[Sarge2]

Actually did the measurement.. looks like counting the edge that the trim ring slips onto it's a wee over 6 1/4" What took me by surprise there is a fairly large amount of room between the ceiling and the roof...
What I also found was the cabling for the roof mounted antenna and there was not a tight fitting in the bundle... so tightened those while I was there and insulated the outter nut from each other with electrical tape...seemed to improve the FM reception...was fading on a few channels before.. didn't try the TV reception.... but never seemed to have any problem with the TV's..

Also checked to be sure what Ohm rating and they are (4ohm.)So being that I could not find any 6 1/4" ones I went with 6.5...from Crutchfield... Being the gluten for punishment that I am, I figured I could make those fit...The head unit is a Jensen so I went with Jensen speakers... I'll let you know how much trimming, if any, I'll have to do to make them work...