http://www.tciauto.com/Products/Tech...eedo_gears.asp
There's one calculator. According to it (and another I found) 700r4 + 3.73 + 275/60-15 = 15 tooth drive gear 40 tooth driven gear. Looks like I had 18/38.
Here's pics of my adventure in changing speedo gears:
http://faith.dixiesys.com/85-Silverado/PICT0005_001
Starts there with a pic of the old gears.