Regearing, where does the power come from?

[Zachanadandy]

The transmission torque input doesn't really matter since it's fairly equal as the pentastar torque curve is flat from 2500 to 6k. So the torque going in to the diff is basically the same geared vs stock as the rpms are only 12% off on 37s and even less on 38s or 39s. Same tire size and regearing will kill efficiency because you are greatly increasing rpm, bigger tires regeared to offer same or slightly better mechanical advantage will restore efficiency.

Your assumption is that tranny can't drop to 2nd or 3rd at 45 mph in both scenarios, it can because the final drive ratio is only slightly higher so engine rpm at a given speed is only a 250 -300 rpm or less higher. In super low load situations you are again correct but on 38s or 39s you basically never see 8th, instead you are in 6th or lower, which is equivalent to 8th in 5.13s.

Have you ever tried the JT with 5.13s on the freeway on big tires? It makes it near stock again in driveability where as the 4.10s couldn't get out of their own way. The 8 speed ratios are magic for 5.13s and 37-39s

The JT no, but the JLU is the virtually the same. The thing was better on the freeway with 4.10s than 5.38s rolling 80+mph on 38s. Low load situations it would absolutely still see 8th gear with 4.10s. With 5.38s coming down a hill spinning 3k rpms I'm looking at it like it needs another gear. If the transmission gearing didn't matter, our ice rigs would be direct drive like an electric motor. Again it's not the torque, it's the horsepower output you need causing those downshifts. The jeep has virtually the same power output at the wheels with 4.10s in 7th gear as 5.13s in 8th gear as the final drive ratio is the same and therefore engine rpm and output are the same. The engine doesn't know if it's connected to a .84 overdrive gear driving a 4.10 axle gear pushing the load or a .67 2nd overdrive gear driving a 5.13 axle gear. It's the same load, same rpm, and same output. The difference is the 1 jeep can still upshift and drop rpms and the other can't. I'll stick to the higher end of the gearing and use 8th less while not looking for a 9th gear that doesn't exist for the way I drive personally. A JLU sport has 3.45 gears and 32s, numerically equivalent to 4.10s and 38s. The idea that the jeep can't "get out of its own way" is ridiculous. DOWNSHIFT. Again our JLU actually got .3 seconds slower 0-60 on 5.38s and 38s vs 4.10s and 38s disproving the whole "restored power" argument.

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[bleda2002]

The JT no, but the JLU is the virtually the same. The thing was better on the freeway with 4.10s than 5.38s rolling 80+mph on 38s. Low load situations it would absolutely still see 8th gear with 4.10s. With 5.38s coming down a hill spinning 3k rpms I'm looking at it like it needs another gear. If the transmission gearing didn't matter, our ice rigs would be direct drive like an electric motor. Again it's not the torque, it's the horsepower output you need causing those downshifts. The jeep has virtually the same power output at the wheels with 4.10s in 7th gear as 5.13s in 8th gear as the final drive ratio is the same and therefore engine rpm and output are the same. The engine doesn't know if it's connected to a .84 overdrive gear driving a 4.10 axle gear pushing the load or a .67 2nd overdrive gear driving a 5.13 axle gear. It's the same load, same rpm, and same output. The difference is the 1 jeep can still upshift and drop rpms and the other can't. I'll stick to the higher end of the gearing and use 8th less while not looking for a 9th gear that doesn't exist for the way I drive personally. A JLU sport has 3.45 gears and 32s, numerically equivalent to 4.10s and 38s. The idea that the jeep can't "get out of its own way" is ridiculous. DOWNSHIFT. Again our JLU actually got .3 seconds slower 0-60 on 5.38s and 38s vs 4.10s and 38s disproving the whole "restored power" argument.

HP and torque are linked, its not downshifting for one or the other, its downshifting for both. HP keeps you going, torque gets you there. Going up a hill, accelerating, passing, its downshifting for more torque and more hp since HP is just the amount of torque delivered in a specific time and torque is how hard each of this is. The HP lets you take a given load faster, the torque lets you get that load up to the max speed faster.

There is no argument to disprove, physics is physics it does indeed restore lost power (torque) at the wheels and no amount of anecdotal evidence will change that. Your slower speed was most likely torque management cutting power actually as the computer manages torque in 2hi and will cut power to limit how much torque goes through the diff. In 4hi gained 3 seconds 0 - 60 going from 4.10 39s to 5.13 39s. From 12 seconds to 9. Jlu on 5.13s and 38s gained 2.5 seconds 0-60 in 4hi as well. With out some outside force (torque management or an extra tranny shift, which for the 8 speed both will hit 60 in 3rd so it shouldnt be that if you put it in manual mode), it is physically impossible for 5.38s to be slower than the 4.10s, either that or engineers should be putting lower torque engines to increase 0-60 times.

 

[JTGuy]

I now have 37s and my speed is 10% slow. With my 4.10 rubicon diffs if I go to 4,56 I will be back to stock rpms with 33s. I have been told by about everyone that 4.88s are the best for 37s.
In our 8 speed trans 6 gear is the best at 1:1 and 8th is the worst being .67. Climbing long grades loaded in 8th gear might not be a good idea. But my MPG now sucks so going with 4.88s can't hurt and will probably be faster 0-60 with the torque increase.

