The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post - speedometer reading slower than actual speed

85raiderburban · 09-11-2017, 08:37 PM

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.

09-11-2017, 08:37 PM	#2
85raiderburban Registered User Join Date: Nov 2014 Location: between AZ & NV Posts: 67	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. Attached Images __________________ UNDER CONSTRUCTION '80 Suburban C20 (C6P) (Silverado) - '68 472 (Edelbrock 1813, PH8026 LT's, H pipe) / '91 4L80 (manual shift,vac modulator, 350 converter) / 14B FF 3.21 G80 34' s/ '96 GMT400 interior Last edited by 85raiderburban; 09-12-2017 at 02:19 AM.