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post #1 of (permalink) Old 01-22-2007, 04:22 PM Thread Starter
 
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gearing for an auto

Currently I'm on 35's with an auto and a 34:1 crawl. I'm building a HP D44 front and plan to gear up and run some 37's once my current tires wear out.
I don't like the hunting that my trans does with the 4.56's I have now, and in low range it just doesn't feel low enough.

I'll be at 40:1 with the regear and find myself questioning if that's going to really be enough. So I could always regear the t-case and get it to 51:1 and wanted to find out from others just what they find to be a good ratio with an auto on a variety of terrains.
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post #2 of (permalink) Old 01-22-2007, 04:42 PM
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Re: gearing for an auto

Well i dont know what your going with for gears or what you have for a drive line but my YJ with the 4.3 v6 and 700 trans it works well with 4.56 gears and 36 TSLs I have a 4:1 low And i love that..
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post #3 of (permalink) Old 01-22-2007, 04:45 PM
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Re: gearing for an auto

Is it hunting, or is the torque converter locking and unlocking?
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post #4 of (permalink) Old 01-22-2007, 04:52 PM Thread Starter
 
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Re: gearing for an auto

4.3L, 4L60E, and an EB D20 is my drivetrain.

As for the TC, I hadn't really thought about that.
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post #5 of (permalink) Old 01-22-2007, 06:07 PM
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Re: gearing for an auto

Since we are talking 4.3L & 4L60E, (or 700R4), I'll throw in my two cents since I built a similar rig for my brother-in-law which is pictured below. It is sporting 33" tires on front and rear D-44's. It also has a D-300 with 4:1 low range. With the 4:1 low, wheeling is not a problem. What comes into play, since it's a daily driver, is highway drive ability. The key player here is having a useful overdrive or fourth gear. Gear too high, (lower numerically), and the overdrive becomes all but useless except going downhill, (hence your hunting problem, I suspect). Punkin has 4.56 r&p's and does okay, but I wish it had 4.88's. Using this logic for gearing, (4.88's w/33's), one can go to a gearing calculator, (http://www.ring-pinion.com/Calc_RPM.aspx), to come up with similar #'s for larger tires.

All comparisons assume engine rpm's for drive/overdrive at 60 mph:

33" tires 4.88 r&p's 3061/2087

35" tires 5.13 r&p's 3034/2068

37" tires 5.38 r&p's 3010/2052





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post #6 of (permalink) Old 01-22-2007, 07:58 PM
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Re: gearing for an auto

WOW!
Sweet Jeep.
That pic could be a commercial.
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post #7 of (permalink) Old 01-22-2007, 09:36 PM
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Re: gearing for an auto

I currently have a 34:1 crawl ratio with my YJ. That is with 4.56 gears. I have the 999 auto and stock gears in my tcase. I think that my ratio is perfect. I wouldn't change a thing.
With 37 inch tires you will be needing about 5% more gearing if you wanted to keep it equivalent to what you have. 37" tires have a circumference about 5% more than 35s.
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post #8 of (permalink) Old 01-22-2007, 11:05 PM
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Re: gearing for an auto

[ QUOTE ]
I currently have a 34:1 crawl ratio with my YJ. That is with 4.56 gears. I have the 999 auto and stock gears in my tcase. I think that my ratio is perfect. I wouldn't change a thing.

[/ QUOTE ]


Since your TF-999 final drive ratio is 1:1, your 60 mph engine rpms are 2697 with 35" tires. If you had 0.7:1 as do the 4L60E/700R4, then the rpms drop down to a rather unusable 1839, (might be okay for a V-8, but not for a V-6). To get the 6 cylinder engines to work well, 60 mph rpm's should be between 2,000 to 3,000, which in your case, your spot-on.
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post #9 of (permalink) Old 01-23-2007, 11:44 AM Thread Starter
 
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Re: gearing for an auto

Just to clarify there is absolutely no question about the axle gearing. I know what's going in there once I have the front D44 built.

I'm just curious if it's worth the effort to go from a 40:1 to a 50:1 crawl with the auto.
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post #10 of (permalink) Old 01-23-2007, 11:54 AM
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Re: gearing for an auto

The obvious answer is to wait until you can do some wheeling on the 40:1 crawl and see how it works. There's no reason to make two changes at the same time.

My guess is that you'll find it to be somewhat under geared. I run about a 40:1 on 31s and find it quite satisfactory. On 37s you're going to be 20% taller - about the equivalent of 33:1 on my 31s, and I don't think I'd like that.

On the other hand your automatic will make up for the deficiency somewhat while under power. But on the third hand, it'll be worse trying to use engine braking on the downhill side.
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