whats the best trans for my 6.9idi. i need help to deside, its an auto and i want to keep it that way if i can. i need a better gear ratio too. can you help?

 

your looking for the E40D.it sports a locking converter,over drive and is electronically controlled.you'll need the harness and ecm from a parts truck or an aftermarket controller w/harness.
skip '89 and look for diesel donor trucks with this trans from '90-94 for a bolt in swap.search other things in the idi section such as trans cross member and drive shaft length etc for details.
you can find a PSD diesel 94-97 donor e40d trans as well,but you would need an early solenoid pack,the ecm and harness from an idi truck (unless going aftermarket) and 4 stud diesel converter to mate it behind the idi so in your case,finding an idi/e40d donor is your best bet.

this is a costly swap and very seldom done,due to the swap costing most likely more (perhaps several times over) than many idi trucks value.
those who do add OD, opt for the more affordable zf5 manual trans option to get an over drive trans in an older idi truck.often it's simply easier and more cost effective,if you just have a basic oem pickup setup that's easily/readily available, to just buy a more modern truck with an od already.take a good objective look at your rig first.good luck.

 

You might consider getting something like a Gear Vendor under/overdrive unit. They're not cheap (around $3000 new), but will do what you need and will give you twice as many gears. Once in a while they pop up second hand on places like eBay too.