No difference... Usually shift in the middle of the cassette so at least there's less friction. Shifting into the small chainiring will help with noise if you have an older kickr. My first gen kickr is louder than a 747 taking off.

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What's your CdA?

It is a complex answer, but the gist is that it does and doesn't matter.

In summary, ERG will dynamically adjust the resistance to arrive at a target power. So, the gear is irrelevant, and you just ride your cadence.

However, there are some considerations...

• When you are riding in a gear with a high wheel speed, the trainer us using a much lower resistance. This means that your pedal stroke needs less force at the dead areas to maintain a smooth stroke. The nut is you have a very different muscle engagement in gear with a low wheel speed versus a high wheel speed. A low wheel speed has muscle engagement like you are climbing a hill, because you need higher pedal force all the way around to maintain a smooth pedal stroke.
• A gear with a lower wheel speed is quieter.
• Some trainers have a resistance floor that is affected by gearing and wheel speed... for example my Tacx Neo 2 seems to have difficulty getting below 175W (I cannot remember) when I am riding a 52-11 gear.
• The Tacx Neo and Neo 2 exhibit the virtual tire slip at high power outputs and lower wheel speed. (This may be fixed in the 2T. It is not a thing with the KICKR.)
• TrainerRoad recommends you ride in a gear with a lower wheel speed on a smart trainer.
  

To the OP, may I add or clarify that by stating "wheel speed", we are referring to flywheel speed of a direct drive smart trainer.

GPLama goes into some detail about this topic. I believe most have their bike in small ring up front with a mid range gear in the cassette. This seems to simulate "hill climbing" as stated below, due to muscle recruitment. A nice benefit, is that a slower flywheel speed equals a more quiet trainer.

*hope I'm correct...the above is my understanding of the topic.

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Not everything is as it seems -Mr. Miyagi

I always ride in small front ring and middle in the rear when in ERG mode......As mentioned above with the flywheel speed of the trainer.

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-Brad Williams
Website | Twitter: @BW_Tri |Instagram: @BW_Tri | Strava | Co-Founder & Coach at: KIS Coaching
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Funny, I was just listening to one of the old TR podcast where Nate (CEO of TR) said he rides in his biggest gear to get the most flywheel effect. Like others have said, it's equivalent from a training standpoint but it makes some difference in, "feel".

Personally I pick a gear so that the miles ridden that gets loaded in my training log are somewhat the similar to the miles I would have covered outside.

The correct answer is it depends on what trainer you are using. Many will be more accurate at a particular rear wheel rotation rate. There are some posts about which trainers are affected and have power drift if you are in too big of a gear.

I just pick the quietest gear on my Hammer, which is 46 (1x) and 16t. For some reason any other gear in the back, including 15t and 17t are really noisy. I wish 15t was the quiet one because that would give me a more accurate virtual distance compared to riding outside

I leave mine in the small ring and the straightest chain line for the rear, keeps it nice and quiet

As others have correctly mentioned, the difference is how much momentum you put into the system. Apparently some are sensitive to effects of this, others aren't. Higher gear is more akin to flats with more intertia, but even the heaviest flywheel in the heaviest trainer will not compare to road cycling in the flats since there is no 60-80kg human mass moving down the road, offering momentum.

I for one very much prefer the larger gear/higher flywheel effect. Even if I concentrate, I get this robust tibalis anterior workout in a low gear/ high resistance setting, sometimes even some pain. Part of me going with a Full Kickr and not a Core or other trainer for my recent upgrade was that it had this nice heavy flywheel. When I am in 52/14 or so, it is louder but it is totally worth it for the feel compared to a wheel-on Tacx. I'd second GPLamas videos for more info.