V6 Crawl

As one that has been through quite a bit of this a few years ago with my v6/T14/D18 combo....A couple things:
1) as Doug above says, the T14 and T90 have essentially the same first gear, so no benefit to crawl ratio there.

2) I run a T14 and tera low gears in mine, and that does significantly help with crawl. I also have 4.88 gears so that helps. Just to give you numbers (maybe you already did this), CR with your current setup is 3.1x2.46x3.73 = 28.4 (which is not particularly good). If you go tera low gears in the D18, that becomes 3.1x3.15x3.73 = 36.4. That is an improvement, but overall that is still not a lot of crawl - but it also depends on what type of wheeling you are doing, and also, the tire size matters as well. Before I added the tera low gears, I was at CR of 37, and that was okay for mild to moderate trails with 30-31 inch tires. Basically, adding the tera lows would be the equivalent of getting the optional 4.88s gears. Now as for Herms comment about needing and OD....Technically, you don't need an OD, but I assume he is getting at the difficulty in finding the correct tera low D18 drive gear that mates with the T14s unusual 10spline output shaft. There are some of them around somewhere, but you may have to scour the various forums and be patient - they certainly are not common. Because I have 4.88 gears, I run the OD so I can cruise at highway speeds, and yes, its probably easier (and more expensive) to use the OD that comes with the correct tera low and T14 10spline compatible planetary/barrel gear drive unit. And, yes, its probably hard to justify the cost of the OD with 3.73 gears unless you are really running a small tire.

3) There is going to be a lot of discussion about doing a truck 4 speed swap to get the granny first gear for crawl - such as the sm465 you mention. I put an sm465 in my 58...so I've now gone that route as well. Other common options include the T18 and SM420. In theory, they are the BEST solution....you get great crawl, you get a stronger transmission, you don't need tera low gears (keep the D18 as is), and your highway cruise stays exactly the same with 1:1 final drive ratio. In your case, just getting something with a 6:1 granny gear would get your CR to 55 (actually, it would be higher as all 3 of the options listed have a granny low in the 6.3-7 range). BUT, there is nothing 'bolt' in about the swap no matter what transmission you use. At the time I was working on this, I was not comfortable doing all the modifications needed, so I went tera low....it was something I could do at the time, it wasn't an overwhelming project, and it was more in my price point (as I didn't need a new OD - but I did have to buy the tera low compatible overdrive barrel gear to go with the actual tera low gears.

As you can see....the crawl ratio challenges of jeeps with 3.73 gears is not a trivially simple solution. My cost estimates might be a bit off these days as stuff is probably more expensive than I remember, but TLs will probably cost you $1500 (adding in the OD), you will get a small to modest gain in CR, but have no other changes needed in terms of driveline, cross-member etc. It can be done in a couple weeks by a relatively inexperienced person - it will basically be a rebuild of the D18 and replace the gears while in there, and you'll get new seals and bearings etc, and bolt it all back together. The 4 speed swap is going to be at least $2000 given you will probably need an adapter of some sort (either on the front or back end), cross-member and floorpan work, clutch work, and driveshaft changes at a minimum. The end result will be awesome, but its going to take months unless you've done this stuff before. Having gone through this myself, only you can decide what the best plan is (including just leaving well enough along and enjoying driving it as is), and yet, its really hard if you are not experienced or can't drive other people's examples to feel the differences, to know what is the best plan of attack. Just my useless 2 cents, but unless you can find the tera low drive gear and skip the expense of the OD, I'd probably just drive it as is until you know you want to go all out and swap the transmission. good luck....

 

the 26 tooth, 10spline drive gear for the 18 is VERY hard to find....

I understand this is pretty standard numbers in the industry, but even for all the wheeling I do, I've still yet to get rid of my T14 for a 4speed and have just enough CR with with 33s to do what I need. its not ideal, but depending on the OPs usage (and my personal experience), there is a lot of pressure here to go 4speed, when in some cases, its not really necessary - just nicer to have - and also depends on skills and budget. In the case of the OP, if CR in the 30s is enough (plenty good for most easy and moderate trails, especially with smaller tires), it certainly is the easiest. If I had tried to do a T18 swap years ago instead of the TLs, I'd probably still have an unfinished jeep. Instead, I have a jeep that is not ideal, but its just good enough and has been driven A LOT over the last 10 years.

 

I agree the final number is small, but disagree that its not noticeable....That is a 20% improvement - definitely noticeable. When I went from 37 to almost 48, it was VERY noticeable.

 

There is/was a D20 a tera low kit - assume herm is carrying that as well now....you may have the same challenge with the drive gear. T14 has the rare 10 spine.