accelerate than light weight 28" tires. The AX-15 trans is 1:1 in 4th gear and I believe 0.78 in 5th, which is 22% different. So as you found out, 4th is new new 5th.

Should you want 5th gear back, the gear ratio number to compensate should also be 18% larger, which is 3.62. The nearest ratio number available for a stock YJ D30 and D35 is 3.73.

I agree, 3.73 is the way to go for stock performance

 

[fishadventure]

actuality, the You lose much more performance, mileage, and enjoyment across the entire spectrum, in every gear, and at every speed

I have some 3.73 gears somewhere for the 8.8
And a 3.73 D30. I put a 4.0 in my 89, running 4.10s. Eventually I bought 4.10s for the spare D30 because whenever I get around to rebuilding it I have discovered with 33s that even 4.10 in hilly Vermont is a bit tall especially towing, and the worst hills on the highway needed 4th gear now and again anyways.

Gearing at 4.10 gives me a nice middle of powerband at 65mph and decent gas mileage - just about the same as my 87 4.0 on 31s. In fact, the 87 (3.07 gears) gets the same mileage it did with 205-75x15, but hills want 4th gear more often.

I think now 3.73 would be counterproductive- other than flat-state highway running, the gas mileage is pretty good with my ‘89 (for a jeep). Plus, first gear is decent, but at 65mph you’d be fueling 3000rpm+ for nothing but a bit of pedal feel. “Pedal feel” is the illusion of needing acceleration when with the immense torque of the 4.0 you can maintain 64-67mph by merely just stepping on the skinny, or occasionally downshifting to fourth in hilly country.
i think 4.56 would trade gas mileage for an illusory feeling of horsepower and ~5% sooner wearout on the motor (with 33s). 4.10s put you right in the mid-bottom rpm range of peak torque with a 4.0. 4.56 might be warranted with the 2.5
My opinions. YMMV

 

[ericscher]

33's ain't really that big, and if downshifting is getting the job done for you, then you're fine the way you are.

 

[riiz]

You could probably find a set of axles from a 4 cylinder, they either had 3.73 or 4.10 dependent if they were auto or manual. I got a shop near me that sells sets for 500ish, cheapest way to get 4.10 gearing.

 

[ChiliPalmer]

I have some 3.73 gears somewhere for the 8.8
And a 3.73 D30. I put a 4.0 in my 89, running 4.10s. Eventually I bought 4.10s for the spare D30 because whenever I get around to rebuilding it I have discovered with 33s that even 4.10 in hilly Vermont is a bit tall especially towing, and the worst hills on the highway needed 4th gear now and again anyways.

Here's a good example of why 4.10's and 33's don't go that well together, sure, flat road's are not an issue but the moment you add weight and big tires you are no longer in the "stock" range. If all you ever plan on doing is driving on flat ground in a bone stock Jeep then 3.73's or 4.10's would be fine but if you're adding weight or going off-road you will regret 4.10's and 33's.

Read some comments in the following thread, and pay attention to post #8, Jerry has been around for a LONG time and has more experience then a lot of people on here.

NO POWER IN 5TH GEAT

HELLO. I HAVE A 2003 TJ 4.0 5 SPEED , I RECENTLY UPGRADED IT WITH A 4IN LIFT AND 33 IN TIRES AND RIMS , COMPLETE EXUHUST FROM HEADERS BACK , WHILE DRIVING I HAVE GOOD POWER IN 1ST,2ND,3RD,4TH BUT WHEN I SHIFT TO 5TH GEAR THE SPEED DECLINES AND I HAVE VERY LITTLE RESPONSE FROM THE GAS PEDAL. THE...

www.jeepforum.com

Also, do a google search for "best gears for 33" tires on a Jeep TJ" and you will find overwhelming evidence that 4.10's are for 31's, 4.56 for 33 and 4.88 for 35's.

Good luck to the OP.

 

[Boojo35]

Jeeps have come stock with many different gear ratios and different tire sizes over time. If you have had the opportunity to drive your Jeep in the unmolested state with stock gearing a tire size and are happy with the performance it is easy to decide on the correct gear for you. Run an RPM calculator and put it back into the same RPMs at the same speed as closely as possible. Like so close that you do not have to re-calibrate the speedo.

On a 4.0L I agree that 4:10s are 31s. 4:56s are 33s. 4:88s or 5:13s are 35s. That puts it close to a Jeep that came stock with 3:55s or 3:73s on stock tires. On a 4 cyl I would go higher.

I prefer to have a Jeep that is more fun when pressing on the skinny pedal.

i think 4.56 would trade gas mileage for an illusory feeling of horsepower and ~5% sooner wearout on the motor (with 33s)

Loading an engine harder in lower RPMs causes more cylinder and piston wear than spinning it a little faster IMO. I get your idea but you have to look at the big picture and cylinder pressures and their effects on piston loading. It is highly debatable for sure. I think that there is a middle ground. This is the very reason why some strokers in a round about way have short life expectancies.

I have a 1995 YJ with stock manual transmission, stock 4.0l, 3.07 differential gears and 33 inch tires.
The YJ seems fine driving around town, lots of giddy up and go.

I KNOW that if I drove your Jeep I would say it has NO giddy up and go. 33s on 3:07 gears would be a complete pig to me. It just would. I have driven a lot of Jeeps as a Jeep mechanic. Some are way more fun than others. Your gearing and tire size is no fun. Your gearing and tire size are for sure hurting your performance and your fuel economy.

You are only using 4 gears. It hurts your economy. The easiest way for me to say this in a few words is that vehicles went from 3 and 4 speeds to 5 speeds with OD. The onto 6, 8, 9 and even 10 speeds with OD. The point is, the closer the gear ratios of the trans, the more gears that it has, and the more time you spend in the optimal power band, the better.



It is all subjective and there are going to be numerous opinions.

I know that I said all of this. My new to me JKU has 3:21 gears. That number says pig all over it. It performs well. I am happy with it. In fact, I LOVE IT!

As far as driving a brick on the highway at 80ish plus, you are not going to get good economy. It is not aerodynamic at all and the wind drag on a vehicle goes up exponentially over about 50 MPH no matter what you drive. I climbs more exponentially in stuff that has flat glass in the front. (JEEPS)

 

[fishadventure]

If all you ever plan on doing is driving on flat ground in a bone stock Jeep then 3.73's or 4.10's would be fine but if you're adding weight or going off-road you will regret 4.10's and 33's.

YMMV
i live in Vermont. I wheel. I DD a 1989 jeep. With 33” tires. We have A LOT of hills.
I don’t regret 4.10s in the least, not dissing Mr Bransford in the slightest fashion, either.

Do what you want. I did. I’m happy.