700R4 Transmissions Swap and Gearing Questions for M1008

[TheControlVoice]

Apologies upfront if these are stupid questions. I know very little about gearing.

The M1008 I bought had the 4.56 gear and Detroit locker removed and replaced with 3.73 gearing (If memory serves). I was looking at doing a 700R4 transmission swap and I wanted to know if I should put the gearing back to 4.56 and if the 700R4 would allow for highway speeds? I know it adds overdrive. (the seller gave the me original 4.56 gear and Detroit locker)

My understanding is that the original gearing will red line the RPMs around 60MPH but the 3.73 gearing will allow higher speeds but will have slower acceleration/less torque.

Side question, I read that some people also install the Detroit Locker in the front diff. Is this recommended?

Thanks in advance.

 

[adf5565]

I wouldn’t recommend going back to 4.56. I did the 700r4 swap recently and it allows highway travel at 65 at ~2200 rpm which is great. However since I have the original 4.56 gears, it limits highway speed downshifting options and my speed has to drop to 55 in order to shift into 3rd with the torque converter locked up and even then the 6.2 is screaming. With 3.73, you’d be able to downshift to 3rd and still be in an appropriate rpm range.

I’ve considered going to 4.11s, however it is quite pricey for that change and might not be worth it. From what I understand 4.11 and below are a different carrier than the 4.56. So I’d have to change carriers as well. But in your case, you should have the different carrier already, so if you really wanted, the 4.11 would get you a little more pep while still allowing for 3rd at highway speed. But would cost a few bucks for minimal benefit.

In my case, I’m going to add a turbo in the near future so that ideally I have enough power the 700r4 rarely has to shift out of OD on the highway.

With regards to the front locker, if you’re changing gears it would be easy to add, however again not sure it’s worth it unless you’re planning on going through some real sh*t. Assuming you still have the Detroit in the back that and 4wd will get you through most things. Just my opinion there.

 

[chevymike]

Tire size is also a factor you have to consider. If you are keeping the tire size 33" or under (stock was about 31") then leave the 3.73's. If you are going to 35" or taller, then I would swap back the 4.56's.

 

[TheControlVoice]

I wouldn’t recommend going back to 4.56. I did the 700r4 swap recently and it allows highway travel at 65 at ~2200 rpm which is great. However since I have the original 4.56 gears, it limits highway speed downshifting options and my speed has to drop to 55 in order to shift into 3rd with the torque converter locked up and even then the 6.2 is screaming. With 3.73, you’d be able to downshift to 3rd and still be in an appropriate rpm range.

I’ve considered going to 4.11s, however it is quite pricey for that change and might not be worth it. From what I understand 4.11 and below are a different carrier than the 4.56. So I’d have to change carriers as well. But in your case, you should have the different carrier already, so if you really wanted, the 4.11 would get you a little more pep while still allowing for 3rd at highway speed. But would cost a few bucks for minimal benefit.

In my case, I’m going to add a turbo in the near future so that ideally I have enough power the 700r4 rarely has to shift out of OD on the highway.

With regards to the front locker, if you’re changing gears it would be easy to add, however again not sure it’s worth it unless you’re planning on going through some real sh*t. Assuming you still have the Detroit in the back that and 4wd will get you through most things. Just my opinion there.

Thanks for the info! How is the acceleration with the swap? I dont have the 4.56 installed but I feel like the acceleration is really slow. Feels almost bogged down to me. Does the gearing and or 700r4 change the acceleration performance?

Side question on the turbo you want to buy. I was going to post this question but I might as well ask here since you brought it up. What turbo are you going with? The Banks? I have read over and over that people use the Banks turbo but I cant find it for sale unless I am misunderstanding.

 

[Keith_J]

700R4 has a deeper 3.06:1 first gear ratio so it is about identical with 3.73 axle as the THM 400 2.48:1 with 4.56 axles.

Now the OD with 3.73 axle is very steep for stock engine torque and tires over 33" in diameter.

 

[TheControlVoice]

700R4 has a deeper 3.06:1 first gear ratio so it is about identical with 3.73 axle as the THM 400 2.48:1 with 4.56 axles.

Now the OD with 3.73 axle is very steep for stock engine torque and tires over 33" in diameter.

So are you saying if I did the swap and kept the 3.73, the performance would be identical with the original transmission and having the stock gearing?

 

[adf5565]

Thanks for the info! How is the acceleration with the swap? I dont have the 4.56 installed but I feel like the acceleration is really slow. Feels almost bogged down to me. Does the gearing and or 700r4 change the acceleration performance?

Side question on the turbo you want to buy. I was going to post this question but I might as well ask here since you brought it up. What turbo are you going with? The Banks? I have read over and over that people use the Banks turbo but I cant find it for sale unless I am misunderstanding.

I think acceleration is pretty good. Not a hot rod but I can make it to driving speeds in a normal amount of time. Towing stuff is a little slow but truck itself is fine.

They don’t make the banks kit anymore so you’d either have to find a used one or the next easiest is to use 6.5 parts, which is what I’m probably doing. Lots of posts on here on how to. It’ll still never have a lot of power but enough to help

 

[TheControlVoice]

I think acceleration is pretty good. Not a hot rod but I can make it to driving speeds in a normal amount of time. Towing stuff is a little slow but truck itself is fine.

They don’t make the banks kit anymore so you’d either have to find a used one or the next easiest is to use 6.5 parts, which is what I’m probably doing. Lots of posts on here on how to. It’ll still never have a lot of power but enough to help

Ah ok, that's what I figured since I could never find it for sale. And I remember reading about using ones for the 6.5. Same issue with the after market headers. The ones everybody talks about are no longer for sale since the company went out of business.

 

[Keith_J]

So are you saying if I did the swap and kept the 3.73, the performance would be identical with the original transmission and having the stock gearing?

Yes, 3.73 axles with 700R4 is close enough to equal as 4.56 axles with THM400. 11.4:1 vs 11.3 first gear ratio X axle respectively. You won't notice the difference.

 

[TheControlVoice]

Yes, 3.73 axles with 700R4 is close enough to equal as 4.56 axles with THM400. 11.4:1 vs 11.3 first gear ratio X axle respectively. You won't notice the difference.

ah ok. Got it. And the overdrive wont add any benefit?

 

[Keith_J]

ah ok. Got it. And the overdrive wont add any benefit?

Overdrive would be a tad steep for 33 inch diameter tires. 2000 RPM at 70 MPH so expect shifting out of OD.

 

[Keith_J]

Detroit Locker in the front makes for some handling quirks. M1031 and M1028s came with friction disk type limited slip as even these have a little squirrely handling. I drove my M1031 about 70 miles during the February 2021 snow Armageddon..I needed 4wd often, keeping hubs locked and shifting the NP205 when needed.

I had to visit my mother, some 35 miles away. Average speed 15 MPH. One detour had me going through an industrial park with a steep uphill drive and right turn in deep snow. Like a tank in 4 high but in 2 high with locked hubs, I could not turn on ice. Tires are Cooper Discovery ST Maxx.

 

[nyoffroad]

I kept the 4;56 gears and added a 700R4 within the first year and never looked back! I did have the tranny gone thru and added a corvette servo and a 0.5" TV (?) valve along with the hydraulic lock up on the converter and tires in the 31-32" range can 'comfortably' run 70MPH If you can stand the tire/wind noise. With different gearing your mileage and cruise ability should go way up. I had a 2 wheel civvie truck with 6.2, 700R4 and 3:73 (i believe) and it drove and towed GREAT and got 22MPG!