Hey guys, need some help from people smarter then me. I am putting a 6.0 in my '85 swb C10. I just got a th350 rebuilt a few yrs ago with less then 10 miles on it. Can't decide if I need to go 4l80 or if I can keep the trans. Truck will be a driver. Would like to get around 2000 rpm/70mph. Truck has 3.42s and will have 285/35/22s on the back 275/35/20s on the front. What rpms will I run at 70mph with the th350 vs 4l80e??

 

With the 285/35/22s and 3.42s in the rear you will be at 69mph at 2000rpm with the 4l80e, and 52mph at 2000rpm with the th350.

Or said another way, 2000rpm at 69 for the 4l80e and 2700rpm at 70 for the th350.

Might want to consider a gear vendors overdrive if you want to keep the th350.

 

How did u figure that up? I don't know which route is best 4l80 or overdrive in the th350 let me know some opinions

 

Bump! If I go the 4l80 route I was planning on using 91 burban guages bcuz their electric to make the speedo and all work, what options do I have for making the speedo work with 285/35/22s and a rear gear swap with the th350?

 

Nevermind, I'm going with the 4l80e, that overdrive is almost $3k