Spedo is off.

mine too

my speedo was right before i put a new engine and transmission in now its 10 off...got my wife to pace beside me its a wonder i havent gotten a ticket since i been running 10 over the whole time....i thought it seemed like i was going faster than what the speed was reading......how do i correct this....mine bounces also around 20..if i am running 50 my speedo shows 40......like i said it didnt get off til i got a new engine and trans installed

See http://www.corral.net/tech/gearcalc.html for an online calculator to figure your RPM and MPH using a given tire size and gear ratio.

See http://www.geocities.com/mgnedell/notchback/tech/speedo.html for help choosing the speedo gears.

The speedo bouncing around is usually a bad speedo cable or drive gear missing teeth..

The stock range of speedo gears work correctly with a limited range of tire sizes. The 23 tooth white gear works good for some guys, but self destructs in a few months for others.

See http://www.corral.net/tech/gearcalc.html for more help

Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy within 1%-3%. The drawback is that it isn't cheap.
Please excuse the typo (their's, not mine) in the following picture - it should read Custom ratio adapters, not Custom radio adapters.

See http://www.gaugeguys.com/ratioadapt.htm for more info.

If your speedo is reading slow, then you need to go down on the number of teeth on the driven gear, or up on the number of teeth on the drive gear (or both).

If your numbers are correct then you'd need to go down to a 16 or 17 tooth driven gear to get an accurate reading. In reality you probably need to change the drive gear on the tranny's output shaft. A change from a 6-tooth to a 7-tooth would probably get you within 4%.

You could try lubing the cable with something (graphite worked for me) before you go replacing it. My brand new speedo cable only worked fine for a couple of months before it started bouncing again, so I puff a bit of graphite in there every few months and it keeps it in check.

speedo

mine was working fine until i had the engine replaced maybe it got damaged or something...dont know for sure....i know that it was correct before the engine install...oh well at least i found out before i wound up with a ticket.......i knew i seemed to be going faster than it was saying that i was going