Fill Flash?

I am not sure I can explain my problem, but here goes.

I am trying to get my head around flash and Manuel mode. I have garage door opened standing at rear of garage facing out into strong sunlight trying to expose vehicle directly in front of me. Reading for proper ambient expose was 1/360 at f5.6, flash on auto.
The resulting shot exposed at 1/250 f5.6 and the vehicle was way under exposed, HSS was set.

I know it is operator error so can anyone tell me where I went wrong?

BTW flash was bounced off of white 9' high ceiling.

Take a look at

Flash Photography 101
The EOS Flash Bible
Techniques for Better On-Camera Flash

Since the camera used 1/250 shutter speed, my guess is the flash was not in HSS mode.

Bouncing flash off the ceiling greatly reduces the flash unit's efficiency. It cannot properly expose a vehicle at F/5.6 ISO 100 when used that way. It just doesn't have enough power. Maybe try this at dusk, so you can use a higher ISO. It takes a lot of juice to compete with the sun.

Curtis N wrote in post #5803015
Since the camera used 1/250 shutter speed, my guess is the flash was not in HSS mode.

Bouncing flash off the ceiling greatly reduces the flash unit's efficiency. It cannot properly expose a vehicle at F/5.6 ISO 100 when used that way. It just doesn't have enough power. Maybe try this at dusk, so you can use a higher ISO. It takes a lot of juice to compete with the sun.

Thanks Curtis...

Yes. The light is best then & you don't really need the flash: A few Car Lighting Tips - Updated

But, if you still want to fill flash, 1st get the exposure right for the car. Set that in M mode. Then try different distances for the flash AND different settings for it. Like Deanphoto did: MKI Audi TT Coupe

Don't you think this is just a situation where the strong backlight fools the evaluative metering? I would try partial or spot metering and/or add two stops of plus flash exposure compensation and try again.