I have a 69 Camaro with a TH350 transmission with the small bullet gear it has a 410 gear in the rear and is off about 30 MPH when you are going 50 it is showing 80.Did all 69's have rear axle frame bumpers?They are 3 holes above axle tubes on both sides that i cannot figure what goes there.Could it be exhaust related?

Check this guys site out.

http://members.core.com/~faldrich/speedo.htm

Might help you.

Try these links I've compiled..
Best solution is to find out what your Drive and Driven gears are, will take you less than 10 minutes to do, just remove the speedometer cable from the tail end of the transmission and count the teeth on the driven gear and note the color on the drive gear, then with that info and my links you can compute and figure out the precise gears that will work best for your combination.

Brian's Speedo Links (http://69camaro.nextmill.net/temp/speedo.html)

26" tires:
Drive Gear 7 Driven Gear 22 = +1% off (pretty accurate)
Drive Gear 8 Driven Gear 25 = +2% off (pretty accurate)

27" tires:
Drive Gear 7 Driven Gear 21 = +2% off (pretty accurate)
Drive Gear 8 Driven Gear 25 = -2% off (pretty accurate)

There are 3 rubber bumpers above the axle on 69's not sure about other years. One centered over the pumpkin, the others are bolted to the frame as it goes above the axle, the radius part. I got new ones to install this winter now.
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Here is a pic of the 3 holes you probably are seeing.
http://www.brainerd.net/~knudsen/69RS/69ShockPassRear.JPG