 

[bleda2002]

I now have 37s and my speed is 10% slow. With my 4.10 rubicon diffs if I go to 4,56 I will be back to stock rpms with 33s. I have been told by about everyone that 4.88s are the best for 37s.
In our 8 speed trans 6 gear is the best at 1:1 and 8th is the worst being .67. Climbing long grades loaded in 8th gear might not be a good idea. But my MPG now sucks so going with 4.88s can't hurt and will probably be faster 0-60 with the torque increase.

4.56 will get you back to stock rpm's but you've also increased load by increasing rotational mass. It will get the engine back in the same power band for a specific speed, but the power its delivering is pushing a heavier load so it still wont feel the same as stock. Thats why folks overgear a bit is because you're not just losing power band but increasing force needs with extra load.

On the JLU, which is 100's of pounds lighter than our trucks, jeep goes 4.56 for 35's and AEV goes 4.88's for 37's.

 

[Wheelin98TJ]

....Again our JLU actually got .3 seconds slower 0-60 on 5.38s and 38s vs 4.10s and 38s disproving the whole "restored power" argument.

I'd have to see it to believe it that 4.10 is quicker 0-60 than 5.38 with all other factors being equal. That doesn't make sense.

 

[JTGuy]

I had my JT weighed for shock tune. It's 2720 front and 2620 rear. Total 5340. ARE camper is about 165 pounds. I'll be going with the 4.88s as recommended by everyone.

 

[JTGuy]

For all you math guys, what will my RPM be at 70mph with 4.88s and 36.5 tall tires? In 8th gear that is .67 overdrive.

 

[Zachanadandy]

I'd have to see it to believe it that 4.10 is quicker 0-60 than 5.38 with all other factors being equal. That doesn't make sense.

I didn't believe it either, but repeated testing proved it to be true. The 2.0t does recline at 6k, so I do believe there was an extra shift right before 60mph. My point is, gearing too low can absolutely have drawbacks. If it didn't every jeep would come with 5.38s. You aren't gaining power through mechanical advantage anywhere but 1st gear as that is determined by final drive ratio and as I pointed out, 7th gear with 4.10s is identical to 8th gear with 5.13s. I guess technically you also gain mechanical advantage in 8th gear, the exact opposite of what I want or need coming down a hill on top gear. Everywhere else your just a downshift away from the same output.

 

[Zachanadandy]

For all you math guys, what will my RPM be at 70mph with 4.88s and 36.5 tall tires? In 8th gear that is .67 overdrive.

2107rpm, or 2700 rpm at 90mph. 4.88s are a good combo for 37s. The guys suggesting 5.38s for 33s are off their rocker if you ask me. 3300 rpm in overdrive on the freeway? Hard pass.

 

[Wheelin98TJ]

I didn't believe it either, but repeated testing proved it to be true. The 2.0t does recline at 6k, so I do believe there was an extra shift right before 60mph. My point is, gearing too low can absolutely have drawbacks. If it didn't every jeep would come with 5.38s. You aren't gaining power through mechanical advantage anywhere but 1st gear as that is determined by final drive ratio and as I pointed out, 7th gear with 4.10s is identical to 8th gear with 5.13s. I guess technically you also gain mechanical advantage in 8th gear, the exact opposite of what I want or need coming down a hill on top gear. Everywhere else your just a downshift away from the same output.

I thought you were talking about a 3.6L JL.

Turbos need the load to generate boost. Deeper gears that rev quicker do not help this.

2107rpm, or 2700 rpm at 90mph. 4.88s are a good combo for 37s. The guys suggesting 5.38s for 33s are off their rocker if you ask me. 3300 rpm in overdrive on the freeway? Hard pass.

Who is suggesting 5.38 for 33s? I don't even see people suggesting 5.13 for 35s except for rare circumstances.

 

[Zachanadandy]

I thought you were talking about a 3.6L JL.

Turbos need the load to generate boost. Deeper gears that rev quicker do not help this.


Who is suggesting 5.38 for 33s? I don't even see people suggesting 5.13 for 35s except for rare circumstances.

Post #3 in this thread? Dude is going 5.38s with 33s.

 

[Wheelin98TJ]

Post #3 in this thread? Dude is going 5.38s with 33s.

Thanks, I see now. That's just one person and I'm not sure why they want to go that deep. I don't think I have ever seen someone recommend this for a 3.6L JT.

 

[Coldtoes]

I think this is a great tool to understand gear ratio, tire size and it's affects on rpm.

http://www.grimmjeeper.com/gears.html

This looks like a data heavy site which is very much my kind of site, however to use it we need to know a whole ton of numbers. I expect somewhere in this forum they can all be found if you dig enough and are lucky, but do you happen to know any of these already or know any of the threads that would have them?
Also, it has a field for "under drive", is that the same thing as a doubler?

 

[JTGuy]

I know that many go 5.13s with 37s but you loose a tooth off the pinion with that. I am going 4.88s so that if I had to I could run 35s easily. With 4.88s I could even go 38s but would have to re do the suspension. IMO, 5.38s are for 40+.

 

[bleda2002]

This looks like a data heavy site which is very much my kind of site, however to use it we need to know a whole ton of numbers. I expect somewhere in this forum they can all be found if you dig enough and are lucky, but do you happen to know any of these already or know any of the threads that would have them?
Also, it has a field for "under drive", is that the same thing as a doubler?

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