Interstate 75 used to go through Tampa and St. Petersburg, but then in the early 1980s, the powers that be in the Florida Department of Transportation decided to build the bypass through eastern Hillsborough County and changed the number on that portion to 275, indicating to traffic bound for Fort Myers or Miami that they should take the shorter route and stay on I-75. However, since the bypass didn't open for several years after the number was changed, motorists were no doubt even more confused in the intervening period.
There used to be a sign on southbound I-275 near the Ashley Drive exit where the edges of an old "75" shield were visible underneath a newer "275," but that sign has since been replaced.
Between Atlanta and Tampa, I-75 is a very boring drive, with nothing but trees and billboards to look at.