I'll bet you could interpret this data to tell rpm
vs mph in a stock V6 automatic Mustang:
http://farm3.static.flickr.com/2177/2196907768_4331b47c4a_o.jpg data - looks like fourth gear to me -
http://farm3.static.flickr.com/2296/2196119507_07b9b8f4b4_o.jpg graph
(Stock in this case means:
K&N drop-in filter, FRPP M-5230-GTA axle-back muffler, b@m@chips 91 octane Performance tune, 26.57-inch tall Michelin Pilot Sport PS2 275-35-19 tires{six-tenths of an inch shorter than the GT Pirellis}. Data from 11Jan08 JBA "rolling road" dyno run #2. First run gave 191.2 rwhp, 234 tq. b@m@chips charts show a stock-stock V6 puts down 176 rwhp and 190 tq, or near there.)
In the olden days one discovered the safe cruising speed of an engine by calculating piston travel in feet per mile. I remember that the VW Beetle of that era had a top speed of 72 mph or near there, and a theoretical safe cruising speed of 120 mph.
I'd guess that a car built with modern technology wouldn't be bound by the same rules as the 50s and 60s cars were, due particularly to advances in the areas of metallurgy and lubrication; however, piston travel in feet per mile would be a good research set for insight into the questions of wear and tear on an engine in high-speed cruising.
I'll see if I can find my spreadsheet that shows MPH vs RPM for (insert here) tire sizes and (insert here) differential ratios for automatic and manual-shift Mustangs.