My Possible Project (1990 Suburban)

[cal30_sniper]

Semper Fi Brother. Nice looking truck.

I have about 0 experience with the diesels in these trucks, but I do have a lot with the transmissions. If it were mine, I would not swap out the TH400 for a TH700. The 700 is a good trans when done right, but it's hard to get there (and often expensive as well). If you're looking for a gas mileage boost, check out gear vendors overdrives. The TH400 is one of the most common applications for a gear vendors, and you can find them pretty regularly on craigslist and ebay. I'm in the process of finding one for my 454/TH400 powered C20 burban right now.

The TH400 is one of the strongest automatics ever built for passenger cars/light trucks. It's right up there in strength with 727 torqueflite, and way ahead of the TH700. With a gear vendors, you're basically adding a .78:1 overdrive to the back of your TH400. In my opinion, that's a better 4th gear to have than the .70:1 overdrive of the TH700. That's a lot of overdrive to be pulling in a big vehicle with really bad aerodynamics at highway speed.

If you're planning on going to 35"ers from the 32"ers that are on there, better look at 4.10:1 gears or better. You should have 3.73:1 gears in there now (possibly shallower due to the diesel). You'll want to increase the gearing to get your cruising RPMs back up to where they should be. Increasing the tire size and adding an overdrive without changing the gears could be result in a 4th gear that you can't really use.

That's just my suggestions, I'm sure others on here will have some great input too.

-cal30sniper