that doesn't seem right. my old '86 5th ave with 3.55's, 42RH (.69OD and lockup) with 255/50R17 (27" tall) was 2000 RPM at 65/66 and 2150@70. your tires are almost 2" taller. it should put 2k rpm right at 70...
I was on the highway yesterday and watched closely as I kept it right on 100kph which is 62mph. It was just over 1900 RPM, and 110kph which is really close to 70mph was about 2000RPM.
