Quick Poll: 3.91 Diff w/4 speed?

What motor, what wheels?
What diff ratio do you have now?

242DLs came with 4.1 diffs, 4-speed and 14" wheels. They are bearable, but pretty agricultural on the highway.

My 84 245 ended up with a 3.9 diff in it. When the auto refused to go into OD anymore, it became properly unpleasant and fuel consumption was horrible.

Do you know what's in there now? Should be 3.73, I think? Maybe 3.54?

If its a 3.7, then you'll probably not find the difference to be worth the effort - the difference is less than 5%. That said, if your current diff is noisy/broken/whatever, then it would be worth the effort to track down a 3.9.
Another way of looking at it, is that a 5th gear will give a rev drop of around 20%. So 4-speed & 3.54s are still geared shorter (more revs) than a 5-speed and 4.11s.

I guess the point I'm trying to make is that diff ratios are a compromise - whatever you gain, you lose somewhere else. If you gain a lot, you'll also lose a lot. More gears in the gearbox is a better solution.

The auto to manual swap will make WAY more difference to the way the car goes, than the diff ratio swap. If it was me, I'd fit the 4-speed, and make a judgement from there.

It may not seem like much but I can handle 100 in the 144 with a 4 speed but 110 is just too high in the revs to be comfortable.

Just fit a 5 speed and don't use 5th ...or use it when you are on the hwy. :P

No but a M46 is still worthy! Just change the shifter knob and pretend it's a 4 speed, then when you're sick of the high revs change it back to the original knob and hook up the OD wire. Easy!

It must be noted. Dauntless removes all mufflers from any car he owns. They are pretty loud at idle let alone at speed.