The T176 is simple to rebuild. If you have some basic skills, no problem
If you swap in a T176 to replace a T4/T5/SR4, you will need the following parts for a complete "bolt in swap":
1) Transmission
2) Bell housing
3) Drive shafts
4) Dana 300 shifter handle for a T176
5) T176 reverse light harness
6) T176 floor plate
7) T176/Dana 300 combination shifter boot
I kept all of the above in the back of my Scrambler when I went to the Kentucky National event a few years back when I was still running a T5
I blew three T5's which were subjected to moderate use/abuse with 33's and MPI equipped 4.2. For a stock Jeep/stock tires they are fine. You can upgrade gears and spend all kinds of money on "upgraded parts", but there is nothing you can do about the little sliver of case material located between the front input shaft bearing and the front cluster bearing. I have three broke cases in my garage, with the corresponding wiped out cluster gears/main shafts
All three of my T5's blew up on the highway, just cruising in 5th gear at around 75 mph.
Depending on axle gears/tire size/engine health, overdrive might not even be necessary
I bought and drove my "now red Scrambler" from Indiana back to Biloxi, MS with a T176, healthy 4.2, 32" tires, and 3.31 gears. I got around 17-18 mpg on I65 the entire trip home. "Yard Scrambler" is set up the same, same performance. Neither Jeep is/was a "rock crawler, but for daily driving, light to moderate off road use, and highway use, I don't know how much more you could want
The T176 is a much stronger transmission then any of the other three 80's CJ manual transmissions. The parts are bigger, and in a transmission, bigger gears/shafts is generally better. The T176 main shaft runs on ball bearings, the others on taper roller bearings. The T176 case is much stronger/thicker/heavier.
The "granny gear" 4 speeds are probably stronger then a T176. I had a T18 in my old CJ7. They are tough, but I eventually wore out/damaged my T18, resulting in a full rebuild. The "granny trannies" are not as fun/useful for daily driving. The T176 uses all four gears, easy/close shifts. The HD 4 speeds have long throws, and you typically drive them as 3 speeds on road. Depending on power/tire size/gearing, this can result in sluggish performance.
The only problem I have with the T176's is the sloppy feel of the shifter. The shifter parts wear out over time. The crappy non OEM replacement parts suck. But, as long as the shifter parts are good, the T176 shifts fine.
I have a T5 in my stock green Scrambler. I like it there, it shifts nice. This Scrambler does not see hard use. It should live. It has 125,000 miles on it, so it is time for a "pre-emptive rebuild".
I now have a 4:1 first gear AX15 in my red Scrambler. I love it. The ratios and shifting characteristics are almost identical to a T5, BUT the transmission is much stronger
If you drove my red Scrambler and did not know I swapped the transmission, you would think it was a T5.
Before doing a swap, maybe rebuild what you have. What you have for your uses might be fine. A rebuild can buy you time to find all the parts/save up $$$ for a future swap
As for the MPG gains of the overdrive transmission, depends
Low axle gears will almost certainly require an Overdrive, unless you want to drive slow on the interstate. My "real world experience here:
1) Carbed 4.2, 3.31 gears, T176, 32" tires = 17-18mpg at 75mph (two separate Scramblers like this)
2) Carbed 4.2, 4.10 gears, T176, 35" tires = 15-16mpg at 70-75mph
3) MPI 4.2, 4.10 gears, T176, 33" tires= 16-17mpg at 65-70mph
4) MPI 4.2, 4.10 gears, AX15/T5, 33" tires = 16-18mpg at 75-80mph
It all depends on how far you want to go, how fast, how much RPM you want to spin, how much gas you want to burn
Another important factor to think about: what type of geography do you do most of your highway driving at. If you live somewhere really hilly, at altitude, with long steep grades, and you run a 6 cylinder with conservative axle gears and tire size, OD might be a waste
I swapped in the AX15 into my red Scrambler because I drive it cross country and I want to drive fast. When I had the T176, it drove fine on the highway, and got the same MPG as the OD tranny IF I kept it between 60-65mph. The OD only allows me to go faster. I still burn almost the same amount of fuel. A Scrambler is a brick. No way around it. Unless you do a bunch of highway driving, it might never make economic sense to do a transmission swap.
Keep records of how you drive your Jeep, and the MPG. You might just find that not having OD is not the end of the world, unless you want to go fast, or have low axle gears.
