Spedo Gear Question

[LUV2XCLR8]

Is the gear in the tranny what calibrates the dash spedo?
I was coming home today doing 70 and people were trying
to run me over I'm thinking I was doing more like 50ish.
Truck had a 250 and now has a 350 engine, not sure if the
tranny is the same one or not. Thoughts on this? In recent
years I have heard people talk about a plastic gear that is
on the side of the tranny, is this my answer? I also seem 2
remember something about colors, are they color coded?

[cdowns]

theres the speedo sender gear that the speedo cable screws into and the gear inside the tranny that drives that gear and speedo// depends how off you are sometimes you can change to a different sender gear which is an easy one bolt pull out and slap the new one in then theres worst case that you have to go into the tranny and change the internal gear too in order to correct the speedo// also available is a gear reduction box that just screws onto the tranny and the speedo cable goes into

[Longhorn Man]

Post your rear gear ratio, and your rear tire height, and 60 MPH cruise RPM, and what kind of tranny... (you can leave out the gear ratio, or tire height.... but gotta have one or the other) and one of the math wiz kids here can tell you exactlywhat you need.
Parts should be avalible at the dealer, but you'll have to go to the old guy who remembers how to use the books... unless some one here can come up with the GM P/N's.

[LUV2XCLR8]

Well chit. No idea on the gear ratio and no tach, guess this ain't getting
solved overnight. I do need a new rear diff seal, guess I'll find out then

[Wrenchbender Ret]

Watch the tenths reading on the odometer as you pass a mile marker on the freeway. Go 2-3 miles & look again. If it gains miles pull the gear out where the cable goes in & count the teeth. Get a gear with 1-2 more teeth. Some are available from the parts store or you can get it from a trans. shop. If not go to the dealer.

[jimfulco]

http://novaresource.org/speedo.htm