Why can't I use umbrella insurance instead of auto insurance ...

I currently have my auto insurance liability coverage at 250/500/100.

Playing with the quotes it seems cheaper if I put my auto insurance coverage low (ie 50/100/50) and just buy an umbrella policy on top of that for $1MM.

What am I missing?

Never got into an accident all these years, and only 1 speeding ticket (2 miles over limit, which the judge threw out based on my excellent driving record)

Assuming I did, get into an accident, my umbrella would just come down and cover what my limited auto couldn?t. Am I missing anything? Thanks!

Source: http://finances.solve-up.com/insurance/vehicle-insurance/why-cant-i-use-umbrella-insurance-instead-of-auto-insurance/

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