if you are intrested in speed look up world speed record for a bicycle and good luck in getting over that. lol

15 mph = 79200 feet-per-hour = 1320 feet-per-minute

Mountain bike tire diameter = 26 inch
Big chainring (front) = 44 tooth
Small cog (rear cassette) = 12 tooth

Cadence for 15 mph (MTB)
= (road speed) / (tire circumference) * (small cog/front ring)
= 1320 / 6.8 * (12 / 44)
= 52.9 rpm

Road bike tire diameter = 700 mm = 27.56 inch
Big chainring (front) = 52 tooth
Small cog (rear cassette) = 11 tooth

Road bike speed (at same cadence)
= (cadence) * (front ring/small cog) * (tire circumference)
= (52.9) * (52/11) * (7.2)
= 1800 feet per minute = 20.46 mph

This is a really good question. I believe the answer depends on several factors.

With respect to drag, road tires will let you roll faster. However, not all mountain bike tires are created equally. In fact, some knobbies with a nice center rib roll real smoothly on pavement. On the other hand, some knobbies create horrible drag. So, your gain from tires alone will be dependent on what you are currently running on your mountain bike.

A road bike will allow you to get into a more aerodynamic position on the bar drops. How willing and able you are to get into a tuck on your road bike will affect your over all speed.

A road bike, of course, has no suspension so you will have no energy loss due to bobbing. Of course, if you lock out the suspension while on the road, this will make no difference.

A road bike is almost certain to weigh less than a mountain bike so you’ll get a gain there, especially if your ride is hilly.

A big gain, though, is going to come from the lighter wheels and tires of the road bike. In terms of the energy requirements, a gram of rotational mass is much heavier than a non-rotating gram. Thus, reduced rotational mass will allow you to go faster and accelerate the bike more quickly.

So, all in all, how much difference can you expect? On weight, aerodynamics, and reduced rotational mass I think you can expect to pick up 2-3 miles an hour. Depending on the other factors you may be able to pick up even more.

Hope this helps and I hope you have a lot of fun finding the answer to your question. Good luck to you.

Bradhondarose provides the only realistic answer to your question!

Your right in saying that the tyres add drag, so too does the suspension and general weight.

You will find that on a road bike you can go further with far less fatigue than on your current steed!

Hey, just look at the Top Pro’s and you will see most of them started on mountain bikes.

You may have to venture a little further than around the streets, to raise the adv mph though!!

When you say ‘pavement’ I assume you mean ‘tarmac’ because in the UK a sidewalk is called a pavement and the police will reduce your adv mph considerably!! Lol.

Go for it mate!

With me, about 16 MPH on a mountain bike and aout 22 on a road bike. Both on the flats at about a heart rate of 140.

Funny, I thought of that a few years ago. So I did a little experiment. I have a high end road and MTB. I rode the same course at the same perceived difficulty 8.5 of 10. This is my conclusion, based on this one level of exertion, I traveled 8 miles on lightly rolling terrain for both bikes.
Results:
Road Bike av. speed: 19.5 (tires 23c, 100lbs)
Mountain bike av. speed: 17.6 (moderate knobbies, 70lbs)

This is mostly based on your ability, with no change in tires (mtn bike w/ offroad tires) expect to go approx. 3~7 mph slower than on a road bike. Change the tires on your mtn bike to a set of road tires and don’t expect a drop-off till you get over 25mph (this is due to being in a more aerodynamic riding position on a road bike). If you can spin 90~110rpm and have the leg power, going 25+mph is attainable on a mtn bike w/road tires. After 25 mph aerodynamics is the biggest problem as you need to increase your power in large amounts to see only a few mph gain.

You could expect to be in the 18 – 20 mph range. I ride both types quite a bit and if I’m on my mountain bike on pavement I typically average 15 – 16 mph. On the same route on my road bike I will average 18 – 19 mph. I try to only ride my road bike on pavement anymore and the mountain for light trail riding. Don’t knock the aerobars, it’s amazing how much faster you go with them.