Do I have 2.94 or 3.23 ?

I just bought a 68 valiant 360/380hp create engine with a manual shift 2800 stall 727.

I'm not a hundred percent if the speedo is accurate but after talking with my dad since he followed me home seems it's close.

Mathematically it works out to 3.23 but since a trans has slippage wouldn't that mean its 2.94 ? Revs 2500 at 59 mph with 245 60r 15. 2.94 would rev about 2250 which i would think theres probably 250 rpms of slippage. All the previous owner knew there they were highway gears.

When I was driving it back you'd have to drop it into 2nd to cruise through towns anything under 35 mph especially 30 mph.