From what I've observed with my 1960 235 over the past 6000 miles, the optimum operating rpm, where I get my highest vacuum reading is at about 2300 rpms. Cruising at 55 mph with a 235 at 2500 rpms should present no problem for a sound engine and should yield a relatively high vacuum reading. In fact, I would go so far as to say that 65-70 mph if under 3000 rpms would not overtax your engine but wouldn't be as economical.
Dave
