Crank vs. rear wheel

1979 cordoba factory 318 four barrel orange box better valve springs good tune up on thermoquad == 17.80 @ 84 mph. Removed a/c pump and brackets. 4.56 gears, cheep headman headers. early non lockup 904 with 2800 stall.==15.40's @ 90+ mph. 3905 lbs with driver. Apples to Oranges? Remember show room stock 66-67 383 B bodies only ran 16.10's.