I want bigger tires... but

Mr. Bills said:
I tried the "I can use the OD button" rationalization on my LJ with 42RLE, 4.88 gears and 35" tires. The problem was the voice in my head every time I used the OD button that said, "You wouldn't need to do this if you had the right gears."

I re-geared to 5.38 and the world is perfect once again.

I will say this to those with 4.88 gears, 42RLE transmission and 35" tires: If you think your jeep is driving just fine you are proving the adage, "you don't know what you don't know." Try to drive a similarly equipped jeep with 5.13 or 5.38 gears. It will be an enlightening experience.
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I play with the OD button even with 33s and 4.88s.
I am constantly clicking that thing while going up hills and merging on highways. I hate to hear the motor lug. Lower gears is now on my wish list along with 35s. Next order of business is a Besttop Frameless Trek top for summer. Half tempted to sell my factory hard top to get some cash up but I fear I'll regret that one day especially next winter. I have a guy with a standing $1k offer for it.
 
I hate to be the newb of the group, but can someone explain the "feel" of the drive between a stock set of gears (like 3.73) and 5.38? I've been casually looking through the can-of-worms when it comes to getting beefier axles and upping my tire size to my 35" goal.
 
ErikOffroad said:
That video should explain a lot . I just would not follow his way of calculating gear ratios. Use chris’s Chart
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About 1:23 into the video, he says a 3 to 1 gear ratio would be .33 revolutions per drive shaft rotations and a 5 to 1 gear ratio would be .25 revolutions per drive shaft rotations.

Wouldn't 4 to 1 be .25 and 5 to 1 be .20?
 
From the video...

(Old Ratio x New Tire Size)/Old Tire Size= New Gear Ratio

(3.07 x 35)/29= 3.70 gearing
(3.73 x 35)/29= 4.50 gearing
(4.10 x 35)/31= 4.62 gearing

Which one is correct, @Mudding101?
 
jjvw said:
From the video...

(Old Ratio x New Tire Size)/Old Tire Size= New Gear Ratio

(3.07 x 35)/29= 3.70 gearing
(3.73 x 35)/29= 4.50 gearing
(4.10 x 35)/31= 4.62 gearing

Which one is correct, @Mudding101?
Click to expand...

No one on this thread is going to tell you 4.10s for 35s. I have 4.88s for 33s and that's the limit. You want 5.68s for 35s according to everyone.
 
mgroeger said:
No one on this thread is going to tell you 4.10s for 35s. I have 4.88s for 33s and that's the limit. You want 5.68s for 35s according to everyone.
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Here's my recommendation for a 35" tire. :)
32rh/34"/4.10 = 3039rpm @ 75mph
AX-15/34”/5.13=3004rpm @ 75mph
NV3550/34”/5.13=2966rpm @ 75mph
NSG370/34"/4.88=3038rpm @ 75rpm
42rle/34"/5.89=3012rpm @ 75mph
 
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Jeffreybomb said:
About 1:23 into the video, he says a 3 to 1 gear ratio would be .33 revolutions per drive shaft rotations and a 5 to 1 gear ratio would be .25 revolutions per drive shaft rotations.

Wouldn't 4 to 1 be .25 and 5 to 1 be .20?
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I bet he was a product of Norco High School. Socializing seemed more important than education.
 
jjvw said:
Here's my recommendation for a 35" tire. :)
32rh/34"/4.10 = 3039rpm @ 75mph
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^^ A sound recommendation based upon the parameters given and the rationale discussed by @jjvw at length in other gearing threads. Ironically, it is also an illustration of how one must pay attention to the transmission being used and that one size doesn't fit all.

@jjvw lives and wheels in Colorado where gearing that has the 4.0 with 32rh transmission (1:1) at about 3,000 rpm at 75 mph works well. Parts of Colorado are flat; the rest is anything but flat which requires keeping the engine higher on the torque curve to keep from lugging.

