speedometer reading slower than actual speed

[85raiderburban]

i swapped a '95 GMT gauge cluster into my squarebody. Everything is working great minus the speedo. i have the VSSB I yanked from the same truck and wired it without an ECM. My 33 inch tires (295/75/16) and 3.21's out back equal an input ratio of 0.605xxx. I calibrated the VSSB to that nearest jumper settings (0.604492) found on the chart at http://tbichips.com/?page_id=504. BUT the speedo reads 5 mph slower than actual speed.
i.e. If im going 40 on the GPS, the cluster speedo reads 35.

[85raiderburban]

UPDATE!! i got it working with the 0.632 input ratio ont he chart. Its dead on all the way thru, matching my GPS speedo. KEEP IN MIND for those who are doing taller gear swaps and keeping bigger than factory tires, that the calibration calculator is not always accurate. Try different ones on the chart whether its a lower than your input OR higher.

In my case:

0.591xxxx - 7-8 mph slower
0.598xxxx - 5 mph slower
*0.604xxx - closest to my calculated input (3.21s and 33.42 inch tires--> 0.605) read 5 mph slower than GPS
0.611xxx - 0-20 dead on, at 25 mph went 5 mph slower
0.618xxx - read 7 mph too fast
0.625xxx - read 5 mph slower
*0.632xxx - correct reading, dead on with GPS and school zone speed sensor.

Now at 0.632xxx, the CALCULATOR specs are off on either rear gear or tire size. 32.24 in. tires/3.21's ***OR*** 33.42 in. tires/3.35s. I know damn well I have the factory 3.21's out back, so just a warning for those who get the calculator giving ratios that are off compared to your trucks specs. Keep trying them and mark em down 'till you get it right. Good luck eveyone.

[sntrym95]

Yeah I've found that chart isn't completely accurate too. You can install the jumpers instead of having to resolder that way you can swap tires and just flip the switches.

[Hankster]

How hard was it to make that cluster work? Are there any write ups on it anywhere? Thanks

[85raiderburban]

Originally Posted by Hankster

How hard was it to make that cluster work? Are there any write ups on it anywhere? Thanks

It was easy to setup. Its just like the original gauges. 12v, GND and sensor wiring splicing for the volts, water temp, and oil pressure. The tach goes right to GND side of HEI distributor like any other. Fuel level didn't work with the original squarebody sending unit but neither did the original squarebody fuel gauge. I ended up swapping it out for a 1995 suburban sending unit. EFI pump deleted and replaced with clamped fuel hose. The speedo was the hardest part but as I mentioned it in this thread it is doable.

My modified schematic thats been working flawlessly for the past year


Also, just a heads up , this will NOT work for:
- '98 1/2 and 3/4 ton pickups
- '98-'99 suburban -
- '99-'00 escalade
- '98-'00 3500 dually.

Those years mentioned have the speedo controlled by the PCM, so if your planning on converting to carb on any of those, you will need a 95-97 cluster with speedo wired like my schematic.