A lot of factor affect speed. The primary factors of interest here are aerodynamics and rolling resistance. CX bikes typical provide a more aerodynamic riding position than mountain bikes, but you could tweak the MTB a bit to improve that if you really wanted to. If you're talking about a 29er, it has the same wheel size as a CX bike, and so can use the same tires. A 26" mountain bike, on the other hand, doesn't have the same tire availability. Although there are slick 26" tires available, there are much better 700c tires available, so the CX bike has an advantage here.

Another issue is weight. Going uphill, a CX bike will have an advantage because it's lighter. Going downhill, the weight of the MTB will give it an advantage. In general, wind resistence on the downhill means that the uphill advantage is bigger.

Finally, there's suspension. If the MTB doesn't have lock-out suspension, you lose energy with every pedal stroke.


Yeah, this is no problem. You probably would want two sets of wheels to make the tire change easier, but otherwise no problem.

I like road bars better because of the wrist angle and the number of hand positions available. Riding technical offroad trails, the flat bar is often easier on your upper body because it's wider and offers more leverage. On gravel and smooth-ish dirt trails this isn't really a problem.

There are also "dirt drop" bars with offer a bit of both advantages.

As for "how fast", it's worth mentioning that for most people a MTB with suspension will be much faster than a CX bike on technical terrain. On dirt or gravel roads, I think the CX bike has the advantage.


A 53x11 gear would get you 56 kph at 90 rpm. CX bikes don't typically have that gear, but road components are compatible with CX bikes so it's an option. A 46x12 top gear is more typical for a stock setup, which puts you at 45 kph at 90 rpm, but 60 kph at 120 rpm, which isn't an unreasonable cadence on pavement.

A typical CX crankset has a 110mm BCD, so you can put a 34T small chainring on it. If the bike comes with a 38T small ring, that's often an indication that it has a 130 BCD crank and can't go smaller than 38T. You can get CX bikes with triples, that typically have a 30T small ring but can go smaller.

With Shimano components, at least, you can put a MTB rear derailleur on the bike and go to a 34T or 36T gear on the rear cassette. The SRAM Apex road group also has a rear derailleur that will work with a big cassette like this.