Gear ratios and bad advice

UpperMI

TJ Addict
Supporting Member
Jun 9, 2016
1,556
San Carlos, CA
This also brings me to mention a funny conversation I was involved in that could maybe be helpful to someone.

On Facebook, I was talking with a genius who asks questions and then likes to turn down the answers people give him. Every week or so, he posts a picture of this Eaton blower kit for $2699 and asks if anybody has experience with it. Nobody answers, so he usually "bumps" it, and finally someone might answer, usually telling him to just regear the Jeep.

He has a 3-speed auto, 4.0, on 33's. apparently, he's been racing corvettes since he was 10 years old (he's 31 now), so I guess he thinks that makes him the master of all things Jeep, engines included. I, and a few others, have recommended just regearing to 4.10 and calling it a day. If he still thinks the performance is lacking, then do the supercharger. He refuses, the rpm would be "way too high." I always ask the same question, "what about when the tres were stock, were they too high then?" He never answers it, he only claims that 33/4.10 will keep the rpm too high. Today, the conversation ended up with me blocked because I wouldn't shut up about gearing. Fine, it's his Jeep, I really don't care that much.

I do wish I weren't blocked though so I could see him drop 3 grand on a Super charger when he's still ultimately going to want to regear when he realizes a supercharger on 3.07s is the equivalent of lipstick on a pig.

3.07 generates a ton of heat in those transmissions and isn't good for it. I say regear first to put the engine back in its normal operating range, then supercharge it or stroke it or whatever to get it to the power that one's heart desires.
33's and 3.07 gears is a terrible combination. Especially in An auto. My first jeep TJ had 3.07 gears. When I bought my second TJ I made sure that it had 3.73 gears so I could run 245/75/16 or 31s and not worry about it. I tried to read as much as I could before buying the second TJ so I knew what to look for. Research and Education is a powerful thing if you apply it. I highly recommend it.
 

Bigbill

Member
Mar 11, 2017
39
Connecticut, United States
Years ago in our drag cars no matter what gear ratio we never were on the highway above 3,500 rpm for long periods of time. Keep your rpms 3,500 and under. Above 3,500 the main bearings wearout quickly. Caution.

Gearing vs tire height vs how much power the engine has. In a auto tranny what stall speed converter do you have. Everything plays a role in turning those big sneakers.

In my one ton 4x4 truck I had 34" tires with 4:10 gears but a 400+ hp sbc with a 2800 stall speed torque converter.(400th) no shortage on power.

Now on the other end I had a 80 cj7 304 v8 with 4:27-1 gears with 32" tires, four speed. This wasn't so great mileage wise. It never passed a gas station. But the power was there.

My '55 willys has 5:38-1 stock. 45 mph is max, the little four banger is slow but a stump puller.

My 350v8 four speed pickup k10 had 307 gears. I ran 31" 1100-15 Armstrong tru tracks.
No shortage of power.

My point is the engine hp, tranny setup, gearing, tire height all works together. I know the larger tires of today means big changes in the drive train to get it right and still be driveable with decent gas mileage.
 
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Jake Portillo

New Member
Apr 16, 2018
3
Corona, CA, USA
Ok, I know this is an old post, but there is a lot of good information and experience on here. I own a 2002 TJ 4.0 with a 3 speed auto. I bought it used from a private sale with 33” tires and I believe stock gearing in D30/D35 axles. (I know upgrade the axles), but I was thinking to just re-gear and run these until destroy them. My question is would it be ok to run 4.56 instead of the recommended 4.10 or split the difference and run 4.27. Sorry if this is a stupid question, but maybe someone could chime in on the pros and cons of running a 4.56 with 33’s. Thanks in advance
 

Rob5589

Certified video trained differential rebuilder
Supporting Member
Aug 29, 2016
6,028
W Sacramento, Kalifornia
Ok, I know this is an old post, but there is a lot of good information and experience on here. I own a 2002 TJ 4.0 with a 3 speed auto. I bought it used from a private sale with 33” tires and I believe stock gearing in D30/D35 axles. (I know upgrade the axles), but I was thinking to just re-gear and run these until destroy them. My question is would it be ok to run 4.56 instead of the recommended 4.10 or split the difference and run 4.27. Sorry if this is a stupid question, but maybe someone could chime in on the pros and cons of running a 4.56 with 33’s. Thanks in advance
You can but it will be pretty deep, especially on the highway. Somewhere in the 3300 rpm range at 70. If planning on 35's then yea, go 4.56. If staying on 33's go 4.10.
 
