I'm thinking about changing the rear diff in my '73 240 from the stock 3.36, to a 3.90. I'm only gonna drive the car around town, not much highway driving, so I'm looking to add some "cheap" acceleration.

Three questions: 1) is there a "best" 3.90 (a particular year or model) to look for, 2) is it a straight bolt-up, or are there any mods necessary and 3) will the same driveshafts and half-shafts work with the new rear differential?

Thanks

Cheaper would be a swap to a 3:54 R180. 3.90 geared R180s are rare and moving to a 3.90 R200 adds weight and requires a moustache bar change.

When I installed a 5 speed in my 73, I had a hard time finding an R180 diff. In the end I found a 4.11 from the front axle of a Nissan 720 4x4 pickup. It bolted right up after I swapped the flanges with my original 3.36.

There is a great thread (sticky) on this at hybridz.org.

Unless you improve the breathing of your engine at high revs, I suspect a 3.90 could leave you frustrated. Check out:

http://www.geocities.com/z_design_studio/transmission.html

If you still have the stock 4 speed and stock cam, your torque will rapidly fall off after 5500 rpm. With 195/70-14 tires this combination is good for

1st gear: 29 mph

2nd gear: 47

3rd gear: 73

I suppose 2nd will be very "entertaining" but perhaps a bit "brief". John's suggestion of using a 3.54 might be a good compromise, and would definitely be much easier to source.

This place will help you a bit in your decision.

http://forums.hybridz.org/showthread.php?t=114798

Basically, if you go with a R200 3.7, 3.9 or 4.11 diff, it would be ideal to get a 5-Speed tranny I believe from a 1980zx.

It seems that either one of these diffs with that tranny provides the best acceleration increases. Also with the R200 you get into the LSD area which is always a plus to have.

The R200 will add some weight and required modifications, but I think that the gains will far out way the added work and the wee bit of added weight.