The Truth about Gears and Tires

[Timmy]

Hey guys, I'm ordering my 4.56 gears today and thought I'd post at how I came to my decision on the gear ratio I selected in the event that it helps someone else out (lookin' at you Pickles, for when you finally upgrade to 37" tires, ba hahaha. See Pickles Tire Decision thread for the background here.)

My Jeep: 2014 JKU Sahara, 3.6L manual 6spd, 37" tires. Only future upgrade is a new front bumper and winch (around 200lbs. more weight) and possibly a supercharger if things work out.

First, I'd like to thank some of the forum people here for allowing me to chat with them. I spoke with FerrariFast (FF) and jt1911, who was kind enough to chat with me by phone. Both have rigs with 37" tires, but both were Autos running 4.88 gearing versus my 6spd. Both of the guys love their 4.88 gearing and are recommending I go that direction versus the 4.56 gearing that JL recommends.

What I have been experiencing overall is that the majority of people making recommendations are running auto transmissions and then broadly indicating their experience should translate just fine for people with 6spd's. Then I keep reading and re-reading what JeepLab has said and experienced, and putting much more weight behind his comments because he is running a manual as well. The reality is, the 6spd's do have a different experience because the gearing is different. It's too easy to focus in on the fact that sixth gear in the manual is close to the fifth gear in the auto and call it good from there. Truth be told, the 6spd has steeper gears from 1st-4th, 5th gear is equal to the auto's 4th, and 6th gear is longer than the auto's 5th gear. If I were to match the axle gearing of someone running an auto, I'd find my 1st-4th gears to be shifting much sooner than they are, all in an effort to say that my overdrive gear is now matching that of an auto transmission. To help demonstrate this, here is part of my discussion with FF. He very kindly sent me the following showing when his Jeep was shifting on his way to 60 Mph.

Auto Transmission...

1st gear shifts around 18-20 MPH
2nd gear shifts around 35 MPH
3rd gear shifts around 47 MPH
60 MPH cruising at 2250 RPMs.

I went ahead and whipped up a formula to calculate what speed my manual 6spd would be at if I shifted at the same RPM as FF with 4.88 gearing and then with 4.56 gearing. (Sorry the formula is not more simplified, it is written this way to dump into my programmable calculator.) His auto is shifting around the same time I personally shift (obviously on a 6spd, shift point is unique per person but I do find in every day driving I typically shift around 3000 to 3200 RPM naturally, which matches what his Jeep was doing.)

Formula

MPH = (((([RPM/({Trans Gear Ratio}*{Axle Ratio}))*{Circumference of tire in inches})/12[inches])/5280[feet])*60[minutes]

Example

What speed would I be traveling when at 2250 RPM, in 6th gear, with a 4.88 axle on 37" tires (36" measured)?

61.95 Mph = ((((2250/(.797*4.88))*113.09)/12)/5280)*60

All calculated with 37" tires, accept the stock Rubicon which is calculated with stock tires.
1st Gear @ 3100 RPM (FF's shift point to 2nd gear)

• 4.88 Axle, Auto [3.59:1] - 19 Mph
• 4.88 Axle, Manual [4.459:1] - 15 Mph
• 4.56 Axle, Manual [4.459:1] - 16 Mph
• 4.10 Axle, Auto Stock Rubicon [3.59:1] - 19 Mph

2nd Gear @ 3500 RPM (FF's shift point to 3rd gear)

• 4.88 Axle, Auto [2.19:1] - 35 Mph
• 4.88 Axle, Manual [2.614:1] - 27 Mph
• 4.56 Axle, Manual [2.614:1] - 29 Mph
• 4.10 Axle, Auto Stock Rubicon [2.19:1] - 36 Mph

3rd Gear @ 3000 RPM (FF's shift point to 4th gear)

• 4.88 Axle, Auto [1.41:1] - 47 Mph
• 4.88 Axle, Manual [1.723:1] - 38 Mph
• 4.56 Axle, Manual [1.723:1] - 41 Mph
• 4.10 Axle, Auto Stock Rubicon [1.41:1] - 48 Mph

4th Gear @ 2735 RPM (FF didn't provide this, I'm guessing it held 4th to 60 Mph and then it switched to 5th gear once he lifted.)

• 4.88 Axle, Auto [1.0:1] - 60 Mph
• 4.88 Axle, Manual [1.2:1] - 50 Mph
• 4.56 Axle, Manual [1.2:1] - 54 Mph
• 4.10 Axle, Auto Stock Rubicon [1.0:1] - 62 Mph

5th Gear @ 2250 RPM (FF's recorded cruise RPM. I also would have held 4th to 60Mph, but I'd skip 5th and go directly to 6th.)

