Here are the numbers.
Using 3.55 rear/275/50/15 tires(25.9") / trans ratio of .68
75 mph= 2349 rpms
70 mph = 2192
65 mph = 2036
60 mph = 1879
55 mph = 1722
50 mph = 1566 Now this is where you get into 1600 rpms range.
Personnely speaking, I would stay in fourth below 55 mph.
And for the record.
I do have another car that I run with 3:90 gears on the street. I tach 2950 at 60 mph using a Close ratio Super T-10. When I made the statement about beating the engine my comments were sincere. 9 mpg using 94 octane pump gas. 3500 lb car that needed a little boost from behind.
Bud
http://www.richmondgear.com/101032.html