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Serbonze

TJ Neophyte
Supporting Member
Nov 12, 2015
1,747
Florida, USA
Ok, I know this is an old post, but there is a lot of good information and experience on here. I own a 2002 TJ 4.0 with a 3 speed auto. I bought it used from a private sale with 33” tires and I believe stock gearing in D30/D35 axles. (I know upgrade the axles), but I was thinking to just re-gear and run these until destroy them. My question is would it be ok to run 4.56 instead of the recommended 4.10 or split the difference and run 4.27. Sorry if this is a stupid question, but maybe someone could chime in on the pros and cons of running a 4.56 with 33’s. Thanks in advance
If this post has a lot of good information and it's generally accepted that 4.10 is the way to go with the 32rh and 33" tires, why do you want to forsake all of the previously done testing and advice to go with a different ratio?
 
Reactions: Jake Portillo

Jake Portillo

New Member
Apr 16, 2018
3
Corona, CA, USA
Thank you for the responses. My question was just to see what the pros/cons would be of running that low of a gear. I get that there is a lot of previous testing and a lot of knowledge. My point wasn’t to “forsake” any of the advice on here. It was merely to get more information for the setup I have. Beside, now I can find a ford 8.8 with 4.10 and just regear my front end.
 

Mike Jackson

Member
Jan 18, 2018
36
Bellevue, Michigan
My only question is how much will tires grow after building up heat. Say I have 33” that actually measures 32” when cold could it grow an inch when after building up heat.
 

Chris

Administrator
Staff Member
Sep 28, 2015
34,502
Salem, Oregon
Yep, 5.13 in that case. 5.38 would be better, but you can't do that since you don't have D44 axles front and rear like the Rubicons do.

5.13 will be as good as you can get, which will still be much better than stock!
 

tjc2019

New Member
Jan 31, 2019
4
georgia
Yep, 5.13 in that case. 5.38 would be better, but you can't do that since you don't have D44 axles front and rear like the Rubicons do.

5.13 will be as good as you can get, which will still be much better than stock!
I appreciate it, I’ve been debating between the two
 
Reactions: Chris

tjc2019

New Member
Jan 31, 2019
4
georgia
Another question, will I need a new carrier case? I’ve been looking and someone said that I’ll need a new Carrier case for the front Dana 30
 
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billiebob

TJ Expert
Oct 31, 2015
3,164
Kootenays, BC, Canada
Excellant video !!

He covered the need for performance rock crawling and the need for highway mileage in a daily driver.

Compared calculating 33s on an original 3.07 ratio...
Compared calculating 33s on an original 4.10 ratio.
And mentioned the plusses for each.

For those unaware, OD should never be the gear for performance, it is a weaker ratio than direct plus there are performance losses in OD since it is not direct.

One should also consider the history of OD. The 1970s gas crisis was when OD became a standard. And it was developed specifically to deliver better fuel economy. In the 1960s it was common to see performance ratios installed with no consideration for economy or quiet ride.

Today a lot of vehicles come with multi speed automatics where the top 3 gears are all OD. And it is common for those vehicles to hit top speed not in any of those top 3 OD gears but down 3 gears in the best performance ratio, direct. One of those is the Audi A8. With an 8 speed automatic, triple OD, it hits top speed of 150mph in 5th gear, the direct gear. In 6th, 7th, 8th it loses speed with every shift. But every shift improves fuel economy and makes the ride quieter, more luxurious.

My neighbours Tundra is the same, 5 speed automatic with double OD, 3rd is direct, he can go faster in 4th than 5th.

A race car does not have OD, top gear will be direct because that is the most efficient gear to transmit power.

If you want instant, responsive power with a 4.0L TJ, yes, gear for 3000rpm in whatever gear you cruise in. If you want better fuel economy, and a quieter cruise, gear for 2000rpm in OD at whatever speed you plan to cruise at. I aim for Interstate speeds of 70mph plus. AND I drop a gear or two gears if I want to pull out and pass.

Jeep forums are always top heavy with opinions to deliver dragster or crawler performance. But a lot of us are more interested in a practical daily driver. I'm the guy who talks about that experience. But I'm not saying if you want 4.88s with 33s you are wrong, I am saying you need to respect other opinions for different goals when a guy like me who has 4.10s with 33s, wishes for and recommends 3.73s with 33s to guys with similar goals to mine.

As to, an engine lugs at 2000rpm, it only lugs if you fail to shift as a hill appears and the load increases. IF you know how to drive, you will never lug an engine, all you need to do is anticipate the shift.

This was an excellant video. He gave a simple formula and showed how to use it, wether you are coming from a 3.07.... and want to keep that economy, or have 4.10s.... and want to keep that performance. Altho he never actually said it, he also chose the deeper ratio when adding tire size, weight. I wish he had said, when going to bigger tires you need to go a bit deeper than the formula to overcome that increased load.