Need tire circumference setting for classic trainer

as instrution 've recomended  700c tire for classic trainer. for mtb tire is smaller than road bike, make over watt calculation.

i hope u will add adjustment option for tire circumfenece in later patch.


Tire size is definitely planned!


Weel C 2136mm 28C 39 km/h = 400 watts

if zwift C=2096mm 23C 38.2 = 388 watts

Any updates? 

I’m not using Zwift just because my tyres circumference is about 2288 mm (thickslick 29") and my FTP test shows 63 watts and snail’s pace (TR shows 3x more power with same conditions)

Is not the best solution.
TrainerRoad have option TrainerRelay to transmit as a power meter to zwift.
requires 2 ANT+ dongles

Would be nice if this update happened soon. I’m a large guy so use 28mm tyres for a bit of extra comfort and rim protection on the road. I don’t have trainer-only wheels and don’t fancy changing to a 23mm tyre (if I have a spare lying around) each time I use Zwift. In the meantime my power reading is less than it should be and at nearly 100kgs I need all the help I can get.

I’m using my old bike, circa 1985 Miyata Ninety, in a Kinetics Rock and Roll trainer as my Zwift setup; the tire is 27 x 1 1/4 (32-630).  Aside from the obvious mathematical 8mm difference, how much difference is there between my tire and a 700c x 23mm and how much is it throwing off the zPower curve/accuracy?  Right now I’m lucky to be clipping along at 1w/kg…I weigh 210 lbs. so that’s 95.25kg and I’m LUCKY to consistently keep a 100w pace.  It’s usually closer to 90-95w avg. on a good day.  It seems EVERYONE passes my slow butt no matter the course…sometimes it’s a little discouraging.  Maybe I AM just that slow!!! :slight_smile:

2161 mm 27 1 1/4
2096 mm 23c

100watts = - 5watts for tire

zpower Zwift kurt kinetic 23kph = 104 watts +tire = 109 watts
vpower trainer Road 23kph = 131 watts +tire = 138 watts

Thanks Maurizio!

Please explain what you’re trying to tell me here.  What I gather from your calculations is that in order for me to achieve vPower wattage, I have to work over 20% harder with my zPower setup…is that correct??

vpower,zpower = power estimates.

somer user test zpower with powermeter = pretty accurate.

some user test with powermeter wattage was high by about 20-30 watts

some user test with powermeter wattage was low by about 20-30 watts


Hi Eric,

11 months ago you said this feature was planned. Is there a timeframe for its introduction?


Hi Jason,

There is currently no timeline for this feature. It’s still in the list, however. We’re still a small team so it does take time to get to things and new priorities do come up.


Hi Eric,
Is there a way to adjust the classic trainer (tacx twist) in order to simulate a 28c tire as 23?


Hi Petros,

I don’t believe there is but you would have to contact Tacx Support for confirmation as it is their trainer.


I’m new to Zwift and I can’t tell from the thread above whether I’ll be falsely over wattage or under wattage when using a 32-630.  Can someone explain to me in simple terms?  If Zwift says “You’re at 200 W” will my true wattage be more or less than that number?

Hi Bob H.

Zwift assumes a 23mm tyre on a 700c rim for the purpose of calculating your power and then your speed in the game when you are using a so-called ‘dumb’ trainer. As you may already be aware, Zwift uses wheel rotation speed in conjunction with circumference and a known (for Zpower where Zwift have tested the trainer) or assumed resistance curve for the trainer to calculate power. It all falls apart though if there is no allowance for the inevitable variability of tyre and wheel sizes.

If the circumference of your wheel and tyre combination is larger than that of a typical 23mm tyre on a 700c rim (which yours is by a large margin I believe) then the wattage reported on Zwift will be lower than the wattage you are actually producing on a given trainer. The opposite is true if the circumference of your wheel and tyre combination are smaller than that assumed by Zwift. The degree of error will increase as the difference between actual and assumed circumference increases. So you could ride a BMX and blow by other riders like they were glued to the spot.

I use a 28mm Continental tyre on a 700c rim. That gives me a circumference of approx. 2130mm versus approx. 2096mm for a 23mm tyre on the same rim. I have done some unscientific comparisons between Zwift and Cycleops Virtual Training (CVT) using my Fuild2 ‘dumb’ trainer and for a given cadence I have seen a circa 10% difference with CVT providing the higher reading. CVT allows you to enter the exact circumference of your wheel and tyre combo and it makes a substantial difference when riding. Because the power is estimated for both Zwift and CVT it tends to bounce around a bit so it is hard to determine exact differences.

I also believe that there is an extra sting in the tail, but I am happy to be corrected if anyone has done proper testing or crunched the maths using accurate resistance curve data. If you use a ‘dumb’ trainer with a linear resistance curve (although it’s not really a curve then) such as most magnetic trainers then there is no extra sting. If, however, you use a trainer such as the Fluid2 where the resistance curve rises exponentially relative to speed (of the trainer roller) then the larger circumference causes you to suffer the effect of this difference more and more as your wheel speed increases.

So the options are:

  • Zwift allows the ability to set circumference, or

- You put a 700c wheel and 23mm tyre on your bike, or

  • You buy a power meter, or

  • You buy a direct drive (wheel off) ‘smart’ trainer with built-in power meter/estimator.

It seems to me that 3 out of the 4 options inconvenience the paying customer.

Hope this helps clear it up for you, Bob.

Ride On!


Bravo on a thorough and detailed explanation.  I’m sure other will benefit as well.  I’ve found a 700c x 23c wheel and tire and plan to mount that later this week and give it a shot.

For those who haven’t downloaded the Feb 13 update you should do so as soon as possible. You can now set your tyre size when pairing your ‘dumb’ trainer! Woohoo!

I’d like to say a very big thank you to the Zwift team for getting this done. It does make a difference.

Can I suggest that Eric C communicates this to all who have asked the question via the many posts that have been created on this topic? Working in a software company myself, I know that not all customers read the release notes :slight_smile:

Thanks Zwift team. Sincerely appreciated.

Nope… 2 years later and you still haven’t added the 29" tire sizes… Uninstalling and not subscribing. Tyre size is 53-622 and that’s only my trainer wheel. Outside I ride on a 29x2.1 … 2 years guys. Its literally adding the values to the calculations and as a developer can be done in less than a days development (Heck it should take 10min) 

Also using the excuse of a small team … Nope. Lets hope you extend your trail period when you one day cater for mountain bike wheel sizes


Choosing a tire size is already available within Zwift for users using zPower/virtual power. A 29 inch mountain bike tire is close to a 700x32 and it should get you really close to realistic numbers. The differnce between them is 13cm in circumference, which should be close enough for zPower and should get your numbers really close:

Here is a link to an update to Zwift in February on what changed: