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Confidence level in testing is somewhat personal. The question is how certain do we want to be that our tests give are truly representative of what is actually going on. As a quality inspector I would be comfortable with five samples and a standard deviation of 1.4 to accept a batch of product from a proven process. If I was going prove to a major customer that a process line was operating within tolerance 99.9% of the time I would need to take 40 samples or more.
What we are doing is called 'hypothesis testing' in the statistics books. We are testing the claim that a certain product or technique will provide a detectable (and economically feasible) improvement in fuel economy. We may decide that we want to be 99% sure that on average there is a 5% improvement in mpg. Given that test, there are means to calculate the number of trials to run. Most libraries will have books on statistics or quality and you can pick one that matches your reading level.
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