on my 235 new rebuild motor going down the road ( auto power glide ) its 40 or better and when hot ( 80) or so it drops to 10 to 15 just as soon as you touch the gas it jumps to 35 to 40 ,, i was told that was normal

just saying

1958 chevy biscayne
235 6 cyl 1 barrel
2 speed PG tranny