Here's an online calculator to determine road speed (or RPMs) for a given set of parameters that include final drive gearing, transmission final drive ratio, and tire diameter.

http://www.bgsoflex.com/rpmmph.html

On your automatic you should assume that you're in overdrive at highway speeds and there's negligible slip in the lockup converter. That means the transmission final drive ratio is 0.696 : 1 (for every 0.7 turns of the engine crankshaft the transmission output shaft and driveshaft rotates one full turn).
You'll have to measure your car's tire diameter.