Good point about gear ratio, if you post all RPO codes that begin with "G", (RPO codes are 3 character codes on a sticker in the glove box) we can figure out what axle ratio is in your vehicle. Another thing to consider is if the torque converter does not lock up, it will keep RPM higher. It may or may not set a DTC in memory or turn on the SES light if there's a problem with it. Starting out from a complete stop, driving civilized,
you should be able to feel 4 shift points with the last one around 50 MPH. You should also see the RPM drop on the tach at each shift point. Post your findings.