Gear Ratio and RPM deciding on gears

the math says 4000rpm at zero slip =78.58 mph with 27" tires.
65 shoulddabin 3320@zeroslip.
With an automatic and 65=4000 that would be ~21% slip. That's a lotta slip

With a manual trans. I'd be looking to verify stuff.
My 4.88s cruised at 65=4000 with 27s. Yur right, it sucked so bad I had to get a GVod, to drag it down to 3120