• 4.88 Axle, Auto [0.83:1] - 60 Mph
• 4.88 Axle, Manual [1.0:1] - 49 Mph
• 4.56 Axle, Manual [1.0:1] - 53 Mph
• 4.10 Axle, Auto Stock Rubicon [0.83:1] - 61 Mph

6th Gear @ 60 Mph (This shows what RPM I would be at in the manual's overdrive gear at 60 Mph, ie. MPG savings!)

• 4.88 Axle, Manual [0.797:1] - 2180 RPM (vs. 2250 RPM of auto)
• 4.56 Axle, Manual [0.797:1] - 2037 RPM (vs. 2250 RPM of auto)

Just for comparison, here is how fast the 6spd would have been going if I ran at FF's cruising RPM.
6th Gear @ 2250 RPM

• 4.88 Axle, Manual [0.797:1] - 62 Mph (vs. 60 Mph of auto)
• 4.56 Axle, Manual [0.797:1] - 66 Mph (vs. 60 Mph of auto)

I get what JeepLab was describing now about 4.56's being a nice fit for a 6spd manual. If I ran 4.88 gearing on my 6spd, it's not that it would be horrible or anything, but, it certainly is not the same experience as described by those with autos. The 4.56 gearing is ALREADY a steeper gearing overall for the 1st-4th gears than the autos with 4.88 gears! In other words, it already has more power, I'm already shifting sooner/more, let alone what the 4.88 gears would provide vs. an auto. If the 1st-4th gears are already steeper than those happy 4.88 auto guys, than why not go with the 4.56 vs. the 4.88 gearing and pick up some more fuel efficient high end that is also quieter and smoother at highway speeds? The 4.56 gearing in a manual transmission, with 37" tires, is STILL more aggressive/steeper than a stock Rubicon with 4.10 gears of same transmission (6spd or auto!) New Jeep owners are advised all the time that the automatic Rubicon is a better city driver and not as preferred on the highway as a Sahara if it's your daily driver. Dang! And the 4.56 geared 6spd, on 37" tires, is steeper in 1st-4th than a stock automatic Rubicon, but you get a lower overdrive gear (ie. lower final RPM) than a stock Rubicon!

So, long story short, I'm going with the 4.56 gears. Obviously everything I wrote here is "science" and real-life might play out different, but I really think that the experience in a 6spd manual deserves a slightly different decision or approach than that of an automatic. The process of doing this research has also really opened up my eyes about all the different configurations you can purchase a new Jeep and what your experience might be in each combination.

Thanks to everyone for your feedback and lively discussion. I might actually hold off on ordering the gears today if for no other reason that northridge4x4 has Easter Safari kicking off tomorrow, so maybe a deal will come my way ;-) So if you guys look over this and see some error in my work, or decision making, there is still time re-evaluate and make a different decision.

Regards,

Timmy!

[JJeep]

I recently bought a 2016 Unlimited, auto-trans. I am having 3.5 superflex lift installed with 37" Toyo MTs. Were you saying that the two members that drove the autos preferred the 4.88 to the 4.56. Thanks. There is just so many different opinions and "experts" on regearing.

[JeepLab]

I recently bought a 2016 Unlimited, auto-trans. I am having 3.5 superflex lift installed with 37" Toyo MTs. Were you saying that the two members that drove the autos preferred the 4.88 to the 4.56. Thanks. There is just so many different opinions and "experts" on regearing.

Where do you live? mountains? or highways?

If you want to keep your truck a good highway cruiser, go 4.56. If you are looking for maximum crawl, 4.88.

I had 4.88s, and it was too much gear for me as a daily driver. Turned a rocket into a pig. (well not really, but bad enough to change again)

What are your current gears?

[Timmy]

I recently bought a 2016 Unlimited, auto-trans. I am having 3.5 superflex lift installed with 37" Toyo MTs. Were you saying that the two members that drove the autos preferred the 4.88 to the 4.56. Thanks. There is just so many different opinions and "experts" on regearing.

I have 37" tires and a manual trans. Technically the manual trans is providing a hair bit lower gearing than the auto so my experience may not match yours. I actually have the opposite view of Ross. I put in 4.56 gears and after having it for about a year I wish I would have put in 4.88. I often find that the gearing is just not low enough for all types of driving, city and highway. Sometimes I burn the clutch in reverse if I have to get over/up something big because it's just not enough. On the highway, I hardly ever shift into 6th gear because it just can't handle it. I only shift into 6th gear when I'm over 60 mph. I may use 6th gear at 50mph, but only if the road is totally flat or downhill.

