$ .02 more :
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final drive ratio with both Toploader 4 Spd and C4 3 Spd is
1 :1 to Driveshaft. Late 60's and 70's
small block six cars sacrificed performance with tall rear gears to keep RPM's manageable with new Interstate travel speeds.
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for 'modern' acceleration needs and interstate
Drivability above 70 MPH interstate Cruise speeds an
OVERDRIVE tranny is simplest solution.
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AFAIH - are there really no
automatic-overdrive bolt-on possibilities available for the small block six?. (AOD ?)
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T5 overdrive is so far
simplest option to upgrade overall modern drivability of small six cars.
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'61 250 Tri-Power has 3.80:1 posi gears, with T5
OD can safely drive to the track @ hour and half away at Thruway
cruise speed. ( close to 1/4 end speed ! )
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the '63 170 Falcon longroof evolved through 3 on the tree to a Dagenham 4 Spd and finally an 80's NWC T5 rebuilt for $200. 4 spd Dagenham had great gearing and 'Hypo" lockout shifter but lacked cruise speed with OEM 3.50 :1 rear gears. T5 gearing can match most six builds and lower ratio OEM rear axles match T5 gearing.

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... I never learned to drive them automatics' ...
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