In MANY cases, seeking to keep in the "OEM Application" orientation with ODAutomatic transmission swaps can be the least expensive, all things considered. Still might be some adaptations of equipment, NOT including the driveshaft shortening issues or rear axle change costs.
There are some threads in here about using the later TF from a Dodge diesel pickup. But that needs and expensive bell housing adapter, then various pressure switches to make things work the related internal electronics. Pretty easy, once you get it all understood, but still expensive, which can make starting with a LA engine better than a B/RB, it seems.
AND, there might be some floorpan clearance issues with the 518s?
Back in the later '70s, when fuel economy ratings were important, Chrysler published a pamphlet on fuel economy and how various vehicle specs affected it. Seems like their research indicated that it took a 3% rear axle ratio change to make a 1% difference in highway cruise economy? A VERY interesting little publication which our local dealer got for several consecutive model years.
The generally lower cruise rpm of an OD situation becomes VERY dependent upon cam selection and when the cam give the engine its best "cylinder pressure" (rpm band). Kind of like they used to use a rule of thumb on axle ratios for trailering . . . gear the car for max torque rpm at the speed limit in the state you'll be driving in. OTR trucks can be geared similarly, which is why many fleets had to "re-gear" for the lowered 55mph national speed limit in the 1970s.
As most OD ratios are in the .70 range, 30% drop in gear = 10% better fuel economy. Starting with 20mpg, then another 10% starts to become significant. Less so with lower mpg levels.
Get the engine tuned so it's operating best in the 3000rpm range, for what you've now got. Make it "sound happy", which means it usually is. Worried about engine wear? Get some quality synthetic motor oil and change it when the TBN gets toward 2.0, with a quality oil filter. Tune the carb for 14.2 air/fuel ratio at cruise, but also make sure the power mixture is where it needs to be for good power (12.7, as I recall?). Keep engine temps at cruise between 180 and 210 degrees F. That's WITH what you have now. THEN, learn to drive at more steady speeds, especially if fuel injection is involved, but also realize that getting to "cruising speed" is best done briskly rather than slowly (getting to the most efficient speeds sooner rather than later). Tire pressures and front end alignment are important, too.
I know that having a relaxed cruising rpm is good. Which is why I like 2.76 axle ratios, for road use, but with a little less power at lower speeds. Always did like 2-3 WOT upshifts at the 90mph range.
In the '90s, some dealers ordered their OD Automatic pickups with "highway" gears for allegedly better fuel economy. I got calls about their trans "always shifting" on the highway. After getting their explanation, I told them everything was normal for what they were experiencing. That was then verified when I tracked their rear axle ratio! At that time, a used rear axle assy was over $1000.00, plus labor and swapping out/rebuilding the brakes as needed. A new gearset was about $500.00+ (at that time), plus labor. End result . . . not cost effective to do for the benefits. My suggestion was, knowing that the trans was doing what it was supposed to be doing, be sure they got a pickup, next time, with something like a 3.42 rear axle ratio than a 2.73.
Certainly, there were some aftermarket means of decreasing the "hunting for gears", with some sensor swaps, but at the dealership level, I couldn't advocate that. Some aftermarket shift kits claimed to do that, too.
When we first got involved with the aftermarket OD situation, it was for HD2500 6.2L diesel pickups. With the diesel's lower governed rpm, if you geared them to pull, little "road speed". Of you geared the for the road, decreased pulling power. So an external OD unit was the answer to better use available rpm. That was with manual transmissions, rather than automatics.
GearVendors was one of the earlier add-on OD units, so since back then, they could be one of the better units on the market, I suspect.
Personally, I think it'd be nice to upgrade to a Chrysler 8-speed, a good EFI, and such. But no electronics to run it just yet. In the OEM use, they do work SLICK! Much better and smoother than similar GM uses, by observation! Perhaps an aftermarket software upgrade might make the GM 8-speeds work better. GM had stand-alone controllers for their GMPerformance Parts OD automatics. NOT cheap, by any means, BUT OEM-level items.
Y'all enjoy!
CBODY67
