What are the Differences Between Formula Dé and Formula D?
Formula D is a more recent version with a few rules changes and some tweaks.


Car miniatures are different.  I think the old Formula Dé ones are higher quality; that said, both are perfectly functional.
Formula D comes with some nice new 'gear stick' control sheets which let you track your current gear by sliding a gearstick, alongside a pegboard for tracking car wear.
(I find these more convenient than the old versions; they seem slightly faster and easier.)
Formula D has updated the rules a little; in particular there are more categories of car wear.  In Dé wear is measured by using up a limited pool of points of tires, brakes and fuel.  In D fuel is not tracked, but wear is counted against: tires, brakes, gearbox, body, engine and suspension.
(In play this isn't nearly as complicated as it sounds; our group found it just as intuitive as the old system.)
Formula D is newer and maps are still being published. There are 8 maps for D to date (counting the two in the box) compared to around 30 for Dé.  This isn't important, as the map format has not changed significantly; maps for Formula Dé work fine with Formula D.
Formula D allows street racing as well as Formula 1; map sets so far all have a Formula 1 map on one side and a "city" map on the back.  These use the same system, but have added hazards.
There are also some rules changes (less worrying about weather tires, more use of nitrous oxide... street racing characters also have minor special abilities and different car wear stats.)