Speedometer Gear? How do I know which one to use?

[Rondo68]

My Speedo reads about 10mph more than it should. How do I know what Speedo Gear to buy for my 68'?

[John_Kunkel]

You need to know the rear axle ratio and the diameter of the rear tires. With that info the correct pinion can be computed.

Be aware that not all speedo errors are due to an improper pinion, the speedo head could be bad. It's best to use the odometer to verify if the correct pinion is installed, drive over a known distance (a couple of accurate highway mile markers is a good reference) and compare it to the odometer reading.

[Ghoste]

Or find a friendly cop on a radar stakeout. 

[69fourspd]

Here are two pages for calculating the correct gear.

http://www.moparts.com/Tech/tech/pages/speedo.html

http://www.moparchat.com/forums/archive/index.php?t-72212.html

[Rondo68]

Thanks for the info guys!!