Not to Worry (About That)
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There should be a standard rider in all automobile insurance policies that allows you to cancel the insurance on proper notice. This is so that you can sell or junk the car without having to continue to pay for a policy that can no longer 'pay off'. Also, most companies sell coverage in 6 month chunks (but not shorter) if that is what you want. So I don't really see any problems with your plan. Just make sure what your particular company's cancellation policy is, and more importantly, make sure that your 'beater' is up to the trip or else you'll be paying far more in repair bills just to complete the trip than any insurance policy is going to cost you.
AZBuck