The added weight does make some difference overall. But once you are moving, you dont quite have the same effect as if you were wearing 100 pound pack and hiking. Its the problem with the wheels being involved.
Now if you were going uphill the entire way, you could easily notice the difference even with good gear switching and it would be a decent increase in over all work out.
A good heart rate monitor is about the only way to calibrate the difference between single and towing rides that I could find.
That's one where after you have some sort of data to see how it impacts you on the same route, you could always tack on additional duration to compensate for the difference. Again, only way I could see getting that would be with an HRM.
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