Can I fit a 26x1.25 tyre to wheel with current tire 26x2.1 I've just bought a tacx turbo trainer and want to get a tacx training tyre for my mountain bike.

Can I fit a tacx 26x1.25" training tyre to my wheel with current tire 26x2.1" ?

Thanks

Matt
I regularly run 50mm and wider tyres on narrow rims which is the opposite of what you want to do, but suggests that your actual rim width might be narrower than you think it is. Viz, you might find that you have 25mm or narrower rims, in which case a 32mm tyre is perfectly reasonable.

I have run tyres narrower than the rim in the past but I don't really recommend that on leaning bikes. In this case, the question is for a trainer where leaning isn't an issue. The problem is that when you have (for example) a 45mm rim with a 25mm tyre extreme leaning can make the rim hit the road, at which point you will lose traction and crash. But this is an extreme problem, you're very unlikely to experience it in normal riding.

The increased pressure shouldn't be an issue unless you have exceptionally lightweight rims, but I'm guessing from the question that this isn't the case. If you do have a high-end carbon fibre mountain bike with ultralight, extra-wide carbon rims they should have the maximum rated tyre pressure printed on them or listed in their specifications. You should respect that. But that's not normally done for aluminium rims because the rim isn't the limiting factor for tyre pressure. 

I've blown the sidewall off a rim, and running a narrow rim with wide, high pressure tyres was likely part of the cause for that. The manufacturer replaced the rim without questions, however, suggesting that they agreed that it was a weld fault at the join (where the blowout started). But you're not doing that, you're running a narrow high pressure tyre on a rim that is most likely capable of working with a wide, high pressure tyre. I would not worry about this, especially on a trainer where you're more likely to suffer hearing damage from a blowout than major crash injuries.