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Discussion Starter · #1 ·
Every vehicle I've owned has had a highway cruise speed that seemed optimal, where engine and transmission seemed to hit a groove. Above this RPM and speed things seemed to strain some. I am curious how different axle ratios affect this sweet spot on the STOCK 2012 and 2013 JK's in your personal opinion. Everyone may have a different "feel" for this speed, but I'm betting there won't be great variance.

I have a 4.1 and that spot is right at 55-60mph. I've always wondered where it would be if I had a 3.73 or 3.21.
 

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between 65 and 70 is my sweet spot. Handles well and gets about 20mpg. Over 75 and mpg and handling drop off very quickly. 6-speed manual 4.10.
x2

jkr
 

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Can someone compare rpm's in top gear between a 5-speed auto with 3.73 and a 6-speed manual with a 4.10? Let's use 65mph or something close.
 

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Discussion Starter · #6 ·
can someone compare rpm's in top gear between a 5-speed auto with 3.73 and a 6-speed manual with a 4.10? Let's use 65mph or something close.
Text Blue Line Design Pattern
 

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Between 70-75 mph; 6 speed with 3.73s.
 

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Are you all running the same tire size? That sould be in your info as well. 3.21, auto, size 32" tires between 65-70mph, Haven't checked rpm's
 

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3.73's 6spd, 32" tires, most comfortable between 70-75.
 

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If everyone is running 32's wheels...

It isn't the speed...it is the RPM's.

I wish Wranglers game with an option for 3.55's because in the NAG1 auto, that would put 70mph at 2162 RPM's. (Being that 5th gear is at .083)
And it seems from all the mileage threads...somewhere between 2100 and 2200 RPM's and not exceeding 70mph looked like the sweet spot. The RPM's (and speed) are low enough to help conserve fuel, yet high enough to produce enough power to fight modest winds or slight inclines without down shifting.

Gear Ratio Calculator

Just for comparison...rpm's using 32" wheels at 70 mph.
3.21 NAG1 = 1955
3.55 NAG1 = 2162
3.73 NAG1 = 2271
4.10 NAG1 = 2497

A Rubi with a NAG1 auto and 4.10's runs a bit high IMO at 70 mph.

6th gear was re-geared a bit deeper starting in 2012 for the manuals (from .84 to .797)....so a lower (or numerically higher) axle ratio suits them better.

3.21 N370 = 1877
3.55 N370 = 2076
3.73 N370 = 2181
4.10 N370 = 2397

On the flip side, not even hitting 1900 RPM's at 70mph is way too low for the manuals. The slightest headwind (especially in a 4 door) will require a downshift. 70mph in 5th gear on 3.21's equates to 2355 rpms. Which is a touch less than a Rubi with 4.10's in 6th.

From the responses in this thread....it sounds like the sweet spot might be between 2100 and 2300 RPM's. But that could also vary by engine, tranny, and mods, and 2 door or 4 door.
 

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Since I'm at it....might as well to the 3.8L numbers.

Just for comparison...rpm's using 32" wheels at 70 mph.

3.21 42RLE = 1625 (never offered)
3.73 42RLE = 1888
4.10 42RLE = 2075

3.21 N370 = 1978
3.73 N370 = 2299
4.10 N370 = 2527

I think this really explains why so many 3.8 owners originall thought 3.21's would be awful as a standard for the auto as on the 3.6 before fulling understanding the difference in the gearing of the auto transmissions. 4th gear on the 42RLE is WAYYYY too long (.69). Even on 3.73's you are sub 1900 RPM's at 70 mph.

It also explains why those with manuals and 3.73's or 4.10's never had "huge" issues with the 3.8 (on stock wheels).

It is all in the gearing...
 

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Discussion Starter · #14 ·
It seems panthermark is on it with the 2100 to 2300 RPM being groovy for most. The manual 3.73's and 4.1's are all pretty happy around 70mph and turn rpms in this range. We autos with 4.1 like 60'ish mph. Betting we are turning low 2000 rpms, but have no figures. At 70mph the auto 4.1's are blowing 2500 rpms and feel the strain somewhat.
 

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2,200-2,400 rims is my preference. My speedo is off with 31's so mph's mean nothing to me.
 

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2013 4dr sport ato with 3.21 245/75-16 (wild country XTX LT sports) ~30.5" tires 75-80mph is very comfortable as is 60-65mph. For some reason my tach is showing higher rpms at 70mph in 5th then the chart. Must be my tach as I know my speedo is dead accurate per 3 different gps as well as those speed check machines they have on many streets around where I live.
 

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2,200-2,400 rims is my preference. My speedo is off with 31's so mph's mean nothing to me.
Is that in a 2012/13 stock JK....or a TJ?
 

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Is the chart is based on actual tire size, the stock Rubicon, Sahara and Sport S tires are probably 31" tall not 32". Most tires do not actually measure what they calculate to be by the numbers.
 

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Is the chart is based on actual tire size, the stock Rubicon, Sahara and Sport S tires are probably 31" tall not 32". Most tires do not actually measure what they calculate to be by the numbers.
my tires are measured size I used a level and measured from the top of the tire to a cement floor flat and level. the number was what I used for setting the speedo with the procal and it came out spot on. I just went out and measured again and mine is 29.5 not 30.5" I remember now initially I used the adververtised measurment and my speedo was off so measured it using the level method.
 
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