One thing that most people dont understand is that the more sun light, the MORE flash you need.
With bright direct sun light, you get deep dark shadows. SO, you need a bright flash to come in and "fill" in the shadow.
When the sun is out you have to stop down to get the proper exposure. Which requires more flash.
The sun is behind this car. (see shadow in front) camera settings we probably 1/180th @ f/9.5 or 11
It takes a lot of flash to equal f/9.5 or 11.
If it were an overcast day, the shadows would be softer and the differance between highlight and showdows not be as extream as it would be on a sunny day. On an Overcast day you may not need flash at all. But if you were to use flash, you would not need as much, because your camrea setting on an overcast day might be more like 1/60th @ f/4.0. Alot less flash required for an f/4.0 exposure than an f/11 exposure.