Ccmano
04-05-2010, 07:16 PM
I recently changed out my differential to a 4.10 unit and I am also now running 335/35/17's on the back. The issue of speedometer accuracy obviously comes into play, with the speedo reading 10mph fast at 70mph. In researching the potential solutions I found that there are two ways to correct the readings. The traditional way is to replace the speedo gears. For a 4.10 both the drive grear and the driven gear must be replaced. The only source of prepackaged gears is our beloved Marc Haibeck. As much as I love Marc,spending $120 for 2 little plastic gears was a bit much (I'm cheap and unemployed), not to mention the labor involved of removing the exhaust and driveshaft to mount the drive gear. On top of that, because of the 335's the speedo would still be slghtly off.
The second solution is to install a calibration box. Our speedos are electronic, with a 4000 pulse per mile sensor. The box allows you to dail in the exact amount of correction needed for your exact setup. In researching the available solutions I found that there are 2 units on the market that fit the bill. One made by Cyberdyne at $49.95 and one made by DakotaDigital for $79.95, free shipping on either.
Since I'm cheap, I ordered the Cyberdyne unit from Jegs to try it out. Wiring is simple. Go to the VSS sensor on the transmission, locate and cut the yellow (signal) wire. Splice in the white and blue wires from the box. Run your wires to the passenger compartment. Supply a power and a ground wire and your ready to go... well almost. After doing all that and driving the car I found that the box didn't work. The speedo simply sat at zero and my "Service Engine Soon" light came on after a few minutes. After further investigation I found that the Cyberdyne box does not like early GM electronic VSS systems. So I packed the box up and sent it back to Jegs for a refund.
Next I ordered the DakotaDigital box from a vendor on Ebay (which turned out to be DakotaDigital themselves) This box is wired the same way as the other, so all the connections were still in place. The wiring hook ups and calibration adjustment interface is much nicer on this DD box. Read the directions, wired it up in my center glove box and took her out for a drive. She worked like a charm. I took the car out onto the highway and set the cruise control at a true 70mph based on the reading from my GPS. I simply pushed the "down" button on the box till the GPS and the Speedo matched. As I continued to drive I found that the GPS and the Speedo were within about +/- 1 mph at most speeds. Not bad for $79.95.
I just figured I'm not the only one in this situation, so I thought I'd pass along this info whoever needs it.
Below is a picture of the installed box.
Cheers
H
:cheers:
http://i249.photobucket.com/albums/gg225/hans_meyer/Corvette/IMGP2801.jpg
The second solution is to install a calibration box. Our speedos are electronic, with a 4000 pulse per mile sensor. The box allows you to dail in the exact amount of correction needed for your exact setup. In researching the available solutions I found that there are 2 units on the market that fit the bill. One made by Cyberdyne at $49.95 and one made by DakotaDigital for $79.95, free shipping on either.
Since I'm cheap, I ordered the Cyberdyne unit from Jegs to try it out. Wiring is simple. Go to the VSS sensor on the transmission, locate and cut the yellow (signal) wire. Splice in the white and blue wires from the box. Run your wires to the passenger compartment. Supply a power and a ground wire and your ready to go... well almost. After doing all that and driving the car I found that the box didn't work. The speedo simply sat at zero and my "Service Engine Soon" light came on after a few minutes. After further investigation I found that the Cyberdyne box does not like early GM electronic VSS systems. So I packed the box up and sent it back to Jegs for a refund.
Next I ordered the DakotaDigital box from a vendor on Ebay (which turned out to be DakotaDigital themselves) This box is wired the same way as the other, so all the connections were still in place. The wiring hook ups and calibration adjustment interface is much nicer on this DD box. Read the directions, wired it up in my center glove box and took her out for a drive. She worked like a charm. I took the car out onto the highway and set the cruise control at a true 70mph based on the reading from my GPS. I simply pushed the "down" button on the box till the GPS and the Speedo matched. As I continued to drive I found that the GPS and the Speedo were within about +/- 1 mph at most speeds. Not bad for $79.95.
I just figured I'm not the only one in this situation, so I thought I'd pass along this info whoever needs it.
Below is a picture of the installed box.
Cheers
H
:cheers:
http://i249.photobucket.com/albums/gg225/hans_meyer/Corvette/IMGP2801.jpg