With just 4.62's and 31's, my  speedometer reads 56 when the GPS says 50.

After adding a GRS II a couple weeks ago, I did the following math:

31's            -- about 11 % higher
4.89 GRS II -- about 18 % lower high range
4.62's         -- about 23 % lower
--------------------------------------
total           --  about 30 % lower

Hmm 30% lower? Engine spinning 30% faster than stock, 4000 RPM at 58? So that means the speedometer reads 60 when GPS reads 42, right?  Whats wrong with my math?

My first test drive with the GRS II and my bad math, I was passing traffic wondering why everyone was going so slow. I got real lucky when the ricer blew past me and lit up the speed trap a couple blocks before I got there.

tell me the stock tire size, stock gears, tcase gears and
I can do the math...

better yet, your speedo is out cause it's old?

I think you would multiply not add. say your high range is 1.8   1.8x3.73=6.714   1.8x4.62=8.316  because your high range is not 1to1 like most your final will always be diffrent from the others. to simplify your speedo is hooked to your transfer case, so your gear change is not a direct ratio change it has to be multiplyed by 1.8 to get the diffrence in gear change.

Everything works just fine except the interface between the seat and steering wheel.  

The problem is not really in the addition or multiplication.

hes right about the multiplication

Should be multipling, not adding, but the real trick is that the speedometer runs off the tcase output shaft.

You only include tires and differential when calculating speedometer adjustments

You include tires, differential, tcase, and transmission when you want to calculate to real speed to RPM.

With GRS II and 4.62s, Tachometer reads 30% plus higher than stock, while the speedometer still reads 12% high.

Wow that sounds complicated
 Now I may be cheating, but I would probably just go out to one of the 12 cops that lives in the neighbor hood and offer him a few donuts to tell me how fast I'm going when my speedo says certian speeds.  
 I could probably get away with that as I am one also.

Utah

Should be multipling, not adding, but the real trick is that the speedometer runs off the tcase output shaft.

You only include tires and differential when calculating speedometer adjustments

What he said.  Believe him.

You include tires, differential, tcase, and transmission when you want to calculate to real speed to RPM.

And crawl.

GeoB

My math says you need bigger tires to fix the problem!

Mike