RPM's MPH to gear Ratio/tire size?
Just off the top of my head……
A 275-60-15 is 2+" taller than a stock tire. This kills anything those 3.23s were giving you.
The RPMs at 60 will go up a bit with 3.73 but the car should be way better all around.
You could also go with a 275-50-15 that would give you a stock tire height and restore your gear ratio.
With that tall of a tire your effective rear ratio is probably somewhere in the 2s.