power/best efforts, Strava and Zwift issue

I like to go for PRs as self motivation so when I’m in London sometimes I’ll do the sprints to see if I’m getting any faster. I had a bum trainer before, so I was trying to get some baseline numbers today, but I don’t think I"m understanding the power/best effort relationship (power newbie).

I noticed that while I’m riding Zwift was taking my first sprint as my best effort, even though it was not my best power (not even close). This happened on both the forward and reverse sprints in London. Then when I went to Strava, same thing.

The thing I’m not understanding (since it really comes down to tenths of a second) is why my best sprints when using average power are not my best ranked when looking at my results in Strava?

Today for forward sprint I had 245w, 323w and 318w, consecutively. For reverse I had 279w, 323w, and 302w for the 3rd effort. However, both Zwift while riding and Strava afterward is ranking my first sprints today in each (forward and reverse) as my best for today. Anyone know why my best avg power wouldn’t be my best effort of the day? I don’t know much about power, fwiw.


Did you start with the same speed going into the sprints, if you start the sprint with more speed you can potentially use less power in the sprint section and still have a better time. 

Did you draft of someone for the first sprint that will also give you more speed for less power.


Average power has nothing to do with it, it’s about speed. You can do less power but be faster by getting your speed up ahead of the sprint start line, drafting can also make a HUGE difference in time. This is one of the few times within Zwift that power does not mean all that much.

Well shoot, I was sort of saving up and ramping up my sprint as I went along. Didn’t realize it was purely speed based and so you also want to come in at the start pretty strong. 

So the benefit of increasing the power you can sustain is…? I assume that translates somehow to speed or maybe climbing power (personally I’m a very slow, VERY steady climber for long periods of time; I’m most definitely not a power rider). 

Yes it is just a timed section, if you have more sustained power you then get more speed going in and then keep it high up to the line.

I’d also concur that this has more do do with speed than power. Power plays a large role in determining speed in Zwift, however it’s not the only determining factor. Others here mentioned drafting as well.

This article has some good information on the correlation between power and speed in Zwift.

This article is also informative in understanding w/kg in Zwift and the physics behind it all. 

Appreciate the info! Bike is out for a repair and spring finally arrived (ie, real road riding), so I plan to try heading into the sprint with more speed next time I’m on and London is on the calendar!