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Fail-safes for cars are a gimick. They are not worth the money, period. All you need is a throttle return spring. They cost less than $3 compared to $40, minimum, for a fail-safe. I ran gas for several years and never had a runaway. If you set them up right they don't put any strain on the servo at idle, just put enough tension on the overtravel spring. Its cheap insurance, worth every penny, and then some.