How slow is too slow???

Okay...Hubby's driving is driving me CRAZY.
I know we are old enough to move to a retirement community in Florida and I would love to do just that. The problem is he is driving like we are already there.
It's starting to scare me a little frankly but I am reluctant to say anything to him because I know he will get VERY upset.
He has always been a careful driver, almost to a fault and I can't somplain about that.
(I grew up driving in Southern California and I am very aggressive.)
He is very cautious but he drives SOOOOOOOOO slow, it is almost dangerous sometimes.
Even a few friends we have gone out with have asked me if he always drives so slowly.
If we are on the interstate where the speed limit is 70 mph, he MAY go 55 if we are lucky, it's usually about 50 mph.
If we are on surface streets, if the speed limit is 45 he goes 30, maybe 35. Same thing on a 55 mph highway.
It makes me very nervous to have 18 wheelers barreling down on us when we are on the interstate and cars passing us while we are on surface streets.
As I said I am reluctant to say anything.
He seems totally oblivious to it.
But it is worrisome.