What speedo gear, conflicting info

I searched, and ive seen tables and I see gear sizes ranging from 39t to 36t speedo gears.

I have a 1990 MJ with a 4.0, ax-15, np231, 4.10s and 31" MT/Rs

I have a 39t right now and I think (its hard to keep a constand speed and read a GPS at the same time) I am 5mph off at 60mph.

What speedo gear should I get? Or better yet, how do I figure it out?

I believe you should be closer to a 35 tooth gear.

Is the speedometer reading 5 mph too fast, or 5 mph too slow? If it's reading too fast then you need a speedo gear with MORE teeth. If it's reading too slow then you need a speedo gear with FEWER teeth. You need to move about 3 teeth one way or the other. My guess is that your speedo is reading slow and you need a 36-tooth gear.

Here is a good link with info that should apply.

http://jeepin.com/features/speedogear/index.asp

pete