I'd done a lot of Km's on a Tacx Flow over a number of years. When it got to around 12K km, it gave up the ghost, so took advantage of a special offer and got a shiny new, new-generation Tacx Vortex. Reads power, and is supposed to be very accurate. Peddled through last winter at fairly derisory power levels, and following a stint this season with a Powercal (HR Power meter … highly recommended!), have upgraded to a Powertap wheel. In spite of doing everything I can in terms of calibration and adjusting of roller pressure, tyre pressure, who-knows-what, it turns out that the Tacx reads under. Way under. I've had to put in a manual override of the calibration of around 115% to get it to (more or less) match up with my Powertap.
Against that - though this is an established fact! - I can't even begin to get the power numbers indoor that I manage outdoors. Feel like I've lost 20 watts or so!