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-   -   Gears and Tire size with Auto and 6 Speed (http://www.jeepforum.com/forum/f96/gears-tire-size-auto-6-speed-1075016/)

MilitaryJK4455 07-23-2010 01:59 PM

Gears and Tire size with Auto and 6 Speed
 
To potentially stop this current influx of threads about the same subject everyone tune in here... If any of this is wrong please discuss and prove me wrong...

You guys are not listening to what we are saying just by reading some of these responses.... 3.73's or 4.10's in a STANDARD 6 speed transmission is just fine... You will get around 15 in town and 17-20 interstate depending how you drive.... This is not applicable with the AUTOMATIC transmission... An auto with 3.73's or 4.10's is going to be a dog with bad gas mileage... check out this chart...

http://www.jeepforum.com/forum/attac...atio-chart.jpg

-An automatic with 35's would have to gear to 4.56's just to get to the performance of a standard tranny with 3.73's and 35's.
-An automatic with 35's would have to gear to 5.13's just to get to the performance of a standard tranny with 4.10's and 35's.
-An automatic with 35's would have to gear to 5.38's just to get to the performance of a standard tranny with 4.88's and 35's.

-A standard tranny with 35's and 3.21's is a dog...
-An auto tranny with 3.21's does not exist...
-A standard tranny with 35's and 3.73's is a doable set up (not a dog).
-An auto tranny with 35's and 3.73's is a dog...
-A standard tranny with 35's and 4.10's is a decent set up (lower end of stock performance)
-An auto tranny with 35's and 4.10's is a dog...
-A standard tranny with 35's and 4.56's is a good set up (higher end of stock performance)
-An auto tranny with 35's and 4.56's is a doable set up (not a dog)
-A standard tranny with 35's and 4.88's is an ideal set up (lower end of high performance set up)
-An auto tranny with 35's and 4.88's is a doable set up (close to stock)
-A standard tranny with 5.13's should be running 37's (getting to the highest end of high performance)
-An auto tranny with5.13's has just gotten to a stock set up (the lower end of stock)

Catch my drift?

Even some of these calculations are giving the auto the benefit of the doubt...

1398653 07-23-2010 02:12 PM

I have 3.21 6 speed with 34 inch Goodyear MTRs which measure the same size as a lot of 35s out there. It is ok on the highway and around town, not great, I would describe it as livable and not that annoying. Off road it is a pain in the arse. It takes quite a bit of skill crawling rocks and keeping momentum going up steep hills. BUT it is doable. That being said I am regearing to 4.88s to get more off road performance. I just thuoght I would add some personal obervations involving 3.21s and 34-35s. a lot people make it sound like 3.21s with 35s will be like driving your jeep with square tires. It is not as bad as some make it out but is certainly isnt ideal. When I went on a jeep jamboree the guides kept asking me if I was in 4 wheel low. I had to explain it to several people that my gearing sucks.

Imped 07-23-2010 02:13 PM

Nice thread. To add to it:

The 6 speed manual offered in JK's is the NSG370. It's a Mercedes-sourced unit that was also used in 05-06 TJ's.

Gear ratios:
1st - 4.46
2nd - 2.61
3rd - 1.72
4th - 1.25
5th - 1.00
6th - .84
Reverse - 4.06

The 4 speed auto used in the JK's is the 42rle. It was used in 03-06 TJ's as well.

Gear ratios:
1st - 2.84
2nd - 1.57
3rd - 1.00
4th - 0.69
Rev - 2.21

Now look a little deeper. The OD on the 42rle is 15% higher than the NSG370. So, for any given tire size, RPM's will be 15% lower. This is why the axle gearing needs are very different for the two transmissions. Just saying "you need 4.88 gears for 35" tires" doesn't do it. You're missing out on half the story. Axle gearing is simply a function of engine, transmission, and tire size. The NSG370 also makes use of a much lower first gear, which is awesome for crawling. Lastly, it has 2 more gears before OD, so the RPM drop between gears is reduced considerably compared to the 42rle, helping the engine stay in its powerband. Combine all of that and the numbers speak for themselves--4.10's and 35's are very liveable with the NSG and not even close with the 42rle. To give you the same performance as an equivalent JK with the NSG, you should gear ~20% deeper with the 42rle.

MilitaryJK4455 07-23-2010 02:19 PM

Quote:

Originally Posted by Imped (Post 9838879)
Nice thread. To add to it:

The 6 speed manual offered in JK's is the NSG370. It's a Mercedes-sourced unit that was also used in 05-06 TJ's.

