Earned Run Average
Every pitcher in RBI has an Earned Run Average. Much like a hitters Batting Average and Home Runs, the ERA is purely a cosmetic stat. The various pitcher speeds, drop rating, left curve, right curve and endurance determine how good of an RBI pitcher they are. ERA has zero actual use in RBI out of the display.
Low pitcher ERAs
In RBI, your maximum ERA can be no higher than 3.55 (sans nightwulf hacks). Unfortunately in real baseball, many pitchers had ERAs well above that mark. So all RBI did was loop the ERA - if your ERA is 3.56 in real life, your ERA in RBI was 1.00 (the lowest you can have in the game). So basically if your ERA was above 3.55, the formula is used:
Real ERA - 3.56 + 1.00 = RBI ERA
Don Sutton 1986 ERA 3.74, RBI ERA 1.18 Shane Corbet 1986 ERA 3.66, RBI ERA 1.10 Bert Blyleven 1987 ERA 4.01, RBI ERA 1.45