http://www.hmfengineering.com/yfz450pg.html

Seems that the FMF dyno was wright, FMF claims 37. HMF claims 40.1 HP with full exhaust. Seems like Yamaha spread wrong rumours with the 50 HP... Strange title : tested with stock tires, 49 HP with non-tred tires... Didn't know that tires made that hp difference !

 

All Yamaha specs would be measured and the out put shaft, not the rear wheels. They did dyno some bikes and the ran 40-43 RWHP(but they don't publish those numbers), however all dynos are different and bikes too. As time goes on all the magazines will do their own Dyno runs and you will probably see several different numbers.