^^^ This. I made the same switch in my Dart with 255/60/15's The car preforms better with the 3.55's but on a highway drive of an hour or more its a little much for me, keeping up with traffic. I usually end up running at 60 mph. Your results might be better than mine though, you being 4 speed vs my auto with 3800 converter. On a once a year drive I would probably just keep my speed down and enjoy the drive. This is my opinion, you might not mind running 70 or so.
