One thing about these speedos, is the faster you go, the more inaccurate they are. I used an adapter as I had a similar issue with the housings. It got me right on till about 25-30 mph, and it runs about 5 mph slow at about 70...all checked with GPS tracking. I was happy, probably wasn't much more accurate from the factory.

Here is the site I got the adapter from.

http://www.transmissioncenter.net/speedometer_calibration_______va.htm

You just need to find out the percentage you are out and then select from the drop down the closest match. It's the GM / Dodge / Ford Ratio Adapter. This way you avoid taking the trans apart to change the drive gear.