i have a 93 dodge diesel 4x4 with an automatic and 355 gears. i am wondering if anyone has ran 307 gears behind an overdriven tranny, what kind of mileage was achieved and if there was a big difference in what and how you could tow stuff.

 

i'm working on a 47RH conversion on my truck. and to my knowledge my truck has the 3.07's... i'm lookin to run a gear close to the 3.55, i believe the parents truck with thr 47RE is around 3.8_ somethin gears. they can turn 2200 rpm down the highway at about 75-80 mph and get around 19mpg on a bad day. i dont see why 3.07's and an overdrive unit couldnt get you around 25-28 mpg. it'd be perfect for flat land. any kind of hills tho its going to put a good load on the engine. it wont "maintain" speed and pulling power will be reduced. how soon are you looking to do the gear swap? if things work out and i re-do the gears anytime soon i'll part with the 3.07's

 

It will be sometime this summer. Still getting snow here in Utah. Thanks for the info though. One more question, what all has to be done to do the 47RH conversion in the first gen's?

 

The problem is you give back what you gain on the top end to the bottom end. You'll actually lose MPG in mixed driving if you push tall gearing too far. The only potential MPG benefit is in long-distance highway miles.

 

307 are pretty hard to find on top of it... But if you drop the rpm to much at highway speed you will be lugging the motor down and taking it outa it's idea towing and cruising rpm. Fine if your just driving but towing is going to suck horrible with the tranny jumping in and out of od hunting for more rpm even on slight grades. If you want an overall drop in rpm with towing power yet look into a gear vendors overdrive unit.

 

A non-lock-up auto with 3.07's means that by the time you get above stall and into the efficiency range, around 2200 RPM, you're going 80+. And by the time you're going 80+ you're loosing as much as you are gaining due to aerodynamic losses.

I average 12 driving through the desert averaging 90, and that's with a Goerend's converter, Suncoast 518 and 3.07 rear end.

 

I think a 3.23:1 would be a good gear for the 5spd.. but i'm looking into splitter boxes myself, hate knowing my top end is only 88mph, great if it was a delorean with a flux capacitor, but sadly it's not *sigh*

 

I've been running a Gear Vendors OD in the 91 for over two years. It seems like a perfect match for the Getrag with 4.10s and anything around a 33" tire. It runs 1900 rpm in 5th over at 70mph with the 34" tires I have on it now.

I don't think it would work as well with 3.54s unless you were cruising in the 80mph area, and then like Begel said, the air resistance basically rules out any possible fuel economy.

 

dang i didnt think i stressed that new rebuild THAT bad... my truck only does like 65 at 2300 rpm hahaha. it only had a few thousand miles on it with a little bit lower stall convertor when i got the truck. when i first got the truck on the road i could run 75 @ 2500 rpm and averaged 19mpg on my trip to VA

to address the 47RH conversion: this is whats needed to the best of my knowledge and how far i've gotten

engine to transmission adaptor plate- got mine from ebay for like 93 bucks shipped
starter to match adapter plate- still need to get one
flex plate- 160 bucks shipped from PATC on ebay
torque convertor- comes with goerend transmission i plan on getting
transmission- getting from goerend
relocating trans mount- idk wtf i'm going to do about this... scrap steel and a welder most likely
OD on/off switch- local auto parts store
optional: manual lock up switch- undecided
Trans fluid: ATF+4- i think i'm going to get some amsoil, royal purple doesnt make an equivelant and bully dog fluid and PATC's "raptor blood" is rediculously expensive

the "grey areas":

i've heard drive shaft shortening is required.- there's a drivetrain shop not too far away from my place that does driveshaft work
idk if stock cooler lines will interchange, i dont see why not- i opted for re-routing the lines to an auxillary cooler and filter
NP208 tranfercase is said to bolt right up- rob it off the backside of the 727

 

Tower_ofpower --

Relocating the rear mount is easy...
2 options :
1. Drill new holes in the frame - easiest;
2. IF someone at the factory had a bad day and decided to screw around and put the bolts in from the TOP of the frame where they cannot be removed - get a torch and elongate the holes ~ 1 1/2" toward the rear.

Just got thru installing mine 8 - 9 mos ago and I had to use option 2. Takes about 20 min per side.