Chevrolet Repair: 1987 Chevy Truck Speedometer problem, vehicle speed sensor, speedo gear

Question
I bought this 1/2 ton pickup used. It has a 305 TBI and TH400. The speedo was always off. It works, but it would read (approx )15 mph under actual speed. I replaced speedo cable (new) and speedometer (new), but it still does the same thing. I noticed that there is a sensor that plugs into the back of the speedo and have been told that it is a Vehicle Speed Sensor. Problem is, nobody can tell me what it does. Some say it's for Cruise Control. The cruise control works fine. I've asked if it controls the speed that the speedo shows, but no one really knows. Some mechanics, I've had look at it, say the speedo (reduction) gears in the trans are the wrong ones, but I tell them the transmission is the original one that came with the truck. I realize the gears could be worn, but I'm tired of replacing parts and would like to just fix the problem. What do you think?

Answer
HI RANDY,THE SESNOR ON THE BACK OF THE SPEEDO IS A PROTECTOR THE SPEEDO GEAR HEAD FROM BOUNCE..THIS IS WHAT I WAS TOLD BY GM...I DO NOT THINK THIS IS YOUR PROBLEM..DO YOU KNOW IF ANYONE HAS CHANGED THE REAR GEEAR RATIO IN YOUR TRUCK,TRNASMISSION EVER BEEN REBUILT ?? ARE THE TIRES THE SAME SIZE FROM OEM SIZE..YOU MECHANICS ARE CORRECT ABOUT CHANGING THE TRANSMISSION TAIL SHAFT GEAR IF NEED BE.IF IT IS WORN THIS WILL NOT CAUSE YOUR PROBLEM. BUT IF THE GEAR RATIO HAS CHANGED YOU WILL NEED TO REPLACE THIS GEAR. NOT THAT HARD TO DO,JUST REMOVE YOUR TAIL SHAFT COVER AND REMOVE THE CLIP THAT HOLDS THE GEAR TO THE TAIL SHAFT AND SLIDE IT OFF THE SHAFT..TAKE THIS GEAR TO YOUR CHEVY STORE AND THEY CAN CALCULATE WHICH NEW GEAR YOU WILL NEED....IT GOES BY THE NUMBER OF TEETH ON THE GEAR AND COLOR. I HOPE THIS HELPS YOU AND GOOD LUCK,,,TOM