Quote Originally Posted by nscrbug View Post
Well, it appears they all use the "horizontal" or effective TT length...so I would assume that they are all measuring the same way. But I suppose it's always a possibility that they aren't.
They may all be "measuring the same way", but that doesn't mean they are measuring equivalent lengths.

Imagine a bike with a very vertical seat tube. To set up a proper saddle to crank relation, this bike would use a seat post with a large setback. Now imagine a bike with a very tilted back seat tube. This bike might have a seat post with no setback. With identical saddle to handlebar length, the second bike will have a much longer (effective) top tube.