3.91 rear-end causing incorrect speedometer reading

[Hood]

I installed a 3.91 rear-end over 40 years ago in my cuda. I never let the speedometer being off bother me. But now this old man wants it to read more accurately.  At 50 mph I'm reading around 70 on the dial.  What exactly do I need to do to correct the false speedo readings?
I'm running 14" BFG's around the car.

[Bullitt-]

  Replace the speedometer pinion gear in the transmission,,, 

215/70/14  26" tall tire?  Chart indicates a 40-tooth gear would be correct
https://www.manciniracing.com/mrespeedo7.html

https://board.moparts.org/ubbthreads/upload2/5690904-MoparSpeedoGearChart.jpg

[Hood]

Hi brother Wade, good to hear from you. The thermoquad is running as sweet as ever. So I  need a different speedo pinion gear. How deep into the trans will this adventure take me?

[Bullitt-]

Hi brother Wade, good to hear from you. The thermoquad is running as sweet as ever. So I  need a different speedo pinion gear. How deep into the trans will this adventure take me?

  Good to hear Larry    Nothing to it, check this video  https://youtu.be/_6pxoIW-DGc?t=262

[Hood]

I just googled it.... sounds easy enough 

[Bullitt-]

    One important thing he doesn't mention is orienting the pinion housing
  There are #s that represent the tooth count range that must be aligned properly

[Hood]

Thanks a million 

[Dakota]

I admit I didn't watch the YouTube video, so my apologies if this advice is a repeat:  unless you have really quick hands, drain the transmission case before pulling the speedo gear out.   Don't ask me how I know this is important.

[340Challman]

I admit I didn't watch the YouTube video, so my apologies if this advice is a repeat:  unless you have really quick hands, drain the transmission case before pulling the speedo gear out.   Don't ask me how I know this is important.

I learned this the hard way as well. It was many moons ago, but you'll never forget cleaning up that mess. Now I just jack up the a$$ end of the car. I need access underneath anyway.

[Bullitt-]

I admit I didn't watch the YouTube video, so my apologies if this advice is a repeat:  unless you have really quick hands, drain the transmission case before pulling the speedo gear out.   Don't ask me how I know this is important.

@Hood  You need to know.... I didn't

[Hood]

It's all good. I need to have my trans flushed so that would be a great time to swap the speedo gear. Two birds....One stone. Thanks and appreciation to all.

[dodj]

Backing up on a couple ramps is enough to prevent the mess. Just FYI in case you decide to put off the fluid swap.

[Jay Bee]

Here's a couple pictures for clocking the adapter