Bike tyre size/pressure question

Tried Zwift for the first time yesterday, so am very much new to this whole thing! I’ve been searching online to try to find an answer to this question, but without any joy, so was hoping someone here could help please?


I’m currently using my cyclocross bike on a “classic” trainer (Tacx Booster), with Garmin cadence and speed sensors, and HRM. My issue is to do with the tyre size - because I’m on a cyclocross bike, my tyres are a fair bit wider than the Zwift recommendation of “700c x 23mm”, and I’m using 35c tyres (in fact the minimum recommended tyre size on my rims is 25c). I also can’t inflate the trainer tyre to Zwift’s recommended 100psi, as the max stamped on the tyre is 75psi.


So, my questions are:

  1. How is this tyre size/pressure likely to be affecting my ride on Zwift? When I tried it yesterday my speed and power seemed particularly high, so I’m wondering if this is the cause, or there’s something else I could be doing wrong?; and


  1. If I changed to a smart trainer (e.g. Tacx Vortex), would the wider tyre / lower psi still be an issue, or is this only a problem related to using zPower (which a smart trainer would presumably eliminate?)


Many thanks in advance! Sorry if the questions seem daft, as I said I’m very new to all of this!

I’m curious about the effects of a 700x35c tyre on a dumb/zPower trainer as well - two friends of mine will have that setup and we’ll be riding together after Christmas. ^_^ Given the exponential resistance vs speed curve of most trainers, my guess is that the 3% wheel size/speed underestimation will give about a 5-10% lower power.

I don’t believe that tyre size/PSI would have any meaningful effect on a wheel-on smart trainer - the power differences between my kickr snap and power meter are within the ±5% accuracy stated for the snap for both a 23c trainer tyre and my usual 30c road tyre.

Don’t use knobby tires though - since all of my bikes are 142x12mm, out of curiosity I threw on the hardtail a few weeks ago. A Rocket Ron 29x2.25 @ 30psi felt … slow.