Distance on Zwift does not parallel distance measured by Garmin

(Dan Perkins) #1

Not sure if this is an issue or not, but . . .

Since riding counterclockwise, I have found that the distance measured by Zwift is consistently and significantly shorter than distance measured by my Garmin. Did not have this problem when the direction was clockwise around the island.

Here are my constants:

1-hour rides
dumb trainer (Kinetic Rock & Roll)
Quarq GXP power meter (it’s old, but has been reliable)
Generic HR strap paired with Garmin
Speed/cadence sensor (can’t recall the model)
Garmin 500

When riding clockwise, I would find that distance in-game was periodically off from distance on Garmin, but it seemed plausible that this was due to the changing grade of the terrain in-game, while my trainer remained constantly flat. This was confirmed by watching the distance measures both in-game and on the Garmin. I’d find that during uphills, I’d slow down in game and get ahead in distance on the Garmin, but catch up on the downhills and flats. Results were that final distances were always pretty close – usually within .1 or .2 miles of each other.

When riding counterclockwise, the same phenomenon occurred (unsurprisingly); however, I was never able to “catch up” in-game. On a 7-lap ride, I would continue to slowly lose ground between in-game distance and distance measured on my Garmin. An hour-long ride usually ended with the Garmin showing between .6 and .9 more miles ridden than the game.

Wondering if this is a glitch in how speed is calculated in-game, or is it somehow due to the longer hill going counterclockwise? It seems to me that regardless of the hill, the distances should still even out in the end. Minor issue, but I’ve noticed it consistently since the switch to counterclockwise.

(Mark Duncombe) #2

I would imagine the speed and distance in Zwift is done purely by the amount of power Zwift sees you producing which is input into their physics model to produce speed and distance in game. Zift would not look at how far you wheel travels, just how much power is needed move the in game bike a certain distance. Thats how I see it anyway.

(Dan Perkins) #3

Well, that is likely true; however, I’ve done some rides in both directions with fairly constant power, but still see those gaps going counterclockwise, but not clockwise.

Suppose I ride a full lap in either direction at a constant rate of 200 watts. Theoretically, even if there’s a difference between the distance on my Garmin and the distance in Zwift, it should be the same regardless of direction. I’m not getting that. I’m getting a significant discrepancy going counterclockwise, but not clockwise.

(Mark Minn HPP) #4

Might have something to do with the grade of the hill being different from each direction.

(J P. Miller) #5

You climb for longer at a shallower grade counter-clockwise which gives you less distance/time to make up for it on the steeper downhill after Col Du Zwift.

(Eric C. (Zwift HQ)) #6

The distance and speed measured on your bike computer will never quite match Zwift as it does not take in to account all of the virtual terrain, wind resistance, and more on Zwift Island.

We are only using your power output (in this case, zpower) to determine speed and distance.

(Dan Perkins) #7

I recognize it will never quite match, but it seems to be significantly different going counterclockwise, but only marginally different clockwise. I suppose that is due to the longer, shallower climb on the counterclockwise loop, and that the distance isn’t really off, it’s just that I’m able to ride further on the “flat” course being measured by the Garmin than I am on the rolling course on Zwift.

It just seems odd that the rolling course going clockwise pretty closely approximates the flat Garmin “course,” while the other is so far off. Just not what I expected given the close approximations I experienced going clockwise.