With a DC battery, electricity actually flows from the negative (where the electrons are repelled) to the positive (where the electrons are attracted) side of the battery. It doesn't matter which grounding method you use (positive or negative ground), the electricity will still flow the same direction.
On the Illinois Education website the question of electricity flow is answered thus:
Electrons are negatively charged, and so are attracted to the positive end of a battery and repelled by the negative end. So when the battery is hooked up to something that lets the electrons flow through it, they flow from negative to positive.
For the most part vehicles use a negative ground (earth). Manufacturers quit using a positive ground (how things started in the automobile industry) because with this setup, galvanic corrosion occurs. Some vehicle still use a positive ground, but they are far and few between.
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