Math 101: your gear ratio should be the same change rate as your tire change to keep things equal. So 39.5/31.5= 1.2539. This rate of change is now multiplied by your rear gear to find your "new" gear, 3.73 x 1.2539= 4.67. While you're speedo won't be 100% correct you'll be closer with a 4.56 gear than a 4.88 gear. In gearhead speak, go 4.56 if you daily drive and care about mpg, 4.88 if those don't matter.
By the numbers, your truck is performing as if it had 31.5s and a 3.00:1 rear gear. YIKES!