I live at the north end of the Sacramento River Valley in Northern California and frequently drive long straight flat stretches of Interstate for hours at a time where the prevailing speed is 75-79 mph. 3,000 rpm all day long would drive me nuts, 2700 rpm not so much. Also, it is easier to accelerate out of a lugging condition on flat ground than on a mountain grade at high elevation. So my gearing recommendation based upon my particular transmission with 0.69 overdrive and the 17.25" rolling radius (34.5" dia.) of my particular tires is:
42RLE/(17.25x2)/5.38 = 2712 rpm @ 75mph.

For comparison, at 75 mph in 3rd gear @jjvw is at 3039 rpm. If I drop out of OD into 3rd gear at 75 mph I will be at 3930 rpm. His gear choice is based on rpms in 3rd gear; mine is based on rpms in OD.

YMMV
 
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The list I provided is for gearing that puts all available 4.0 transmissions at about 3k at 75mph, which assumes the final drive gear. Mine is geared similarly for its tire size. I personally like the lower gear/higher rpm for mountain travel as well as crowded city driving. But it should be noted that most out here follow the traditional recommendations which would be one step (numerically) lower across the board. I am the anomaly in more ways than one.

There are two reasons I like to list 3k at 75. One is to show that the gearing/rpm is no where near as terrifying as many want to feel it ought to be. Another is to suggest the idea that a TJ can indeed safely accomplish 3000rpm and 75mph without becoming a fiery car crash. Mine certainly has with no ill effect that I can identify.

Another aspect of this worth considering, and something Blaine astutely brings up, is that the early TJs with a 3 speed were geared for much higher rpms right off the assembly line than the tractor engine crowd would ever feel comfortable with. Yet Jeep was fine with it.
 
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jjvw said:
There are two reasons I like to list 3k at 75. One is to show that the gearing/rpm is no where near as terrifying as many want to feel it ought to be. Another is to suggest the idea that a TJ can indeed accomplish 3000 rpm and 75 mph without becoming a crashing fireball in the ditch.
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X2. Many Jeepers came from cars with small tires and sometimes bigger engines where they were used to cruising at 1700 to 2200 rpms at 70 mph. That's a perfectly normal cruising rpm for most cars. But for running big Jeep size tires, those rpms aren't enough.

I actually had 3k rpms at 65 for several years. Not on purpose. I had installed 4.88 gears for my 35" tires with the 5-speed transmission. It gave 2750 rpms at 70 mph which was absolutely perfect. Then that 5-speed was replaced with the 3-speed 32RH. Losing the 5-speed's Overdrive ratio raised the rpms from around 2750 at 70 mph to 3000 at 65 with the 32RH. My cruising slowed down a bit after that, usually to 55-60 mph but when I needed to go 65 or 70 the 3000 and slightly above rpms were not a problem. The 2.5 and 4.0 can cruise at that rpm all day every day without problem.
 
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Another likely reason people are wary of low gears is because of YouTube videos like BleepinJeep and the one posted earlier. The formula so many use assumes the earlier tire size and ratio were good to begin with.
 
jjvw said:
Another likely reason people are wary of low gears is because of YouTube videos like BleepinJeep and the one posted earlier. The formula so many use assumes the earlier tire size and ratio were good to begin with.
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To add to that, after installing big tires many mistakenly calculate what ratio is needed to restore the rpms back to what the engine turned at with the factory size tires. That doesn't work. To turn bigger diameter and heavier tires, more engine rpms are needed. Like bigger arm muscles are needed to lift heavier weights.
 
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Found this from 1970..

Not something I paid attention to in Grade 11 but 36mpg was exceptional in 1972.
Overdrive and a 2.94 rear end.

This special Duster garnered its name thanks to a 187-pound weight loss compared to regular Duster models, and it had a Dodge counterpart in the Dart Lite. A 225-cubic-inch inline-six sits under the aluminum-braced hood, with the trunk and bumpers featuring the lightweight bracing as well. Power from the 100-horse engine is transferred through a four-speed manual to a 2.94 rear end. Rated at 36 mpg, the Feather Duster was never confused for a performance car.

75759
 
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