Just get some gravel tires and put them on your current wheels. Unless you are going super-skinny on tires or have really wide rims, a 700 gravel tire will mount right up on 29r wheels. I have some nice 40mm gravel tires that I run tubeless on my mtb rims and with tubes on my cross bike wheels. Other than looking funny on my mtb, they are fine.

You can try to find a gravel bike with the same front and rear hub spacing as your mtb, but that could limit your choices.

Also, there are a lot of folks riding pretty wide “mtb” tires these days on gravel. It’s certainly more comfortable and can be faster depending on the surface. Some of the new gravel bikes are basically hard tailed mtb bikes w drop bars.

Schwalbe thunderburts and continental race kings are both around 2" wide and very good tires for a 29er gravel rig that should mount no problem to any existing mountain rim. IMO the thunderburts are a faster tire but the race kings can normally be found for much less money. Both set up tubeless well, although you may need a little more sealant in the lite sidewall versions.

Good luck, have fun!

Your current wheels will likely fit gravel tires just fine. I'd try out a tire swap and see how you like it before pulling the trigger on wheels.
Such a niche wheelbuild is a bit risky... if you're contemplating building a gravel bike in the future, would it obsolete your new wheels if you didn't match the hub spacing of the gravel bike to that of the MTB?

I wouldn't call that a "conversion" anyway, though. Tire width isn't really that big of a factor in an MTB's lack of road attitude, and you're already paying some of the costs of wide tires regardless of what tires you actually mount (like wide q-factor). If your gravel riding has particularly rough spots, some wide fast tires can be a better choice than skinnier "gravel" tires anyway.

For dabbling in gravel on a mountain bike, I'd personally just mount some fast cross-country tires.

I would second the Thunder Burts if you want a super fast mountain bike tire. They roll really well on any dry surface and even wet gravel roads. Wouldn't be my first choice if there's a more pavement than gravel, or if it's super slick mud, but otherwise it's a great tire. I run them on my "gravel" bike, which is a rigid MTB with Jones Bars. The only time I don't use them is like I said, if there's more pavement than gravel.

Also consider maxxis pace 2.1's. Pretty fast rolling for mtb tires.

If you are looking for minimal and really fast tire, Maxxis velocita AR 40mm. It's almost a slick, but good for dirt roads and mild loose stuff. Not a good choice for a chunky course or mud.

Keep an eye on the reach if you plan on installing drop bars. Mountain bike are designed around flat or riser handlebars where the grip is pretty close to the stem clamp axis and stem length under 100mm. That doesn't give much room to go shorter if you go to drop bars, where your grip will be at the hoods/drops, significantly further ahead of the stem clamp axis.

Yes, it's possible to swap out your mountain bike wheels and put on a set of wheels that are compatible with skinny tires for gravel riding. when looking to swap out your mountain bike wheels for a set of wheels that are compatible with skinny tires for gravel riding, you'll want to make sure that the new wheelset is compatible with your bike's frame and fork, that it can accommodate the size of tire you want to use, and that the tire you choose is suitable for gravel riding.

OP asked this question 4 years ago, so hopefully 'gravel' excursion on MTB been and gone and either OP is back to proper trails or N+1.
Or they are no longer with us, having overdosed on Powerbars.

Dang...2019 was four years ago :(

For those whom are interested - even though it's now 4 years later ;)

The Thunderburt is astonishingly fast-rolling. It's essentially the same as a 40c TerraSpeed at appropriate pressures (Terra Speed 40psi / Thunder Burt 25psi) and likely faster at higher pressures (smooth surfaces only). There will be an aero and weight penalty, so I can't speak to over-all performance / speed.

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