Gear ratios:
1st - 4.46
2nd - 2.61
3rd - 1.72
4th - 1.25
5th - 1
6th - .84
Reverse - 4.06

The 4 speed auto used in the JK's is the 42rle. It was used in 03-06 TJ's as well.

Gear ratios:
1st - 2.84
2nd - 1.57
3rd - 1.00
4th - 0.69
Rev - 2.21

Thanks imped... That will do people some well in the future for the F.A.Q thread... You sure do know a lot about JK's... You sure you dont own one?:shhh:

Imped 07-23-2010 02:42 PM

Sure don't. The JK is still a Jeep with solid axles at both ends. Everything is transferrable. Having the same transmissions as TJ's doesn't hurt, though.

MilitaryJK4455 07-23-2010 02:46 PM

Quote:

Originally Posted by Imped (Post 9839056)
Sure don't. The JK is still a Jeep with solid axles at both ends. Everything is transferrable. Having the same transmissions as TJ's doesn't hurt, though.

Yeah, I guess thats true...

Imped 07-23-2010 02:53 PM

I'm an advocate for applied physics, geometry, and just going by the numbers. Numbers don't lie. It doesn't matter if we're talking about YJ's, TJ's, JK's, XJ's, etc. If you understand the fundamentals, those are just letters. Math and physics doesn't change. Your thread does a good job of explaining that. The problem is most people don't view things in black and white like that....the JK section especially.

PS, the gearing charts are excellent. I'm glad someone else got as fed up about gear threads as me.

MilitaryJK4455 07-23-2010 03:05 PM

Quote:

Originally Posted by Imped (Post 9839116)
I'm an advocate for applied physics, geometry, and just going by the numbers. Numbers don't lie. It doesn't matter if we're talking about YJ's, TJ's, JK's, XJ's, etc. If you understand the fundamentals, those are just letters. Math and physics doesn't change. Your thread does a good job of explaining that. The problem is most people don't view things in black and white like that....the JK section especially.

PS, the gearing charts are excellent. I'm glad someone else got as fed up about gear threads as me.

Well, dont give me credit for the gear chart... I did not make it... I was actually confused for the longest time as a newb because all that I had ever seen for the JK was an auto chart so I thought that my Jeep with the 3.73's according to the chart was going to be a complete dog with the 35's on... Then I stumbled upon the standard tranny chart and realized that all would be fine and dandy :thumbsup:

rballman 07-23-2010 03:07 PM

And here I was about to start another thread!!




:shhh: JK

MilitaryJK4455 07-23-2010 03:10 PM

Quote:

Originally Posted by rballman (Post 9839200)
And here I was about to start another thread!!




:shhh: JK

:laugh: Your a **** ball... :highfive:

rballman 07-23-2010 03:14 PM

I plan on running 46s one day. Can you find a better chart that has my size on it?? :D






...ok I will stop now. This is good information; nice work.

MilitaryJK4455 07-23-2010 03:17 PM

Quote:

Originally Posted by rballman (Post 9839249)
I plan on running 46s one day. Can you find a better chart that has my size on it?? :D






...ok I will stop now. This is good information; nice work.

Yeah... You'll need new axles and 5.86's... possibly 6.17's you **** miester :rofl:

Farmdiesel 07-23-2010 08:00 PM

I think if that Tranny chart was a sticky it would help Military JK. When I 1st started looking at JKs I didn't understand the difference b/t the auto and the 6sp. Knowing the stock differences b/t transmissions is huge when you are picking gear and tire sizes. When I drove my 6sp with 4.10s home for the 1st time I was freaked out and thought I had made a big mistake until I did a little more research.

MoparToYou 07-23-2010 08:40 PM

I would add another line at the bottom of your opening post for auto tranny with 5.38 gears. It is still only "acceptable" based on your chart, and I would agree with that. 5.38 gears are acceptable, but not great by any means. For performance gearing, or high altitude, in an auto tranny Jeep, you need lower gears than 5.38. 5.86 gears or 6.17 gears would be more appropriate for performance, and or high altitude use. Too bad they don't make 5.86 or 6.17 gears for our axles.

I live at 6000 feet elevation, at the base of a mountain that is 11,000 feet elevation, and I have 35" tires with 5.38 gears in my auto trans Rubi. Hate to say it, but it is still a dog. My Jeep downshifts just to make it over a freeway overpass. Another hundred horsepower under the hood would help.

CooterBrown 07-23-2010 08:45 PM

so then why does my stock JK rubicon fall into the yellow?


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