The Truth about Gears and Tires

[JeepLab]

Reviving this old thread. Just about to order my 4.56 gears. Just to be sure, I did some more research to make sure I didn't want to go to 4.88's. I really don't do much highway driving above 70 Mph, so it almost made sense, but in the end I decided to just stick with 4.56.

I found the following web site incredibly helpful to making the decision: http://www.4ws.com/gear-ratio-rpm-calculator

It allowed me to drop in the most accurate gear ratios I could find for first-thru-sixth gear. I put in my measured tire dimension of my new 37" tires, and then compared that to my old 35" tires in terms of what I was feeling, and then just kept bopping between different gears to see where things landed. With my old 35" tires, I was moderately happy with my 3.73 gear ratio. I kept feeling that it could use a bump to 4.10, but overall I could handle 3.73. I see that with my 37" tires, at 4.56 gearing, I would be slightly higher in gearing than my 35" tires at 4.10 gearing, so that ended up being the deciding factor for me. I think that 4.88 would have just been too tall. I have found out that Chrysler seems to have changed sixth gear from the 0.83 gearing that is often quoted to 0.797, so if you use the link provided above, make sure you double check the numbers from the built-in "Pre-Filled" drop down list, as those numbers do not appear to be 100% accurate.

Here is what the tool was showing for me between the 4.56 and the 4.88 gearing.
4.56
Attachment 1556

4.88
Attachment 1557

what is going on with these attachments? and im sure you will be happy with 4.56.

[Timmy]

what is going on with these attachments? and im sure you will be happy with 4.56.

That's weird, I'm not sure what happened to attachments. I just re-uploaded them and they came in this time. Go figure...

I will say, I'm still slightly on the fence about the 4.88's. I rarely drive about 65 Mph, as there is just not that much around me where the speed limits are 70Mph+. Also, there is a chance I may tow some things. I think the *safe* answer is the 4.56, as I trust JL on his experience here, but I am itching for the 4.88's if it will help my low range since I don't have the Rubicon transfer case ;-(

[dalearyous]

I wanted to chime in here. I have 2013 JKUR with 35s (nitto trail grapplers that measure very close to 35s) and 6 speed. I recently re geared from stock 4.10s to 5.13s. I was extremely anxious and nervous about going with such an aggressive gear. But 37s are in my future so I wanted to be ready. I agree with initial post that there is a lot of garbage on the Internet about gears and the chart is over analyzed.

I wanted to say (from my experience only, have not driven 4.88 auto) that I am extremely happy with 5.13s. 1st gear is still useful and I start from it. Sure I have to shift a bit more but the 6 speed obviously lets me choose when. The highway is now enjoyable. I can stay and cruise in 6th gear and accelerate uphill at 70+ miles and all under 3k RPMs. If I couldn't see the needle I would think I was cruising at stock RPMs.

My two ignorant cents. I love my Yukon 5.13s. I don't have SC or TC.

[JeepLab]

I wanted to chime in here. I have 2013 JKUR with 35s (nitto trail grapplers that measure very close to 35s) and 6 speed. I recently re geared from stock 4.10s to 5.13s. I was extremely anxious and nervous about going with such an aggressive gear. But 37s are in my future so I wanted to be ready. I agree with initial post that there is a lot of garbage on the Internet about gears and the chart is over analyzed.

I wanted to say (from my experience only, have not driven 4.88 auto) that I am extremely happy with 5.13s. 1st gear is still useful and I start from it. Sure I have to shift a bit more but the 6 speed obviously lets me choose when. The highway is now enjoyable. I can stay and cruise in 6th gear and accelerate uphill at 70+ miles and all under 3k RPMs. If I couldn't see the needle I would think I was cruising at stock RPMs.

My two ignorant cents. I love my Yukon 5.13s. I don't have SC or TC.

not ignorant, but id ask, what is the RPM you run at when driving 70mph?

is your tach and speedo corrected for the gears?

