I just received two axles with 5.12 gears to put under my 87 samurai. One is a front samurai axle and the other is a hybrid rear housing. I have lost all calculater pm's ive received and was wondering if anyone can tell me how far off my speedo may be once I install these with 235/75R15 tires and a 3" spring under lift? I have the stock tcase for now but hope to trade my two stock axles for a lower geared tcase soon.

Also for a very mild wheeling rig that is a daily driver what would be the best tcase gear set up to use? I plan to just go as big as 31's.

Here you go:

http://www.solemnwarning.net/transmission/?id=53

Already programmed for you. With the 5.12 gears you are pretty much in line for 31" tires already.  I wouldn't go lower then a 4.8:1 kit myself.  But you can change the transfer case in the transmission calculator, it has most of the after-market SJ series transfer cases in there.

here is a good online gearing calculator that you can run a couple setups side-by-side to compare them.
http://www.grimmjeeper.com/gears.html

Here is another one that I have used.
http://www.lepayne.com/gears.html

I appreciate it guys thank you.

There's a really crude way to do it.  Have someone go 20 or so down a straight road.  Pace them, and then move your needle the the speed they say they are going.

Of course, use a stock car.  I used my GPS for a round-a-bout estimate of how off I am, then moved my needle.