lower geared tranny Vs gears

/wwwthreads_images/icons/tongue.gif Several things are worth considering here, Andy. First off, if you run lower axle ratios, the driveline DOES have less work to do.....the torque will be less with a 4.56:1 than with a 3.73:1 axle ratio. So in that view, a lower axle ratio is a plus. There is, however, a problem with using a lower (and in this case I mean lower SPEED, not lower numerically)axle ratio, and that is that the pinion speed goes up and that's a disadvantage in a sustained run across country, AND....there is less of the pinion gear TOUCHING the ring gear. Low ratios mean smaller pinions, and that means less metal does more work. We used to get into this when we ran trucks with big gassers for power. The 392 Chrysler Hemi was a big favorite, but it DID have a higher cruise/pull RPM than did the 413 Chrysler non-hemi. The difference in RPM made the rear axles behind the 413 a slightly higher ratio, and they lasted longer because the pinion speed was lower. The other side of that issue is the gearbox, and you can manipulate that endlessly. I set up my favorite truck with a four-speed brownie with double-under for extreme power, but the problem was that if someone else drove it and was not careful pulling through a tough spot, they blew the driveline out just behind the brownie from too much torque caused by the extreme reduction. Of course they couldn't just blow a driveline....it had to whip around and puncture an air tank too and lock up the brakes and.......well, you get the idea/wwwthreads_images/icons/frown.gif Personally, I still like to take the reduction in the gearbox, and have a higher rear axle. I feel that the axle will be stronger for it, and I would rather blow a DRIVELINE and fix it than blow out a rear axle pinion gear and be really stuck./wwwthreads_images/icons/smile.gifWhether they realize it or not, when a Jeeper goes to a bigger axle, he is attempting to over come the shrinkage in diameter that the pinion in his own axle would experience if he went to a lower gear in THAT axle. By starting with a bigger axle, he can have just as much gear surface if not more even after going WAY down in ratio. So you can see that if you are looking for a slow-speed axle, there is absolutely no substitute for size./wwwthreads_images/icons/smile.gif Try to imagine what it takes to put 450 HP to the road at only 700 or 800 RPM like a Diesel truck does. Is it any wonder that they use 1900 Series Spicer 8-Bolt U-Joints?/wwwthreads_images/icons/crazy.gif

CJDave
I never believe any statistics unless my moonguys /wwwthreads_images/icons/crazy.gif/wwwthreads_images/icons/wink.gif made 'em up themselves.

84C-J-7

The answer to your question is no. If you start with 100:1 as an overall crawl ratio and start dividing out the factors, here's what you'd get:

100:1/5.88's (lowest gears, effective anyway) = 17.01

17.01/2.62 (D300 low) = 6.49 first gear.

Now, you say, suppose I throw in another D300 or a 4:1 Terra Low? Yes, then it would work.

Yes, a lower first gear is nice, but then when you leave first it's not so nice anymore. I like a mix (which is also a stronger combination), like you stated, to achieve a "working" ratio. It depends on what you're looking to do for wheeling. For the all around mud, rock, and general woods type stuff, I find 50:1 to 75:1 is about as low as you'd want to go. Leaves a lot of room doesn't it? It's nice to be able to use the lower gears, but I use mine only occasionally. It'd be better to have a well rounded gearset. For instance, with my 4cyl, I don't have a lot of power to sprare, so I have to choose my gears more carefully than someone with a V-8. When I get to a mud pit, I usually choose second, in high gear, hold it to the carpet and hope it doesn't stop. If I try to change gears in the middle I'm done, no power to recover. If I were able to shift when needed, it'd be a different story. I know my problem is mainly power, not gearing, but a gearing change will help compensate for the loss of power. Eventually I'll have the doubler, which will allow me a second low range and thus lower crawl, but this is secondary to the additional ratios I'll achieve. There is such a wide split between first and second it's hard in some areas. One is too fast while the other too slow. With the D300's it's nice, because the ratio is multiplied (or divided) by 2.62, not my 2:1.

I don't know if this made any sense, I gotta stop posting until AFTER the morning caffeine intake. /wwwthreads_images/icons/smile.gif

JEEPN
'81 CJ-8 Scrambled!
GM151/SM465/NP205 twinstick/7" Lift/33" TSL's/IHC Scout II D44's 4.10's & Lockrights

