well heres my question i have a 82 300sd that gets great milage aroud town but today i went to albany from binghamton ny. at about 75 the whole way and got terrible milage at about 18 mpg normal around town i get 25 what are some things to look for? the guy behind me said there was a constant light smoke the whole way.. it always starts good ,shakes at idle (common problem) its got 187000 miles great power uses very little oil any ideas would be great i would hate to have to get rid of it ugly as it is( work car ) i checked the IP it was right on as for the cam timing it is off but not that much PLEASE HELP!!!!!!! THANKS