With the 31's mine could easily do 90+mph and I did 80 on a regular basis with the cruise set. Though I have all new suspension parts lol. The highest I've taken it with the 33's is 75 and it held and rode great.
I've done a full tune up, replaced all my vacuum lines, fresh oil, filter, air filter, fuel filter, cleaned the throttle body, and it is running great. It doesn't feel like a total slug anymore. The vacuum lines made a pretty big difference, I never could find an actual leak but all the hoses were cracked where they slide on so I think there were probably some teeny tiny holes that I couldn't find, and all that building up dragged the engine down.