Poor resistance with trainer tyre on mtb

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I have a Cyclops op2 fluid turbo and have put a trainer tyre on my spare wheel but despite having wheel on tight and riding on smallest gear it feels like im in easy gear and riding on a gentle downhill slope with no resistance on wheel. When used with mtb tyre feels like ive more resistance (due to grips). Any ideas if going wrong with trainer tyre or the set up?