While not an "expert" -- I have set up a few Volvo rears -- both with new parts and in the case of my '69 144S I used a later used '73 rear w/4.3 gears--new bearings & seals--and a saved limited slip from a junked 265.
What you propose is not entirely silly. The differences in gear depth and lash is mostly determined by the differences in the housings---the machined parts should be close to if not perfectly identical. You'll need a housing spreader (try a Jeep guy). Use the shims from your rear on the used gearset and you should be pretty close. Take one set of bearings that you will not be using and grind out the center a bit so they'll slip on and off the shafts easily---that way if you have redo the shim configuration you will not be wearing out the shafts (the bearings are pressed into place--tight). Read a chapter on rebuilding rears with illustrations for interpreting wear patterns with the blue dye. Be patient. -- Dave

I would keep the automatic rear end. I think you'll find that it will accelerate like a rocket (relatively) and I doubt you will sacrifice a significant amount of mileage. This is a guess.

I currently have an automatic with a manual 3.31 rear, and acceleration suffers (but of course, mileage is great). If you do the reverse, combining a 3.73 rear with a manual, I expect it will be a lot of fun, and mileage will still improve.

Too bad you're not in the metro DC area; I'd swap with you.

The diff is Dana 30, and I believe there are two different carrier sizes. You can probably get some good advice on what ring + pinion swaps will fit within your existing carrier at an off-road shop, as Jeeps for years used the Dana 30 for the front axle.

Fuel mileage will be pretty much a wash, IMO. Tire pressure and driving style can easily mask the difference that the axle ratio will make.

Be aware that if your car has ABS, you need to swap in an ABS-equipped diff if you expect the system to work - ABS cars have a different sensor and tone ring to pick up wheel speed. You'll also need a different speedometer to account for the tone ring sensor speed as well.

Hi,


The car is certainly drivable with the automatic rear end. I myself would complete the transmission swap first, then worry about the rear end later. If you have a core axle, then you just take your time rebuilding it until it’s convenient. The carrier will only accept ring gears of specific ratios. If you swap the carrier, it’s usually best to check the bearing clearance which can be a pain.

Without the swap, your car will have better acceleration, but a lower highway top speed / fuel economy sweet spot.


Goatman