Speedo Gear Calibration

[Blue85]

I'm finally getting around to makin my speedo read correctly, I found on this site http://www.transmissioncenter.net/sp...n_______va.htm what gear combos i need, just wondering what "speed" the speedo in these trucks is. It says on the site most speedo's 1965 and up read at 1000 revs per mile, is this true for a 72 truck? BTW, its 700r4 trans, 3.73's, and 28 inch rubber.

[LockDoc]

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I'm not close to a speedo to look but this site says some are marked in small numbers at the bottom of the face.

http://www.bgsoflex.com/speedo1.html

LockDoc

[70GMCer]

Originally Posted by Blue85

I'm finally getting around to makin my speedo read correctly, I found on this site http://www.transmissioncenter.net/sp...n_______va.htm what gear combos i need, just wondering what "speed" the speedo in these trucks is. It says on the site most speedo's 1965 and up read at 1000 revs per mile, is this true for a 72 truck? BTW, its 700r4 trans, 3.73's, and 28 inch rubber.

You have the same setup as mine except I have 27" tires. And yes, the speedo is 1000 revs per mile.

The gears you need are: Speedo driven gear 40 tooth (black)
drive gear 15 tooth (gray)

The "driven gear is where your speedo cable fits into & the drive gear is in the tail shaft. You have to remove it to replace that gear. Check your colors though, you might already have the gray gear in there, & you'll only have to change the speedo gear.

http://www.bgsoflex.com/speedo1.html

[Wrenchbender Ret]

The 700-4R trans. is easy to calibrate the speed. Compare the odometer with the mile markers on the freeway. If it gains tenths take out the driven gear & get one with more teeth. If it looses get one with fewer teeth.

[70GMCer]

Originally Posted by Wrenchbender Ret

The 700-4R trans. is easy to calibrate the speed. Compare the odometer with the mile markers on the freeway. If it gains tenths take out the driven gear & get one with more teeth. If it looses get one with fewer teeth.

It depends if the driven gear fits within the drive gears range. Mine didn't and had to change to get the correct ratio. It's a lot easier to do the calculator method, rather than buying different gears until you get it right. $2 a piece at GM

[Blue85]

ive never changed the speedo gears before, but i do know what has to be done, except, is there any special procedure for makin the gears mesh correctly?

[70GMCer]

That's why you have to be within a certain range for the gears to mesh. When they are within that range, they just couple together in side the case. If the two gears are out of that range, they will just bind up and strip, and most likely break into pieces.