The Truth about Gears and Tires

[Timmy]

should have clarified: ok yeah obviously you can't adjust the tach, i meant the ratio of RPMs to speed. anyway, to clarify the resistance: i was referring to the amount of RPMs need to sustain a speed with the cruise on. your vehicle is obviously going to work harder if you are driving straight into a strong wind and so you get that slight fluctuation (in speed and RPMs). all i was referring to.

i don't like trusting a GPS for my speed. my OBCD dongle has an app that allowed me to basically drive one mile at a constant speed (again as best i could) with a timer and then i did the math. i always found GPS to be 1-2 MPH slower than actual speed.

Ahh, got it.

I agree with you, some GPS can be a little off. Of course, if the GPS is consistent, that's what matters most as that is something you can work with (either consistently above or below.) I have found when I calibrate to GPS and then I cruise past one of those stationary pole mounted radar guns that displays your MPH, that my MPH displayed in the dash matches pretty close to what the radar gun shows on the pole, so I think the GPS is close enough. Also, it's usually good to be a little high on the speedo as it helps save you a ticket. That's why most German cars have a built in 3% margin to the high side ;-)

[KaiserBill]

should have clarified: ok yeah obviously you can't adjust the tach, i meant the ratio of RPMs to speed. anyway, to clarify the resistance: i was referring to the amount of RPMs need to sustain a speed with the cruise on. your vehicle is obviously going to work harder if you are driving straight into a strong wind and so you get that slight fluctuation (in speed and RPMs). all i was referring to.

i don't like trusting a GPS for my speed. my OBCD dongle has an app that allowed me to basically drive one mile at a constant speed (again as best i could) with a timer and then i did the math. i always found GPS to be 1-2 MPH slower than actual speed.

GPS systems have a percentage of error like anything else.

[Snarf77]

My 2 Door with boost will maintain 90 with no problems...not that its a good idea. 70 was a non issue with 37 KM2s and 4.88s. Like Dalearyous said, its probably good advice to go with what feels right. Maybe drive someone else's vehicle with the gearing desired to see if it has the right feel? Might take some time but still, you could potentially avoid a mistake.

[JeepLab]

My 2 Door with boost will maintain 90 with no problems...not that its a good idea. 70 was a non issue with 37 KM2s and 4.88s. Like Dalearyous said, its probably good advice to go with what feels right. Maybe drive someone else's vehicle with the gearing desired to see if it has the right feel? Might take some time but still, you could potentially avoid a mistake.

maintain 90 at what RPM? 3500?

[Timmy]

Just another follow-up to the book I just published (sorry, I write a lot.)

For what it's worth, I took a fairly good look at the gearing options from stock, to 35" tires to 37" tires. It is becoming very apparent to me of the following based on RPM vs. MPH vs. Gear (I'm now realizing I'm re-inventing the wheel here, aka, the chart at the beginning of this forum.)

3.6L JK with 6spd Manual (Auto is probably the same.)
3.23 Gearing on stock tires = 3.73 gearing on 35" tires = 4.10 gearing on 37" tires (basically a base JK feel.)
3.73 Gearing on stock tires = 4.10 gearing on 35" tires = 4.56 gearing on 37" tires (basically a JKU with the max tow package feel.)
4.10 Gearing on stock tires = 4.56 gearing on 35" tires = 4.88 gearing on 37" tires (basically a Rubicon feel.)

In each case, the gearing is slightly steeper to the 35" tires, and then slightly steeper yet to the 37" tires. It's not by much, about 1% change for each, but that is enough to possibly make up for the difference of the added weight of the tires.

Maybe the real question we need to be targeting for people is... If you went to a dealer and test drove a base JK, a JKU with the max towing package and a Rubicon, which did you like driving the most?

The little equation I just posted above is basically what the graphical chart at the beginning of this thread shows, I think it just puts it in a little more in-your-face, easier to digest style as you know you could go to a dealer and experience all three to better understand what you could expect from your Jeep after a re-gear.

[2k13jk]

My 13 jk 6 speed equipped with a ripp i had a buddy slap 4.10s in over my old 3.21s and its a perfeft combo i can use 6th as low as 45mph and still have acceleration power at 70 i doing 2200

[Pznivy]

I have 5.13s. My 37s measure 35 (KM2) I dont have a rubi, and am not on the highway often. Its the best feeling for my situation, Might be too much gear for others, but when I put it in 4 lo and the truck climbs without me touching the pedals regardless of terrain its worth it.

Shouldnt drive too fast anyway. No rush.

[KaiserBill]

That is way out of line with BFG's liturature on their tires which states the tires are 36.8 inch in diameter. I cannot see them having a +/- 1.8inches manufacturing tolerance. I could see +/-.30 manufacturing tolerance at most.

[dalearyous]

3.6L JK with 6spd Manual (Auto is probably the same.)
3.23 Gearing on stock tires = 3.73 gearing on 35" tires = 4.10 gearing on 37" tires (basically a base JK feel.)
3.73 Gearing on stock tires = 4.10 gearing on 35" tires = 4.56 gearing on 37" tires (basically a JKU with the max tow package feel.)
4.10 Gearing on stock tires = 4.56 gearing on 35" tires = 4.88 gearing on 37" tires (basically a Rubicon feel.)

i like this. this is basically how it was described to me.

"Drive what feels best" is no answer at all. Anyone can agree with that. The driver's goals will dictate what feels best, and with gears you rarely know how its going to feel until you've spent the money and ripped your diffs apart.

Im not against 5.13, please dont take it that way, Tonka, our JK on 40s has 5.13 and no SC and she feels fast. there are different opinions with regard to this related to driving style and location. In NJ you want to be able to drive fast as well as wheel. I also have a 4 to 1 transfer case that gives a little more freedom to run a little less gear.

the best information is for those of us who have actually done it to share with others what our trucks do mathematically now. Then others can get in their trucks and run their RPM up to where we say ours go to maintain highway speed, and see if thats livable for them.

"drive what feels best" as in like what was said, try and find and test drive other peoples rigs and then make a decision. i understand and i am not offended at all. your original post was trying to be informative and point out some glaring misinformation that people use to select gears and i agree with it. i also started this late, on my phone, and just now (on my computer) realize what timmy's rig is and that he does not have a SC. so that explains a bit.

[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!