There's a chart, hopefully someone will post it.

Do you plan to stay with 31s or move up in size.

Another way to get close, figure what percentage your speed is off, not mph. Then based on that percentage, see what toothed gear you have now, and change it by the same or close percentage. Use 4th which is a 1:1 ratio

 

Looking at the link below, looks like 43 tooth is a close you can get.

Jeep Speedometer Gear Tooth Chart

If the ring and pinion and/or tire size has changed on your Jeep vehicle, then you will want to replace the factory speedometer gear with the corresponding gear on the chart below. The chart lists the gear ratio and tire diameter to easily guide your purchase into the correct size application.

www.quadratec.com

 

46 tooth is what you need. Good luck finding one. If you want it accurate, you might have to go this route (like Rubicon owners have to).
JEEP Wrangler TJ 1998-2006 SpeedoDRD-J1 SPEEDOMETER / SPEEDO CALIBRATOR | eBay

 

I'm planning on staying with the 31's for now. Thank you both for the info! Looks like the calibrator may be the way to go.

 

it is purely a ratio thing and not a fixed amount
so you cannot accurately say 10 mph off without saying at what speed
all with be spot on at zero and if 10% off will be 10mph off at 100 mph and only 5 mph off at 50 mph
while if 20% off will be 10 mph off at 50 mph

then just apply that ratio to current speedo gear
if you want speedo to read 10% lower you just increase tooth count of current gear by 10%
as 10% more teeth take 10% more transfer case output rotation to get up to same speedo gear rpm

 

it is purely a ratio thing and not a fixed amount
so you cannot accurately say 10 mph off without saying at what speed
all with be spot on at zero and if 10% off will be 10mph off at 100 mph and only 5 mph off at 50 mph
while if 20% off will be 10 mph off at 50 mph

then just apply that ratio to current speedo gear
if you want speedo to read 10% lower you just increase tooth count of current gear by 10%
as 10% more teeth take 10% more transfer case output rotation to get up to same speedo gear

it is purely a ratio thing and not a fixed amount
so you cannot accurately say 10 mph off without saying at what speed
all with be spot on at zero and if 10% off will be 10mph off at 100 mph and only 5 mph off at 50 mph
while if 20% off will be 10 mph off at 50 mph

then just apply that ratio to current speedo gear
if you want speedo to read 10% lower you just increase tooth count of current gear by 10%
as 10% more teeth take 10% more transfer case output rotation to get up to same speedo gear rpm

Thanks for the info.