Currently 42 28 on the front for xc racing.
Switching to a 29er and wondering what gearing to choose?

In theory 10% lower give or take (so 40 or 38 and a 26). In practice, most people just manage – give it a go and then change if you feel you need to.

I’m thinking the same. why wouldn’t you keep the 42 at the front so you can go as fast as you can?

Well there’s not much point having the 42 if you never actually use the top gears so it depends on fitness, etc.

10% larger rolling radius than a 26″ wheel, so gear down by 10% to compensate.

Had been thinking 1 X 11 but reckon I’ll just stick the 42 28 on and see how I get on

to be fair, if it’s XC racing you’ll not greatly notice the difference in gearing. The wheels carry a bit more speed.

I moved from 42/32 and a 12-27 on my 26, to the same on my 29 and to my surprise didn’t notice a difference.