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Old 09-06-2017, 02:23 PM   #1
85raiderburban
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speedometer reading slower than actual speed

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.
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'80 Suburban C20 (C6P) (Silverado) 31's F / 33's R
- '78 RV 454 (Edelbrock 1813, 396 HR alm intake, PH8026 LT's, H pipe) / '93 4L80 TSS / 14B FF 3.21 gov lock / '95 GMT400 gauge cluster

*coming soon* '69 472 Cad swap
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Old 09-11-2017, 08:37 PM   #2
85raiderburban
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Arrow Re: speedometer reading slower than actual speed

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.
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'80 Suburban C20 (C6P) (Silverado) 31's F / 33's R
- '78 RV 454 (Edelbrock 1813, 396 HR alm intake, PH8026 LT's, H pipe) / '93 4L80 TSS / 14B FF 3.21 gov lock / '95 GMT400 gauge cluster

*coming soon* '69 472 Cad swap

Last edited by 85raiderburban; 09-12-2017 at 02:19 AM.
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Old 09-12-2017, 04:47 PM   #3
sntrym95
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Re: speedometer reading slower than actual speed

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.
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