Quick question about TJ Tire size vs. Gear Ratio.

[mingez]

I'm still feeling my way through the TJ. Now, I know this has been done to death, but please post what gear ratio/tire size you re running on you SWB Jeep, and whether or not it's a trailer queen, part time trail rig, or full time city.

I'm currently at 4.10 and 33's, but I'm looking to gear down and want to keep it highway drivable. I have some 4.88's ready to install (from the debacle some of you may remember)....but now I'm wondering if it's too much for the TJ. Any of you have experience with 4.88's and freeway driving?

Auto Transmission BTW.

Any thoughts are greatly appreciated!

 

[Junkpile]

RE: Concrete clean up time... need help!

I ran 4.56:1 in a CJ with 33s. It was okay with the low-mid range power of the 258, but I would run 4.10s if I was doing it again. The 4.0 needs a little more gear because it doesn't quite pull as hard at lower rpms. I'd say stuff the 4.88s in there. It won't be bad. If you think it's a little too much after you do it, you can use it as an excuse to slap on a little BL and some 35s......


I'll be running 4.56s in my auto 4.0 TJ after the 33s go on.

 

[berettajeep]

RE: Re: RE: so if i add..

Have the stock 3.07's with 31's on my g/f's TJ 4.0/5 speed. It sucks. I have 3.54's ready to go in just to bring it back to stock performance, although I wish I would have gotten 4.11's instead but its not my jeep!

 

[mingez]

If you think it's a little too much after you do it, you can use it as an excuse to slap on a little BL and some 35s......

That was my intention, but inspite of the large tire size and weight compensating for lower (not taller) gears, I'm concerned about high RPM's at high speeds....say 70-75 mph. Am I nuts? (about this) :wink:

I need to be able to drive 3 hours to Albuquerque with my 4.88's without fear of damage... I may have to trade them in for some 4.56's.

 

[Inspector-Gadget]

Hi-Ya, Ian,

Mingez wrote:

I'm concerned about high RPM's at high speeds....say 70-75 mph. Am I nuts? (about this)

I need to be able to drive 3 hours to Albuquerque with my 4.88's without fear of damage...

Ian, I slapped together two spreadsheets to give you estimated engine RPM with the three gear sets and two tire sizes mentioned so far in this thread. The first table shows engine RPM for the three gearsets and 33-inch tires; the second table shows engine RPM for the three gearsets and 35-inch tires.

The table entering arguments are MPH (Top row - 60, 65, 70 or 75 MPH, and the gear set (leftmost column - 4.10, 4.56 or 4.88.) The engine RPM, is at the intersection of the table entering arguments.

Do not be overly impressed with the apparent precision of the tables. I used the calculated circumference of the tires (pi times tire diameter) in the table calculations. The "rolling" tire circumference is less. The tires will be squashed a bit on the bottom when supporting the weight of the Jeep; therefore, the "real-life" tire circumference will be truncated.

Like this:

That truncation has the practical effect of raising engine RPM slightly above that shown in the tables for a given MPH and gearset. On the other hand, that "squashing" error is partially canceled by the fact that tires "grow" slightly in diameter (and consequently, in circumference) at high speeds.

There is also a driveline slippage error, but that error is negligible with modern automatic transmissions.

As a practical matter, ignore the mentioned errors and a couple more unmentioned errors. Take the engine RPMs as shown in the tables as ballpark estimates.

Now, with the above info as background -- to deal with your question. I would be uncomfortable cruising for hours at such high engine RPMs, but you will be driving, it is your Jeep - and your decision.

Regards,

Gadget

 

[Bounty__Hunter]

What engine do you have? That determines a lot as the 4banger spins higher easier, and likes it more than the 4.0L engine.

What was the stock gear ratio your jeep came with?

Gear calculators are a good help, and I usually error on the side of being a little lower geared (higher numerically) to help compensate for the added rotating mass of the larger tires.

 

[Snitty]

Re: RE: Concrete clean up time... need help!

i wasn't gonna have anything to say in this thread really.. but i have to give gadget some admiration for taking the time for the explaination and diagrams

 

[Bounty__Hunter]

RE: front locker installed

That rpm range is perfectly acceptable IMO, if you're driving a 4banger. It's a little high for the 4.0L to maintain for an extended period.

