Originally Posted by semtex

Someone else could probably explain better, but I'll give it a shot. The purpose of a dyno is to measure power output. Now, the 'cleanest' way to do that is to take the engine out of the car and mount it to an engine dyno. But that's not exactly practical, right? So most people just dyno at the wheels. Not only is it a tad more convenient than taking your engine out, but you're also measuring how much power you're putting to the ground, which many would argue is what counts anyway. So, that being said, you want to get a reading from the wheels while in a gear that provides a 1:1 ratio, because a gear ratio of 1:1 means that the engine and the transmission’s output are moving at the same speed. You want the engine and transmission to be in synch in order to get an accurate reading. Does this make sense at all? Like I said, I'm sure someone else can do a better job of explaining this.