Go the other way. I have a four speed manual equipped with Laycock de Normanville electric controlled overdrive. Put into 3rd and you get a slow speed in town and higher overdrive if needed. Works well. Automatic transmissions make keeping a steady speed nigh impossible and account for a lot of speed variation. At worse you get braking on downhill slopes with cruise control especially on undulating contours. Also can always drive a manual when the clutch fails if you match revs to moving speed.
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