I have a 90 RS right now. A friend si building a 9 inch for me though with 3.73s in it. BUT I want to keep the speedo giving me the right reading. What size tires will I need to use? I did a search and didn't really find nothing..so if someone could, please help me on this

What ratio do you have now?
If you're going to 3.73's , tire size alone isn't gonna keep the speedo accurate.
You'll need to change the drive gear and the driven gear in the transmission's tailshaft housing.
Do a search on speedometer gear combinations and you should be able to find out what color/tooth-count gears you'll need.
-Rich-

i think they are 3.08s in there now. I am not sure. It's whatever stock size is. All I found is something about changing the vss sensor and a driven gear in the transmission. Is that correct? anybody have some part numbers from SLP online. Also, it is an electric speedometer.

Is this what I need:

Speedometer Gear Set, 3.70 Ratio 1982-92 Electric Speedo

This speedometer gear set is designed for 1982-1992 F-cars. This gear set will correct your speedometer after 3.70:1 gear installation. Sold as a set of 2 gears (drive & driven).

I found that on SLPOnline.com for $26.90 + shipping.

That should do it.
The driven gear goes on the vehicle speed sensor (mounted to the driver's side of the tranny's tailshaft housing) and the drive gear goes on the output shaft of the trans (inside)
-Rich-