It doesn't take an average so much as the fact that your car is still accelerating *ever so slightly* between the beams, for instance maybe you're doing 99 when you cross the first beam and 99.25 when you cross the second one but what comes out is 99.125, hence the "average". It is nowhere near the big deal that Gtech ignorami say it is when trying to explain the Gtech's typical 4-mph optimism because nothing short of a full-on 8-second drag car is still accelerating 4 mph in the last 60 or 66 feet.
Yes, and the reason they do it is because it's easy, accurate, reliable, and uses the same parts and systems as measuring the ET. Some tracks use different distances but 60 feet or 66 feet is most common...which takes less than half a second to cover at 100 mph. I'm not sure why it's 60 feet and not 30 feet but it probably has to do with tradition and the technology available 50 years ago...it's a lot easier to "amortize" a little timing error over the time it takes to cover 60 feet than over the time it takes to cover 10, 20, 30 or what have you. In other words if they get the placement of a timer a little less than perfect, it would throw off the MPH's less on a 60-foot length than it would on 30 or whatever.
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