I would like to be at 2500 to 3000 RPM at those speeds. Hydramatic with a 3.08 rear axle and 710 x 15 tires are what my 51 Frazer Manhattan has. 4.11 rear axle in overdrive with 710 x 15 tires would put you in the 2500 RPM range, and a 3.54 in overdrive with 710 x 15 tires would put you in the 2000 RPM range. If you do not engage or use overdrive, increase the engine RPM by 40% approximately. Almost 3500 RPM for the 4.11's and about 2800 RPM for the 3.54 axle.