Andy,
CJDave is right, if you lower your crawl rate in your transmission then it will put alot more stress on the driveshafts and u-joint. If you go too low in the gears in your front are rear diff then you will put undue stress on the ring and pinion. I have seen this first hand time and time agian. (example)Sure a TH-350 or TF-727 with a Dana 300 and 38" tires and 3.42 gears in GM axles will wheel fairly good but the stress on the drive shafts and u-joints become tremendous, thus twisting your drive shafts intwo or snapping joints will be a VERY common problem. Now, lets go the other extreme with the same setup but with 5.39 or 5.89 gears, you end up with a super crawl rate but you loose the strength of the pinion and under high stress will most likely break. Now, with all this said I am using the smaller of the diff as examples (Dana 44's model 20, Dana 30's ect), you really need to match up your rear ratio, tire size, a gearing in the tranny and transfer case so that all parts absorb the pressure equally so that breakage is down to a minimum.
When you go to the bigger diff like the Dana 60 35 spline, 14 bolt full-floaters, Dana 70 ect. with big tires like the 39.5 Boggers or larger tires then you might need to build in a weak link like lockout hubs and/or u-joint so that they will break before your expensive oversized diffs.
This is just how I go about seting up my rigs and I may be way off base compaired to some of the ways other guys set theirs up but I have had really good luck doing it this way and hardly breaking even smaller diffs and joint like the Dana 30's.
The rig I am building now is going to have a 360, TF-727, Dana 300, 14 bolt rear with a detroit and 4.56 gear, the front is a Dana 60 35 spline with 4.56 gears and a lockright. I have left two weak links in my setup so that they will break before a axle shaft (in theory anyway), the lockout hubs and the 30 spline outer shaft on the front, the rear is a harder pig to tackle, with the strength of the 14 bolt FF I am pretty sure that the drive shaft joints or the shaft itself will give way far before anything in the rear does.

Enough of my rambling, just my 2 cent worth.

Peanut

GUYS!!! i have a couple more questions!! on my cj with a 258 and 31s and also a t4 and stock gears i am not sure i could not find the tag gears! (but that not is so impotnant if u can explain what happened). I went on a trip through the mountains to truck show! and i had a problem with climing the hills!! they were huge long inclines the hill sucked the life out of my jeep and i was going slow! also the hills sucked a lot out of cars but i was remarkbly slower! what could have helped me better matched gears for 31S???
THIS is my plans please feel free to jump at me if i should do anything diffrent!!
I am planning on keeping the 258 for a while in the jeep and T4 so the thing is no power house so i eas planning on running stock axles as i already have one piece axles and a locker for the rear in another axle!
so heres my setup
258 to a t4 to d300 amc 20 and dana 30 and run 4.56 and 35s and 33s i might have to run 33s longer than i want to (to many projects going on and i do not picture 35s under the jeep until almost fall) and i was wondering if this would get me to a respectible jeep for the trail and not be completely a dog on the street? 35s 4.56 and a 258 on the street?? not that i have to race on the street or drive it daily! so i figured as far as upgrading a tranny the only advantages i would have would be strenth and for the fact of lower first gear! but also the lower first gear will not help me as much on the street! as the gears! should i do anything diffrent! I do a variety of off road!
thanks guys for the help!!!!
Andy

/wwwthreads_images/icons/smile.gif I have the moonguys/wwwthreads_images/icons/wink.gif/wwwthreads_images/icons/wink.gif/wwwthreads_images/icons/smile.gif; that is those that can even move after a night of.......well, you get the idea....busy working up a solution to your gearing problem. Here is what they came up with/wwwthreads_images/icons/crazy.gif! Andy.....it all gets down to TURNS PER MILE. If you looked at a completely stock 258 Jeep, and got the turns per mile that the stock tires made, and multiply that by the axle ratio, that will give you DRIVELINE TURNS PER MILE @ 60 MPH. So that is what the factory guys felt that the Jeep engine could handle. If you adjusted the tire diameter such that the turns per mile of the driveshaft was LOWER, well....the engine doesn't make enough HP at that RPM to do the job, and you have to do SOMETHING to get back to "original" turns per mile. You CAN shift down and bring the RPM back up, but you need to understand that the minute that you get away from DIRECT DRIVE, there is a loss through the gearbox. The loss may be small, but there IS a loss through the box, and it takes HP to overcome that loss, so you net out less useable HP out the tail end. Before the advent of twin-countershaft transmissions, trucking companies used to spend a lot of time and money to "gear" the trucks for "Direct-Drive" so that the majority of the time they could run straight through the boxes and save HP/Fuel. Calculating the driveline turns per mile will be a big help in determining where you actually are in terms of useable power. You can see now how this all points to a bigger, lower-speed axle swap. To help you figure this, you can get the tire turns per mile from the books at the tire shop.My moonguys/wwwthreads_images/icons/crazy.gif/wwwthreads_images/icons/crazy.gif/wwwthreads_images/icons/crazy.gifare too hung over to do much more on this Andy, hope this helps.

CJDave
I never believe any statistics unless my moonguys /wwwthreads_images/icons/crazy.gif/wwwthreads_images/icons/wink.gif made 'em up themselves.