3.73 gears installed, trying to figure out how fast I'm going now

I have 4.10s in my car and I just got the new gear put in mine and before when I was doing about 65 it was reading a little over 85... so yours is prolly 15-18 mph fast

was your stock gear a 3.08? since it's auto

Try this:
http://www.csgnetwork.com/gearratcalc.html

Until you get your gear you can estimate speed based on rpm as long as you know your tire height and gear ratio.

rpm = gear ratio x mph / tire height x 336

you can figure out the rpm for a few common speeds. Like 30, 40, 50, 60, and 75. You can estimate in between speeds. Don't forget to factor the tranny gear ratio inthe gear ratio you use in the formula. 3.73 x 1.46 for 2nd gear as an example.

It's just simple ratios. If you switched from a 3.08 to a 3.73, then your real speed is simply 3.08/3.73 X whatever speed is showing up on the speedo (assuming it was accurate before). 30 mph X 3.08/3.73 is about 25 mph. 60 would be about 50 mph.

If you switched from 2.73's to 3.73's - 30 would be 22 mph; 60 mph would be 44 mph.

It's just like Michael said -- it's a ratio. It's not simply a matter of being X miles per hour off. The error is a percentage.

If you went from 3.08 gears to 3.73s, your speedo is reading about 21% too high.
Actual speed........Speedo reading.
100 mph................121 mph
80 mph...................97 mph
70 mph...................85 mph
65 mph...................79 mph
60 mph...................73 mph
50 mph...................60 mph
45 mph...................54 mph
40 mph...................48 mph
30 mph...................36 mph

If you went from 2.73s to 3.73s, your speedo is reading about 37% too high (36.6% to be exact).
Actual speed........Speedo reading.
100 mph.................137 mph
80 mph...................109 mph
70 mph...................96 mph
65 mph...................89 mph
60 mph...................82 mph
50 mph...................68 mph
45 mph...................61 mph
40 mph...................55 mph
30 mph...................41 mph

Hope these numbers help.