With an automatic (AW70) on a 1984 non-turbo sedan, about (very roughly) 70 mph as indicated on the speedometer (not necessarily actual speed -- see below).

But that varies with year (gearing), turbo or not (gearing), and sedan or wagon (different gears AND different tire diameter), hence my qualifications of my particular car.

Also, with an automatic, it will also vary with throttle setting (accelerating or not) and whether you're going uphill or downhill -- the torque converter allows this). I've got a tach on my '84, and its interesting to see the variation in rpm (a couple of hundred, plus or minus).

But 70 mph is in the ballpark per indication on your speedometer, which is strictly driven by your drivetrain (from tranny up to '85, or your differential '86-on).

AND ALSO just in case: if you're trying to use your tachometer in place of a speedometer to avoid speeding tickets, all of the above is also affected by your aftermarket tires! Don't forget that.