Garmin vs Zwift info


I have a Garmin Fenix 3 hr, and use Zwift with a speed sensor, and I know a sensor isn’t real accurate for mileage, but Zwift is always 5 miles ahead, calories burned are way higher. I use both reading to compare but I’d like to use the Zwift because the numbers are better. Haha 

Would you trust the watch over app?

Speed within Zwift for zPower (which you are using) is determined the rear wheel speed of your bike, the power curve of the trainer chosen and this is converted to watts. With those watts, the weight you entered, the in-game bike used, in-game drafting and virtual elevation changes your in-game speed is determined. This speed will never match the speed that your watch is displaying. Calories are calculated using weight and HR over time and can be different between 2 apps.

With using zPower, neither are accurate. Another thing, when using Zwift miles/distance is meaningless, power over time should be what you are working on.

I was never concerned with time. I am training for an event and need an good estimated mileage.

Again, the only thing that matters on Zwift is power over time, that is the only thing that is real. Distance/miles are estimates at best (zPower is less accurate when it comes to estimates of power and distance).

How far is the event you are training for and how long do you think it will take you to complete it?

I do understand that. My question is that if I should choose one over the other, which one? When I’m meant to ride a 50 miles ride, I gotta have some sort of estimate.

It’s an IM

Unless you have some type of power meter or smart trainer the estimate for distance could be way off. With using zPower your estimated watts could be off making the estimated distance way off. 

50 miles will take about 2 1/2 hrs on a flat route on Zwift if you are doing about 2.0 watts per kilogram. 

For an IM I would suggest training with power and HR. Using the same power meter during training and the race will help you a lot. I use a 4iiii power meter ( for both Zwift and races so I can compare apples-to-apples.


If you must choose one then choose the Zwift distance. However I think you’re looking at it from the wrong perspective. Miles ridden are a poor metric even outside as you can do a brutal hilly 50 miles or a gentle cruise over the same distance on the flat. 

Indoors they are entirely meaningless. You need to consider something more like TSS or power output over time etc, not miles.

Hi Erica,

Awesome that you are doing an IM! - your first?

I agree with other opinions, here. Training for an Ironman (or any cycling race) using “miles ridden” as the main bike training metric is never going to end well. Kind of like the swimmers you see at the pool, swimming the same speed non-stop for an hour every day - yes, they have a great base, but won’t be getting faster.

Like Mark suggested, TSS is a great tool, re. how hard was the ride?, how much rest do I need? How does the ride fit in with the week’s training?   BTW, are you currently using Training Peaks?  I am a triathlete myself and also coach 4 Ironman distance triathletes. A power meter is compulsory for any athlete that I coach (zwift, too :wink: because I believe that if you really want to TRAIN and not just RIDE, training needs to be done with power. To me the most important 2 ways to spend your triathlon $, are a power meter and a professional bike fit (most triathletes I see in races have awful fits and then wonder why they have issues running). Also, z power is OK for a brand new person to riding/zwift but that’s about it.

I think that if you invest in a power meter, do a proper FTP test and follow some good workouts on zwift, you will probably see a noticeable improvement in your cycling performance - it’s a lot of hard work, but fantastic to see real results.  One of my female athletes went from an initial FTP test of 148w 2.2 w/kg, at the beginning of September, to 228w 3.4 w/kg, in mid January. Up until September she had been distance and HR training only. She has worked very hard and tells me that she hates me now and then :wink: but is going to be an animal, this season. Now, every ride has a purpose, with no grey or “junk” miles. I also believe her running has benefitted from this improved bike training.

All the best, for your training!

I didn’t see the previous comments.  Thank you so much for the help.  Ok, it makes more sense now.  I do need a power meter.  Sorry, I’m so new to all of this and it is a little confusing.  I also hadn’t cycled at all until 4 months ago.

I know the distance isn’t helpful, but I was focusing on that as my training calendar has me doing miles.  Saturday I road with Zwift for 2.5 hours.  It said I went 50 miles on my Garmin, and only 40 on Zwift.  Blah. 

I’m going to the bike shop today to see about a power meter.  Do I need that along with a speed sensor, or is the speed sensor not necessary?


Brad, it is my first Ironman, and LAST!  lol  I’ve been kicking my own ass, but 12.5 weeks left!


You would only need a power meter for Zwift, your in-game speed will be determined by your watts, weight entered, in-game bike used, drafting (I would suggest using the Zwift TT bike to take drafting out of the equation) and virtual elevation changes. 

There are different power meters that measure at different places: Pedal based, crank arm based (single sided), crank based (usually measure both sides), and hub/wheel based. 

You could still use the speed sensor when riding on the open road, but your Garmin can use GPS so that makes the speed sensor unneeded. 

Make sure you swim more than you think you should.

Erica, make sure you have the bike store people walk through getting your new power meter set-up (calibrating). A good shop will be able to tell you the best type, for your needs and budget. Re - your 2.5 hour ride. Difference may have been that your Garmin detects your indoor trainer ride (zero elevation) and you may have ridden a hilly zwift course (where elevation is factored in) 

Sounds like you’re in great shape - keep up the training and remember to have fun!

Paul, I’m definitely swimming a lot. It’s my weakest link. Scary.

Brad, I am definitely have fun on Zwift! It makes the time go by really quickly, and I’ve been doing some fun challenges. It’s also fun to see my body completely adapt to this regimen.

Thanks again for all your help, guys.