I drove from WA to Lake Powell in Arizona by way of Montana, Idaho and then Utah. Coming in to Utah I was sucking eggs. We were about 6,500 elevation before heading in to Arizona and we had a lot of ups and downs on the Interstate. The Jeep topped out at 85mph and I couldn't go any faster, that was flat out all she had in sixth gear. Going up hill, if I even wanted to maintain 80mph, I had to draft behind a larger vehicle and drop down a gear a two. I was really wishing I had just a few more hundred RPM in sixth gear to help maintain the highways speeds as I was dropping down to 5th gear all the time. I honestly didn't see any better gas mileage in 6th gear vs. 5th.

I do know that Ross has a supercharger in his rig, and it was a manual like mine. With a supercharger, the 3.56 might be perfect, but I really think that without one you need to go 4.88. Instead of switching out my gear, I'd rather the $2k go in to a supercharger and see if I can't solve my situation with more power ;-)

Like I said, if I had to do it all over again, I would go 4.88.

Hope that helps muddy the water for you!


[UPDATE: Durp... forget to mention I had a kayak and two paddle boards on my roof to Lake Powell, so that just added to the aerodynamics of my already streamlined brick on the Interstate.]

[JeepLab]

I have 37" tires and a manual trans. Technically the manual trans is providing a hair bit lower gearing than the auto so my experience may not match yours. I actually have the opposite view of Ross. I put in 4.56 gears and after having it for about a year I wish I would have put in 4.88. I often find that the gearing is just not low enough for all types of driving, city and highway. Sometimes I burn the clutch in reverse if I have to get over/up something big because it's just not enough. On the highway, I hardly ever shift into 6th gear because it just can't handle it. I only shift into 6th gear when I'm over 60 mph. I may use 6th gear at 50mph, but only if the road is totally flat or downhill.

I drove from WA to Lake Powell in Arizona by way of Montana, Idaho and then Utah. Coming in to Utah I was sucking eggs. We were about 6,500 elevation before heading in to Arizona and we had a lot of ups and downs on the Interstate. The Jeep topped out at 85mph and I couldn't go any faster, that was flat out all she had in sixth gear. Going up hill, if I even wanted to maintain 80mph, I had to draft behind a larger vehicle and drop down a gear a two. I was really wishing I had just a few more hundred RPM in sixth gear to help maintain the highways speeds as I was dropping down to 5th gear all the time. I honestly didn't see any better gas mileage in 6th gear vs. 5th.

I do know that Ross has a supercharger in his rig, and it was a manual like mine. With a supercharger, the 3.56 might be perfect, but I really think that without one you need to go 4.88. Instead of switching out my gear, I'd rather the $2k go in to a supercharger and see if I can't solve my situation with more power ;-)

Like I said, if I had to do it all over again, I would go 4.88.

Hope that helps muddy the water for you!


[UPDATE: Durp... forget to mention I had a kayak and two paddle boards on my roof to Lake Powell, so that just added to the aerodynamics of my already streamlined brick on the Interstate.]

OMG TIMMY! Did I screw you? You followed my lead on 4.56 and regret it? That hurts me to my core.

I see no way that it could be the wrong decision for highway driving like you are talking about, but wouldnt disagree with your experience.

How heavy is your rig? lots of steel? maybe thats the difference?

[Timmy]

OMG TIMMY! Did I screw you? You followed my lead on 4.56 and regret it? That hurts me to my core.

I see no way that it could be the wrong decision for highway driving like you are talking about, but wouldn't disagree with your experience.

How heavy is your rig? lots of steel? maybe thats the difference?

Oh man, don't sweat it for a second. I researched a ton from a lot of different resources. One of the reasons I picked the 4.56 is that I felt if I erred in my decision, I would rather error too low than too high as us manual guys can always just down shift if you error low ;-) Also, I was always planning on going with a supercharger and I suspected that your own preference between the two gears was because of your supercharger. I didn't want to buy 4.88's and then toss in a supercharger and wish I had put the 4.56's in. I'd rather have the 4.56's and wish I had a supercharger and move that direction. Sadly, my damn family keeps sucking away all my fun money (haha!)

As for heavy... Compared to yours, yeah I'd be heavier. My Toyo's are heavier than your tires and I also have a full width front and rear ARB bumper. Everything else is pretty well stock so it shouldn't be much more than say 150lbs though. I can easily make that up by taking my top and doors off (which I do all summer) but even with that weight off the 4.56's are still just not enough.