Using what we have for info so far - 1956 3100 3 speed (not heavy duty or OD) with a 20 tooth steel driven gear the following is what I could find in the 1929-57 Chevy Master Parts books:
Parts Group 4.338 GEAR - Plug - Fitting - Retainer - Seal, Speedometer Driven: GM# 3845071 1955-57 1/2 TON (3-SPD.) (exc. hvy. duty) equipped with 6.50-16 or 6.70-15 wheels and tires (20 teeth-steel)
Parts Group 4.343 GEAR, Speedometer Drive: GM# 3845073 1955-57 1/2 TON (3-SPD.) (exc. hvy. duty, O/D) (7 teeth)
There was a 19 tooth steel driven gear listed as available for 1956-57 1/2 TON (3-SPD.) (exc. hvy. duty, O/D), but was listed with trucks equipped with wheel/tire combinations 7 x 17.5 or 6.00 x 18 which don't apply in this situation.
hcb3200, perhaps this will help with an answer to the inaccurate speedo reading and what ratio adapter may be needed to get things back in sync???
