Chevrolet Corvette Forum : “it says its a 4+3 overdrive??”

Never heard of it. Try posting in a Corvette Enthusiast forum to see if they know. Here is one:
http://corvette-nation.com/corvette-forums/viewforum.php?f=10&sid=a113a284cd5ecea2c957e535d0e6ceb3

The car has 2 trannys 4 speed and a 3 speed automatic. Theres 1st gear 2nd gear 2nd gear od when button is engadged 3rd gear 3rd od 4th then 4th od thus giving you 7 gears 4+3 overdrive