I don't have to. This winter I changed my 200g tyres for Marathon Plus tyres at 600g each..... so I'm carrying an extra 800g.
On a 50+ miles ride, the extra effort is hardly noticeable at the beginning, after a few hills and on the last part of the ride I could not keep up with a riding mate and I just had to drop off.
Before I changed the tyres and doing the same ride, I was
always riding in the front but with the new tyres an extra weight I couldn't manage leading the whole ride. The difference, I have to allow for rolling resistance which the M+ aren't too bad at all, much better than I expected but the weight, undoubtedly, played a major role on the hills.
I had the full ride in the garmin and also the video so, at home, I put together the two of them. I don't have a power meter but I could tell that near the top of a longish but not too steep hil I ran out of energy. My heart was pumping ferociously and then my cadence drops down and the video footage captured my mate overtaking me. It was physically impossible for me to overtake him after that.
Would I notice 2kg of water in my tyres? My legs most certainly would