Speedo error

[apache]

Is it GMC`s lack of accuracy or a coincidence that my 84 K5`s speedo said 47 mph @ an actual 65? granted it had 33`s but thats pretty far off. Then on the other side of the coin my new project 85 Burb K2500 that is bone stock with a T400 and 4.10`s shows 75 but im only goin 62mph. Good and bad in that one as it actually means it has about 88K miles on it, also means with mileage error im only getting 8mpg...ouch.

 

[todds302]

the gmc is right, get a hold of a speedo shop there are a couple on line, they can tell you what you need to get them right. around 85 per vehicle. i just use my gps it tracks for free....

 

[michael0584]

I'm sure when your gmc left the factory it's speedometer was accurate. Oversized tires will make a big difference in a speedometer, also if anyone has swapped out transmissions/transfercase, the speedometer gear in the tailhousing maybe the wrong one for your gear ratio or tire size. Most likely in your suburban, the speedo gear is for a 3.42 geared suburban, which would account for it being fast. Mine is fast but someone put in a np208 with a speedo gear calculated for 3.08 gears. You can find everything you need here to correct it as well as a chart to help you calculate what you need.
http://www.transmissioncenter.net/speedometer_calibration_______va.htm