Calculator: gear ratio, tire size, speed, etc.

[AnnDee4444]

So in my 3.6L/8AT I did a full progression from:

4.10's & 33's --->4.10's & 35's--->4.88's & 35's--->4.88's & 37's--->5.13's & 37's

Here are my thoughts:

-- 4.10's & 33's work reasonably well at sea level and on flat roads. At altitude, high winds, or hills the transmission will go up and down from 8th to 6th (sometimes 5th). FCA should have geared this combination with 4.56 from the factory when paired with the 3.6L/8AT.

-- 4.10's & 35's are ok at sea level and on flat roads. It will hold 8th, but if you hit any head wind or hills you will experience the same as 4.10's and 33's at altitude. At altitude, high winds, or hills, this combination is a pretty miserable experience.

-- 4.88's & 35's are a great combination at sea level or altitude. Nice and peppy and handles altitude, wind, and hills very well. It will downshift, but does not drag out the shifts and will quickly upshift as expected.

-- 4.88's & 37's work reasonably well at sea level and in some ways are the same as 4.10's and 33's on flat roads. At altitude, this combination will cause the shifts to drag out on hilly surfaces and will up/down shift from 8th to 6th (sometimes 5th). It's not a bad combination, but it is lacking at altitude.

-- 5.13's & 37's are a great combination at sea level or altitude and behave similarly to 4.88's and 35's, though perhaps not quite as peppy. The shifts no longer drag out as they did with 4.88's at altitude.

Based upon my experience, with a 3.6L/8AT this is why you see me recommend:

33's = 4.56
35's = 4.88
37's = 5.13
38's+ = 5.38

Found this good post in another thread about regearing a 3.6 & 8-speed. Other motors & transmissions will react differently.

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[Slippery Pete]

Going uphill in 7th gear in a rainstorm on the third Thursday of the month, my speedometer is off by 0.28713 MPH. My vehicle is loaded with myself, my wife, a dog, a parakeet, 2 bags of groceries and a tuba. What equation should I use to calculate the distance between the Earth and the Moon? I need 8 significant digits. No less.

Are the grocery bags plastic or paper?

 

[phunktion]

This chart is fantastic despite there not being many comments here. For me it raises some questions though.
I currently drive a Focus ST (manual) with a 2L turbo inline 4. I can shift into 6th on flat ground at about 80kph (50mph).
I recently ordered a manual rubicon with 4.10 gear ratio. With the option of 4.88s giving nice crawl ratios I'm a little torn by what I see in this chart.

At 50mph with 4.10 (33" tires likely staying so it fits in my garage with a roof tent) I'd be at 1500rpm. With the 3.6 not being so torquey at low RPM since it has no turbo it sounds like people try to stay above 2000rpm. 4.88s would bring me up to 1789rpm at 50mph and 2147rpm at 60mph which sounds useable.
Does it really kill my MPG if I got 4.88 on stock 33" tires? I don't often go above 75mph around here and people have complained that the gears are too tall and they sound much taller than the Focus I drive so 4.88 actually sounds like it's closer to what I'm used to. Any first hand experience with both here?

I know I'm digging this up from way past, but there's a possibility you haven't regeared your Rubicon yet if you received it/still have it. This spreadsheet is AWESOME and a huge help - thank you to OP.

IMO the manual is geared incredibly poorly stock. I don't think you could go wrong just jumping straight to 5.13s, but at a bare minimum 4.88s. Like you, I have had mostly small torquey cars and the JL will fall completely flat for you compared to what you're used to. You could drive around in 4th and get a feeling for what to expect going straight to 5.13s in 6th. I'm used to doing 3k RPM+ at 70 w/ my 08 Rabbit w/ the 2.5 anyway - not scared of RPM as long as I'm out of the second stage oil pump on normal highway driving (3k RPM). I wouldn't be surprised if you see an INCREASE in gas mileage going to 4.88s depending on how you drive, not having to fight the gearing so much and letting the engine take it easier (load-wise, despite the higher revs).

I won't regularly break 65, so I'm going 5.13s on my Willys Sport JT with the stock tires (knowing I'll eventually go 37s). I'll be at ~2600RPM at 65 on stock tires (32's).. The 3.6 has nothing to give under 2k RPM. 5th and 6th are all but unusable with 3.73s and the manual, and the gaps between 2-3-4 are laughable. Having to downshift to 2nd on significant backroads uphills in a 6spd offroad focused vehicle with 285hp is ridiculous. Forget towing or needing to reverse a trailer up a slight incline in 2H.

 

[J0E]

For a different view, see my https://bt39.com/GearWheelSpeed
Delete what you don't want to see and edit what you want to compare.

 

[The_Skyo]

I'm happy that I got 4.88 after putting 10,000km on it in the last few months. First gear is short in the city, but it's great when I need it elsewhere. I wish this engine was a bit better at low rpm but I'm adapting.

 

[phunktion]

I'm happy that I got 4.88 after putting 10,000km on it in the last few months. First gear is short in the city, but it's great when I need it elsewhere. I wish this engine was a bit better at low rpm but I'm adapting.

Awesome to hear it. I decided to go w/ 35x10.50s with my transition to 5.13s, so should be similar to your overall performance.

 

[JTDau]

The first sheet appears to be broken on my end.

 

[AnnDee4444]

The first sheet appears to be broken on my end.

What browser are you using?

 

[JTDau]

What browser are you using?

Google sheets app on Android. Heres is a screenshot of what I'm seeing:

 

[AnnDee4444]

Google sheets app on Android. Heres is a screenshot of what I'm seeing:

I just checked it on the Android app... seems to be working for me.

Was there something about the screenshot that is incorrect? Only the orange fields are editable.

 

[JTDau]

@AnnDee4444 Nope, I'm just a dumbass. I inadvertently had low range selected and wondered why 6th was topping out at 85mph... Thanks for your help. Best spreadsheet out there for this imo.

 

[J0E]

Try bt39.com/GearWheelSpeed

 

[AnnDee4444]

Somewhat related:

 

[Ratbert]

Somewhat related:

That was a really interesting video. Thanks for post it.

 

[AnnDee4444]

Really basic transmission video, but the concepts are the same as today.

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