rpm vs. speed question

I've recently acquired a 05 MazdaSpeed Turbo, 6-speed, a retirement
gift for myself. Love it so far. It's been 40 years since as a single
guy I drove a '64 Vette 4-speed. My driver for the last 13 years has
been a 6-cyl '93 T-Bird auto transmission. Now the questions. At 60mph
my Mazda is turning exactly 3000 rpm in 6th gear. My old T-Bird loafs
along at 1750. Both have roughly the same hp. My 300 hp V-8 Corvette,
with a 308 rear end turned about 2750 in 4th if I remember right.
Anyone know a good source for the relationship between rpm, highway
speed, horse power, number of gear speeds, etc. How can that old Ford
engine cruise along at 1750 when it takes the MX-5 3000 rpm? Why do I
need 6 gears in the Mazda when 4 worked fine in the Corvette? I
probably knew all these answers 40 years ago, but I need a refresher.