You do have an overdrive, 5th gear is .811:1 and is by definition an overdrive since the output RPM is higher than the input RPM.
The 4-speed automatic has a .694:1 ratio, a much taller overdrive. Not sure exactly why, but I'm sure it's only possible due to the torque multiplication of an automatic. A manual probably couldn't effectively push a gear that tall.
My 2000 automatic spins just over 2500 RPM at 65 MPH.
Gearing will be negligibly affected by going to 31s, you have the equivalent of 30" tires already, and the same driveline pushes 265/70-16 and 265/65-17 which are 31" diameter.
Brent