The new Nike Vapor Fly shoes improve running economy 4% what does that mean?

On November the 6th Wouter Hoogkamer, Shalaya Kip, Jesse, H. Frank, Emily M. Farina, Geng Luo and Rodger Kram published an article establishing the new Nike vapor fly shoes as 4% faster on average for a group of runners (https://link.springer.com/article/10.1007/s40279-017-0811-2).

  
But what does a 4% improvement in running economy really mean for an average runner? Let me give you an example using Bob.  Bob is an average runner with a 20 minute 5k personal best and Bob weighs 175 pounds. Lets assume Bob is in tip top 20 minute 5k shape and signs up for the latest flat and fast local 5k. Bob will be pacing using his Power2run App at exactly 320 Watts. Even Power will give him his fastest possible finishing time. How much faster will Bob run in his new $250 Nike Vapor Fly shoes?

From the publication we know that Bob will run with 4% greater running economy. That translates into a nearly 15 Watts energy saving. We estimate Bob will need to produce 320 W's to run his 20 minute 5k. But with his new shoes he is now 4% more efficient so an extra 4% of Bobs energy will get put into his speed and his time be faster.  How much? We calculate Bob will now run his 5k in approximately 19 minutes 20 seconds for a 40 second improvement and a new PR.  That is a spectacular time savings of 12 seconds per mile. WOW we hope its true!  

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