Speedometer calibration (correction) - ZR-1 Net Registry Forums

Ccmano · 04-05-2010

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

GOLDCYLON · 04-05-2010

Nice install and write up. I was going to suggest verifying the correction box with a portable GPS unit but you covered that as well.

Another source for that VSS signal is in the original Delco harness for the radio. Its the brown wire with the forrest green stripe. It feeds off the same area and is the mystery wire that allows for the volume to change at speed for the Delco Bose radio (If its turned on that is)

04-05-2010	#1
Ccmano Join Date: Jun 2006 Location: Sparks, NV Posts: 2,843	Speedometer calibration (correction) 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

04-05-2010	#2
GOLDCYLON Join Date: May 2007 Location: Peoria, AZ Posts: 9,120	Re: Speedometer calibration (correction) Nice install and write up. I was going to suggest verifying the correction box with a portable GPS unit but you covered that as well. Another source for that VSS signal is in the original Delco harness for the radio. Its the brown wire with the forrest green stripe. It feeds off the same area and is the mystery wire that allows for the volume to change at speed for the Delco Bose radio (If its turned on that is) __________________ GOLDCYLON - 91 ZR-1 #2014 GOLDCYLON - 90 ZR-1 #2794, 4L60e (Formerly Schrade's) GOLDCYLON - 11 CTS-V Arizona State Director 91 WHITE/BLACK #2014. 380 P&P&PCed,Ported Heads, Jeal Long tubes, Corsa Exhaust/FIKSE FM-5s /LED TLs, LED Headlights, Front Wilwood 6 piston narrowlite calipers and rear Wilwood caliper street shop mod,CNCed Coolant Pipes,TPI Cvr,Filter cover,Stainless Bolts, DRM/DOM PROM /ZFDOC mod build #102,DRLs,BMAD with stainless Debris Screen,Coplan Air Blaster, Pioneer APP Radio 4,Brey-Krause HB,Sub Bar,Fire extinguisher seat mount,DRM Coilovers,LEDs everywhere,Compass mirror (orange),V1 DIC hidden display, Homelink sun visor, Carbon Fiber top x3 and APSIS Carbon Fiber interior, APSIS CF Steering Wheel/NAPA Leather, Banski trailing arms, Guldstrand front suspension,urethane bushings from Prothane (total suspension) ZFDoc drive shaft safety loop, raptor shift light (orange),AO engineering louver front plate, Console seat cushion, 96CE seats with black custom Sheepskins, ss billet catch can,Viper remote entry/alarm,Cragar Rear Louvers,LED side louver lights, Dewitts Radiator with SPAL fans and a Woods 160 T-Stat 90 RED/BLACK #2794. 4L60e Automatic Stage V by RPM Transmission, TCI Dedicated TCM, OBX Stainless Headers, Corsa Exhaust, SAN Secondaries and Haibeck PROM, Exotic Muscle Coil overs, LED Interior Lights, LED Tail Lights, LED Headlights, 94 Sport seats with black custom Sheepskins , Cragar rear louvers, GS Front calipers, Banski Trailing arms, APSIS Carbon Fiber steering wheel, Front and Rear Baer Eradispeeds, DRLs, Guldstrand front suspension,urethane bushings from Prothane (total suspension), Dewitt's Raditor with Dual SPAL fans and a Haibeck 170 T-Stat 11 RED/GREY CTS-V Last edited by GOLDCYLON; 04-05-2010 at 07:47 PM.