Remember that if you want acceleration to improve or to return to where it was at stock, you'll have to go lower than what the charts show as it doesn't take into account the added weight of the tires, only the increase in height.

 

[TwistedCU]

Definately 4.88's.

 

[southtj]

Gadget, you never cease to impress me.

 

[Inspector-Gadget]

Bounty, I believe that Ian's TJ is a 6.

At least, according to this old thread.

http://www.jeepz.com/index.php?name...ic&t=324393&postdays=0&postorder=asc&start=15

Hence, my reservations as regards the high engine RPMs at highway speeds.

For the sake of comparison, my 4.0 Liter Grand Cherokee with automatic tachs at approximately 2100 with overdrive engaged at 60 MPH and approximately 2400 at 60 MPH with the overdrive disabled. I don't believe the automatic in Ian's TJ will have the overdrive feature. It it does, subtract roughly 300 RPM everywhere in the above two tables.

Regards,

Gadget

PS: Snitty and South442: Thank you

 

[mingez]

Thanks guys...

I don't know what the stock gears were, as I bought the TJ with 4.10's installed.

Gadget...WOW! Thanks for that wonderful explanation. I was leaning toward 35's with the 4.88's.

Bounty, it's a 4.0...so do you think that 3500 rpm for an extended period is too much?

 

[TwistedCU]

RE: Catch you all later on down he trail!

Bounty, it's a 4.0...so do you think that 3500 rpm for an extended period is too much?

You can expect approximately 500 rpm higher at 65 mph than you now run with the 4.10 gears with 33" tires. Take it out and see where you are now then let that help you decide.

I would think the 33's would be fine with 4.88's. It is a very common setup. If you want to eventually get a set of 35's then it's a no brainer.

Would it really be worth re-gearing from 4.10's to 4.56's?

What's the reason for wanting to re-gear now? Not enough power up hill? Is it a slug at highway speeds?

 

[mingez]

Re: RE: Quick question about TJ Tire size vs. Gear Ratio.

Would it really be worth re-gearing from 4.10's to 4.56's?

What's the reason for wanting to re-gear now? Not enough power up hill? Is it a slug at highway speeds?

Because I'm a very sick man. And because I have the gears sittin' on my workbench collecting dust. And...why not?

It has the power, but I know I'm gonna want to go 35's eventually. And I'm looking into a locker as well. <Sigh>

 

[Bounty__Hunter]

RE: Re: RE: Quick question about TJ Tire size vs. Gear Ratio

IMO a switch from 4.10 to 4.56 isn't enough of an improvement to warrant the time, trouble, and expense.

Great friend of mine has a TJ Sport ('00) with 35" claws, he installed 4.56 and wished he would have gone with the 4.88 gears. He also has the auto.

Install the 4.88 if you plan on 35" tires. If you're going to stay at 33", keep the 4.10 as it's not worth the trouble for the slight bump to 4.56 gears.

 

[Snitty]

RE: just thought this was a weird pic?

i dont' see a problem with 4.10 on 33" in the first place... if i went to 4.10, i'd be gearing down way too much.. but of course, i have a manual transmission... though, the auto high gear is 1:1... my high gear is overdrive... one would think that the 1:1 high gear would make you look for a mechanically higher final gear... it kinda seems backwards... but of course there are many different factors that are different in the transmissions

 

[Ritt]

Your 4.10s are excellent highway/around town gears. If you were off road all the time with occasional highway, I'd say 4.88 w/ 33s.

The other thing is, I don't think you want to run d35 axles with anything more than 33s. NOW, if you were going to 8.8 axles, I'd do the 4.56s or 4.88s with 35". With d35s............like puttin perfume on a pig. :wink:

Hold on, I missed the auto transmission part...........your 4.10s are like a 4.56 in a standard 5sp transmission. You mentioned 35s with lockers, you'll need to carry a golf bag full of axles with you (assuming d35). I'd stay with the 4.10s.

 

[TwistedCU]

RE: mo pics

Mingez I thought you bought that TJ for a daily driver :lol: 35s??!?!?!

Looks like the Wrangler has become Mingez' new passion
:wink: