Highway RPMS ?

The standard equipment rear axle ratio on a 65 valiant with 225 and auto is 2.93. That info is straight out of the service manual and my 65 matches the info. Of course in the last 50 years the axle could have been changed. If you haven't ever changed your rear diff fluid you should do so and then you can just read the ratio off of the ring gear. The fluid fill plug is a 8 sided plug that you remove using a 1/2 inch extension and rachet. The fluid to use is 80w90 gear oil.