MPH calculation is the way to go for the honest HP value after you make a 1/4 mile pass. To take it one step further if you wanted to compare after modifications to a given car, or to compare two cars you can correct to sea level, making it alike comparison.
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What sort of sucks though is when people take their advertised engine HP and then take off 15% then plug it in backwards to the MPH calculator to tell others how fast their car should be for MPH trap speed. That's just flat out wrong.