I also run 3.9 in my 64, mostly low speed country driving. But like you I run a tall tire in rear to help compensate for the 3.9.
I run a 255/70/15 during summer show season and during start or end of season I run a 245/60/15, you wanna play now, LOL.
Finds the RPM way faster with the 245's. 255's are 29 and 245's are 26.5 tall so it does change the ratio quite a bit.
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64 Lemans hardtop
4spd, buckets
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