Speedometer pinion problem?

[1BAD68]

I replaced my speedo pinion with a 34 tooth gear in my 904 trans. I have 3:55 rear gears and run 25.5" tires.
Now my speedometer is about 10-15 mph off too fast.
I read that I would need around a 34-36 tooth gear and picked the 34 because I figure its better to register a little fast than slow but I thought it would be only 1-3 mph fast..
Am I missing something here?

[John_Kunkel]

The math says you need a 36.5 tooth pinion so either a 36 or 37 would work; I would go with a 36 because it will make it run a little fast...helps avoid speeding tickets.

BTW, the odometer is a better indicator of the correct pinion, a faulty speedo can be off even with the correct pinion.

[dspaulding70]

Where is the best place to get a 34 tooth pinion gear for my transmission?

[will]

Cope Racing Transmissions has them.

[Dino]

Or Ebay. Always a few there.

I've gotten a few from this guy: http://www.ebay.com/itm/SPEEDOMETER-PINION-GEAR-read-description-DODGE-JEEP-31-32-33-34-35-36-833-727-/162622407258?hash=item25dd0d025a:g:q0kAAOSwqu9VTvAY&vxp=mtr

[dspaulding70]

for my 70 727, would i need the long one or short one?

[Nacho-RT74]

long

[Paul G]

The speedometer can be way out of calibration. Like John said, use your odometer to verify you have the right pinion. Best way is to get on the highway and use mile markers to verify actual miles of travel vs. odometer miles of travel.