I think either one is accurate. It took me a few months to get this somewhat close.
13.5 mph means little. Look at the grade, wind, bike, rolling resistance. Mountain bike or road bike, trail or paved street. Lots of variables. You need to find a system that you can control the variables to calibrate yourself.
The best you can do is to find some kind of baseline and fiddle with your HR monitor to get close to the same readings. I lie to my HR monitor so it calculate pretty close, with history it must be close or I would be really fat or really skinny.
Ball park will work. A 160lb person going 20 mph means little, how heavy is the bike, what is the rolling resistance, grade up or down, wind resistance, HR???? So your HR is close you can forget all the other crap. If you go into the wind the HR will rise, down hill with the wind it will decrease.
I forgot to ask about the whole wind resistance thing. We don't have a ton of hills in this part of Minnesota (although it's not as flat as out west of us) but the wind can be a monster sometimes. It never feels like I get as much help from the tail winds as I do resistance from the head- or cross- winds. That may just be me being a wimp.
When you put your activity in the Sparkpeople fitness tracker and it tells you how many calories you've burned riding, say, "x" mph for 45 minutes, is that based on a flat riding surface? Do you adjust at all for riding hills etc... or do you assume you pick up as much 'free' energy going down the hill as 'extra' energy you use going up? I know the only REAL way to know is to use a heart rate monitor but one's not in the budget anytime soon. I don't want to over or under count my calories too much though. It just seems like doing 45 minutes on a flat surface at a given speed is a LOT easier than doing the same amount of time on a hilly surface at the same average speed?
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