Quote:
Originally Posted by Stang70Fastback
Well considering I've done the same thing with my car, and after each tank my car also had read exactly 1.5 MPG higher that what I had actually gotten, I think we can confidently say that the on-board computer is slightly on the optimistic side...
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Oh, I agree that the computer isn't accurate. I track my mileage using an app on my phone, and what's on the dash is usually about 2.5 to 3 mpg better than what the app calculates. It's very easy to prove the computer's calculation is unreliable.
What I'm saying is that he can't conclude that the computers in BRZs generally record 1.5 mpg higher than reality based on one car and one driver. He can only conclude that that outcome holds for him. If he wants to extrapolate to the entire population, he needs to select a desired confidence interval, calculate an appropriate sample size and test a sample large enough to generalize to the entire population.
How do we know from his experience that other people are not getting a 5 mpg difference? How do we know that some people aren't getting better mileage than the computer suggests?
The biggest concern I would have about relying on his numbers is that some people here have reported some rather low mpg readings from the dash without actually doing the math. I suspect that in at least some of those cases, the error is going the other direction, and they're actually getting better gas mileage than the car is telling them. But if they rely on his analysis, they'll think they're even worse off than they really are, because his analysis is only for his car and isn't applicable to their cars or driving habits.
Despite the quasi-scientific approach, this is still just anecdotal evidence at this point.