You're over driving the odometer and the speedometer by the ratio of (new gear/old gear)
If your old gear was the stock 2.73s (well.. auto could get 3.27s, but assuming 2.73s):
(3.73)/(2.73) = 1.37
So, your speedometer and odometer will show 137% of what is actually being put on.
If your trip odometer shows an 137 miles, you will actually only put on 100. If the speedo shows 137mph, you're actually only doing 100mph.
To correct this, you have to change the gear that is on the end of the speedo cable, or the vehichle speed sensor (which is on the end of the speedo cable) that goes into the transmition.
Now, the gear that you need depends on what transmition you're running. But, for my TKO (from an AOD), I needed (I believe I have the right color) the RED gear (the different speedo gears are all color coded). Again, if memory serves me, there are a couple different gears that you might need to use based upon the number of teeth on the transmition's speedo gear (6,7 or 8 tooth I believe).
But, I'm almost 90% sure, for a 7 tooth drive TKO, w/ 3.73s, it was the RED gear that I used.