speedometer recalabration

[g money]

I have an 83 c10 and swapped the rear end out of a blazer with pozi-traction but now my speed o buries out at 85 when im doing fifty, are there any adapters out there that ACTUALLY WORK? Any other ideas?

[DanMcG]

You should be able to change the speedo drive gear if you know your tranny and gear ratio

[rich weyand]

Here's where to get the speedo driven gear for the tranny.  You can either calculate what you need from tires and gear ratio, or look at the one you have and figure what percent change you need from your current reading.
http://www.transmissioncenter.net/speedometer_calibration_______va.htm

[BADAZ chevy guy]

This is a quote from the web site rich weyand just posted. There's some really good stuff there, but this is something a lot of guys miss. Tire height ISN'T just from the ground up!
"To determine your tires true diameter in relation to determining what speedometer gears you need, measure from the ground up to the center of the axle then multiply by two. The actual run out of the tire can be as much as 2 to 3 inches less than what the circumference of the tire measures. In effect your tire has two diameters."

[rich weyand]

Tire diameter for purposes of speedometer calibration is rolling radius times 2.

[Stewart G Griffin]

Another idea is to go with GPS.