My road bike has the PT and I recently got a Garmin that I use for power data and I also take it on my mtn bike which doesn't have power. Riding my mtn bike on flat recovery rides, I would estimate that Garmin produces numbers that are somewhere in the neighborhood of reporting 120%-160% of the calories that my PT would show. Just the other day I went out for an easy two hour spin on the mtn bike and the Garmin estimated ~1000 Cal. More likely around 800 Cal. Don't trust my numbers for extreme accuracy; what I do know is that the Garmin overestimates considerably.