[dalearyous]

i guess i should have said inexperienced instead of ignorant. i only know what i have and i have not driven anything else other than stock rubicons and stock sports.

i have the superchips flashpaq 3872 and have tried to dial in the tach and speedometer as close as possible. i have done the unscientific verification with my wife following me as well as tried to measure distance. basically, at 70 mph im running rite around 2700 rpms. obviously resistance will change that a bit. wind against me its a little higher, wind behind me its a little less. 75mph runs around 2800 ish. 80mph is like right at the 3000 rpm mark. i know the RPMs seem high but the jeep seems to be perfectly fine at those RPMs. like i said, i don't feel like i am taxing it more than lets say 2700 RPMs. i think with 37s it will be perfect which is what i was going for. i am in virginia where the highways are 70 mph speed limit so i go 75 and happy to cruise there.

now if i had your supercharger i would probably be singing a different tune. but most people don't have a super charger so they post about what they have, HOPEFULLY. but yes to your original post, i would completely throw away someone's post about gears when their signature states they are stock or running stock gears on lift and bigger tires. you just don't know until you do it and i think that is the fear and why there are so many threads about it.

btw, snarf and i have had a LOT of conversations on the wrangler forum. seems like a solid guy.

[JeepLab]

i guess i should have said inexperienced instead of ignorant. i only know what i have and i have not driven anything else other than stock rubicons and stock sports.

i have the superchips flashpaq 3872 and have tried to dial in the tach and speedometer as close as possible. i have done the unscientific verification with my wife following me as well as tried to measure distance. basically, at 70 mph im running rite around 2700 rpms. obviously resistance will change that a bit. wind against me its a little higher, wind behind me its a little less. 75mph runs around 2800 ish. 80mph is like right at the 3000 rpm mark. i know the RPMs seem high but the jeep seems to be perfectly fine at those RPMs. like i said, i don't feel like i am taxing it more than lets say 2700 RPMs. i think with 37s it will be perfect which is what i was going for. i am in virginia where the highways are 70 mph speed limit so i go 75 and happy to cruise there.

now if i had your supercharger i would probably be singing a different tune. but most people don't have a super charger so they post about what they have, HOPEFULLY. but yes to your original post, i would completely throw away someone's post about gears when their signature states they are stock or running stock gears on lift and bigger tires. you just don't know until you do it and i think that is the fear and why there are so many threads about it.

btw, snarf and i have had a LOT of conversations on the wrangler forum. seems like a solid guy.

Snarf is more technical than I am. where Im inches, he is millimeters. However, My brain cannot comprehend you having 5.13, AND 35s (which are really less) and still having a comfortable highway cruise. Something is off somewhere. With 4.88s and 37s, I struggled to maintain 75mph.

Thats why I asked if your tach and odometer were calibrated correctly.

I drove for a loooong time with them out of wack and never knew it. Till i tested with a 2 hour drive.

[dalearyous]

snarf helped me with my metalcloak bumpers and suspension when i was making my decision.

i can take a video if you would like. i was thinking about doing one anyway because of all the questions i have gotten with 5.13s.

"comfortable highway cruise" - whats not comfortable to you? we could have a very different frame of reference, what are you comparing it to?

oh also how does it compare to snarf's experience? he went with 4.88s

*edit*

With 4.88s and 37s, I struggled to maintain 75mph.

the way my local shop explained it to me is that they told me to ignore the garbage on the internet and just go with what it feels like (these guys have been 4x4 racing for 15 years). they said 4.88s with 37s (what i will have eventually) is going to feel like stock rubicon with 4.10s and 31s or whatever is on them. if you want to get back to stock feel with 37s, you go 4.88s. if you want something that feels better, go with 5.13s and 37s. this was tough for me, but he laughed and said let your engineer/comp science brain go for a minute and just trust me. if you want it to feel better than stock on 37s, go 5.13s. so i did on 35s for now.

again, more horsepower changes a lot so i have no idea what a turbo or super charger changes but going with "the feel" feels good right now. i would probably never go 5.13s if i had an LS for example.

[Timmy]

With 4.88s and 37s, I struggled to maintain 75mph.

Hey Ross, you've mentioned this a few times now and I was curious... Why do you feel it struggled to maintain 75mph with 4.88's? I'm guessing at 75mph you were around 2900 RPM in sixth gear? (based on that tool I published and punching in your measured tire size.) I'm very surprised that at 2900 RPM it was struggling at 75 Mph? I actually would have thought the exact opposite would happen with the 4.56 gears? 4.56 Gears appears to be hitting 75 Mph at 2700 RPM so I would have thought it would be more of a struggle say if you hit an uphill section? I mean, technically, isn't this like saying you couldn't hit 75Mph by downshifting to 5th because the RPM's went to 3200?

Not doubting your personal experience as I fully trust you when you say 4.88's were not as good as 4.56's on the highway, but I'm wondering if you could elaborate more on why you feel the 4.88's struggled so much?