How do on-road mountain bike speeds translate to road bike speeds?
When cycling on roads on my mountain bike, I generally get around 10-12 miles per hour on a flat surface.
What sort of speeds can I expect to get on a road bike?
I think this is a slightly different question - it seems to be "If I can go X mph on bike A, and switch to bike B, what will my speed be?"
As @Gary.Ray points out, you seem to be asking how to estimate _your_ road bike speed based on _your_ mountain bike speed, but many answers seem to ignore this. Admittedly your title seemed to me to make it pretty clear, but I wonder if you might not emphasise this in the body of the question as well.
On my reasonably flat commute I average around 16 mph on my road bike, but your average speed is dependent on many different factors. A general rule of thumb is that if you are switching from a mountain bike with knobbies to a road bike you will be between 15-20% faster at the same watts/effort. Typically that's only a change of 2-3 mph.
I teach a bike commuting workshop, and one of the most common questions is whether to switch from a mountain bike to a road bike in order to increase speed. Typically, I tell people to try three things first:
- Swap out your knobby tires for high pressure slicks. You can find 1.25" -1.5" slick tires that fit mountain bike rims and run at between 75 - 90 lbs of pressure. These will dramatically reduce rolling resistance.
- If you have a suspension either lock it out, or set it as stiff as you can. Locking out your suspension will cause more of your effort to be transferred directly through the drive-train and translate into less loss of momentum from shock absorption.
- Try clipless pedals. Your pedal stroke will be more efficient, again resulting in an increase in speed.
If you do those three things the only real differences between a road bike and your mountain bike will be the weight (which matters a lot more when accelerating than it does when you are already rolling) and surface area resistance from riding in the drops. But most road bikers don't spend much time in the drops.
Finally, your weight and fitness make a huge difference. I frequently pass road bike riders while on my commuter rigid frame mountain bike with high pressure slicks.
I would also add that in a road bike I find that you can put power into the pedals easier than on a mountain or especially a cruiser bike.
I suspect the difference is even greater for someone in good physical shape. My touring bike, unloaded, is about 25lb and has 26x2.0 slicks. I find I cruise at 15mph on it, whereas on my racing bike it feels like I do 20mph+ without even trying. Don't underestimate the effects of weight, stiffness, and rolling resistance.
This answer ignores the primary difference between a road bike and a mountain bike. That's fine, for commutes or solo rides, but not so good if you ride with someone who's on a road bike while you're on a mountain bike. The difference is gear ratio. For instance, the typical top end MTB gearing, with a 44t big ring and a 12t rear, and a typical cadence of 90 RPMs gives you a speed of 26 mph possible. The same cadence in the top gear of a compact double road bike, a 50 x 11 usually, gives a possible top speed of 33 mph, at the same level of effort. Standard 53 x 12 is about 32 mph.
Of course, you need to be fit enough to maintain that pace. But since you're comparing the same levels of effort, that's a bit more than 2-3 miles an hour. More like 8-9mph faster. Which on a 20 mile commute = 46 minutes on the MTB, versus 33 minutes on a road bike, at the same level of effort. This is strictly a matter of gearing and mechanical advantage, for a roughly 30% increase in speed. BTW, all of the math here assumes the same tire diameter, which is not typical. The difference in wheel size alone, assuming high pressure slicks, is worth the 2-3 mph difference that @GaryRay claims.
I first read your post and was like "clipless pedals", huh? But then I came across this article which explains why they're called "clipless" (when they do actually clip in)... http://gizmodo.com/5990381/why-you-should-switch-to-clipless-pedals
One minor difference is the rider position. Most road bikes have drop bars so the rider is crouched over vs sitting somewhat upright. You can add drop bars to your mountain bike to help with that. I would say this is fairly important because at about 15 mph air resistance is a lot more dominant that rolling resistance.
Just a single data point but these are the speeds I would observe on flat ground for 3 - 5 mile rides- 1)Mountain Bike 26" with knobbies, no clips, suspension normal, bike weight >25 lbs very close 15 mph 2) Mountain Bike 26" flat tires, with clips, locked out suspension, bike weight > 25 lbs: about 18 mph. 3) Road Bike 700cc small flat tires, with clips, bike weight about 12 lbs: 19 mph. So most of the increase you get from flats and clips. But you still get a bit from the road bike.