Gears for 37" tires

[ShadowsPapa]

Good chart indeed...
This also shows how horrible 37's and 4.10 gears would be.. Even more so, with a lower speed limit ( 100 kph (62mph here )
It is not holding 8th at 1746 rpm.....

That's another reason I didn't buy the 34's on Rubicon wheels for sale locally at a tire shop. Mine would never even get to 8th, let alone hold it. Mine will be constantly hunting gears towing.

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[KennyKustom]

See thats the problem with charts.
4.10 and 37s will turn 1877 in 7th at 60mph which is in the powerband that trans will try to keep the RPMs in anyway. The 8 speed is an awesome trans and the programming will keep it in the ideal band for the speed and rpm given if it can support it. at 60 4.10 were not a problem on 37s.

I daily mine, and have a 200km commute. My jeeps spends most of the time in 7th at 65 mph. ( stock rubi )

Irregardless of what any chart or calculator says, my real world daily commute proves to me, that when i put 37's on will be gearing to 4.88.

 

[MrKnowitall]

All this recently posted experience, commentary and charts shows that while you absolutely CAN drive around with 37s on stock gears, 5.13s would in no way be too deep for the automatic. If you’re going to pull the trigger, go as big as you can without negative results.
Oh, and when doing calculations, try to go with tire RPM specs rather than height.

 

[jurfie]

I daily mine, and have a 200km commute. My jeeps spends most of the time in 7th at 65 mph. ( stock rubi )

Irregardless of what any chart or calculator says, my real world daily commute proves to me, that when i put 37's on will be gearing to 4.88.

Interesting. 65 mph on a stock Rubicon is at exactly 2300 RPM in 7th. 8th is 1834 RPM; you mentioned it won't hold 8th at 1746, so I'm going to assume the ideal shift point is 2000 RPM? Less than that and it will hunt gears? I ask because I don't yet have a JT to try for myself.

Whereabouts in Canada are you (to give me an idea re: terrain of your commute). With a 200km commute, I'm going to guess you're either outside of Vancouver or Toronto?

 

[KennyKustom]

Interesting. 65 mph on a stock Rubicon is at exactly 2300 RPM in 7th. 8th is 1834 RPM; you mentioned it won't hold 8th at 1746, so I'm going to assume the ideal shift point is 2000 RPM? Less than that and it will hunt gears? I ask because I don't yet have a JT to try for myself.

Whereabouts in Canada are you (to give me an idea re: terrain of your commute). With a 200km commute, I'm going to guess you're either outside of Vancouver or Toronto?

Manitoba.

 

[jurfie]

Ah. So...flat.

Looking at the calculator, it seems 4.56 will give you the closest performance to stock. Left is stock Rubicon, right is 4.56 gears and 37s (36" mounted). I put 2000 RPM in the middle column for each - you can see the speeds at that RPM is almost identical. The second chart shows your RPMs at different speeds (50/62/70 mph or 80/100/112 kph):

 

[ShadowsPapa]

Interesting. 65 mph on a stock Rubicon is at exactly 2300 RPM in 7th. 8th is 1834 RPM; you mentioned it won't hold 8th at 1746, so I'm going to assume the ideal shift point is 2000 RPM? Less than that and it will hunt gears? I ask because I don't yet have a JT to try for myself.

Whereabouts in Canada are you (to give me an idea re: terrain of your commute). With a 200km commute, I'm going to guess you're either outside of Vancouver or Toronto?

Putting stock Rubicon tires and wheels on my Overland would mean constant shifting on the interstate here and never hitting 8th on most of the roads around me. Hmmmmm, guess I'd never wear out 8th gear.
Even more concerned about towing with those tires and the 3.73 ratio now.

 

[Oscar Indy]

All this recently posted experience, commentary and charts shows that while you absolutely CAN drive around with 37s on stock gears, 5.13s would in no way be too deep for the automatic. If you’re going to pull the trigger, go as big as you can without negative results.
Oh, and when doing calculations, try to go with tire RPM specs rather than height.

513 and 8 gear with 37 inch tires turns the exact same RPMs as 7th gear and 4.10s. That's a lot of money spent to basically delete 8th gear

 

[Oscar Indy]

Ah. So...flat.

Looking at the calculator, it seems 4.56 will give you the closest performance to stock. Left is stock Rubicon, right is 4.56 gears and 37s (36" mounted). I put 2000 RPM in the middle column for each - you can see the speeds at that RPM is almost identical. The second chart shows your RPMs at different speeds (50/62/70 mph or 80/100/112 kph):

I really felt the four tens on stock tires was undergeared. Any headwind would kick me out of 8 and really hurt my mileage

 

[WhatExit?]

Some of you guys need to develop a dial-a-gear differential

At least the no-need-to-regear, denial-is-more-than-a-river-in-Egypt crowd haven’t been in this thread lately

 

[ShadowsPapa]

Some of you guys need to develop a dial-a-gear differential

At least the no-need-to-regear, denial-is-more-than-a-river-in-Egypt crowd haven’t been in this thread lately

I had a couple of trucks like that - a switch on the shift lever would swap the gears around in the differential. But rather limiting, only two choices.
That's why some like the Ford 9" for racing - keep a spare pumpkin or two with different gears, swap at the track with a jack and hand tools.

 

[Factoid]

Good chart indeed...
This also shows how horrible 37's and 4.10 gears would be.. Even more so, with a lower speed limit ( 100 kph (62mph here )
It is not holding 8th at 1746 rpm.....

Yeah, but I have a secret weapon.

6.4L

 

[Factoid]

I had a couple of trucks like that - a switch on the shift lever would swap the gears around in the differential. But rather limiting, only two choices.
That's why some like the Ford 9" for racing - keep a spare pumpkin or two with different gears, swap at the track with a jack and hand tools.

I put a Ford 9” in my ‘64 corvette coupe.

 

[ShadowsPapa]

I put a Ford 9” in my ‘64 corvette coupe.

Sweet. The better days of Corvette. (just my opinion)

 

[MrKnowitall]

513 and 8 gear with 37 inch tires turns the exact same RPMs as 7th gear and 4.10s. That's a lot of money spent to basically delete 8th gear

So you said with 4.10 gears, yours won’t hold 8th with a head wind. My opinion is that if you’re going to bite the bullet and regear, you go as deep as you can tolerate. The deeper gear will let the engine run lean mixture and full ignition advance more, which translates into offsetting any economy lost vs a smaller gear ratio.

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