how to calidrate 1971 speedometer

[barrykathy]

i have 1971 GMC 1500 6 cyl. the stock rear end 3.73 i changed to a 3.07 now the speedometer is off by about 15 MPH is there a way to calibrate to speedomete

[hayhauler71]

You need to change the speedo drive gear and the driven gear in the tail shaft of the trans. There is also a few companies that sell transducers that go on the trans that will change the speedo reading but dont recall the companies name

[geezer#99]

http://www.transmissioncenter.net/sp...____va.htm#350

[blasher]

I just fixed the same problem on my truck. I used a GPS to confirm how far I was off (read 48 when traveling 60), then I bought a vintage Stewart Warner speedometer correction gear box on eBay for $30. I got one that was a 1.250 ratio (48 X 1.250 = 60). Real simple to install and works like a charm. It was a rewarding fix. You can buy a new reproduction one for twice the price. Since mine was used I opened it up and cleaned out the old grease and re-greased it. The innards and brass gears were in great shape. It's a quality unit.

[69GWC]

Thats about how far off my 69s is after installing 33s, 50mph =65...
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[Tim Christian]

Is it a automatic or manual?