How do on-road mountain bike speeds translate to road bike speeds?

On my hybrid (think Mountain bike style but with thinner wheels) I average about 12-14 mph around Bolton (lots of hills) when I go out at my in-laws (in Doncaster where it's fairly flat) I average 15-18mph.

On a road bike you'll be faster still as it's lighter and built for speed.

Fitness is the biggest factor in how fast you can maintain. To provide a point of comparison, a local club here in the Seattle area holds an early spring Time Trial (race against the clock) every March. Looking at the results--and keep in mind, these are racers going all out, just short of barfing when they finish--those that competed averaged from 20 mph to 30 mph. Personally there is no way I can maintain 20 mph on the flats due to my lack of conditioning.

When The Fat Cyclist wrote about riding in the team car for a stage of the Tour of California, he got to follow one of Team Radio Shack's riders. His rider was planning on making it an 'easy' day, and was riding around 30 mph.

In conclusion...Your Mileage May Vary!

edit: btw, my personal speed (48-year-old dude, about 30 pounds overweight) is about 16.5 mph on smooth flats with no wind. Overall speed for a long ride, counting little stops to make a phone call, fill a water bottle, etc. averages about 13.5 mph.

20 to 22 mph (32 to 35 kph) average over 40 miles (64 km) with a light road bicycle, bicycle shorts, clipless pedals, proper bicycling shoes, a helmet, excellent fitness, and a lot of practice. Peak speeds can be as high as 30 to 35 mph (48-60 kph) on the flats in a tight group of 6 or eight riders or so.

As noted...It's mostly the motor... On nice flat roads with a good roadster...20+ even at my age. However, throw in some hills and a headwind and the average plummets. I have a 2-mile road course around the local park that incorporates 2 hills on each leg. When I was 15 years younger and in good shape, I could maintain 18 around that.
Not now...

At those speeds on flat terrain there are other factors that will impact average speed far more than bicycle type.

In particular, stopping at intersections in an urban area will cause your average speed to plummet. 12mph average is actually reasonable for a dense city area and is not much slower than what cars average.

If you're talking about riding in areas without stopping, getting faster than 12mph average is easy with a little more experience.

The average speeds that can be attained on a road bike (or any bike) are determined by several factors:

• Distance covered
• Road surface
• Wind
• Equipment, meaning the bike and components
• Terrain
• Age and fitness of the cyclist
• Teamwork

Based on observation and considering all that, on primarily flat, non-windy, terrain, on a decent road bike, my estimates would be:

• Novice cyclist - a short distance (10-15 mi): 10 - 12 mph
• Casual, fit, cyclist - a short/medium distance (~25 mi): 15 - 16 mph
• Average club cyclist/fitness cyclist - a medium distance (+/-40 mi): 16 - 19 mph
• Experienced club cyclist/amateur racer - a medium/long distance (+/-55 mi): 20+ mph
• Pro cyclist - long distances: 25+ mph

Anecdotally... on one of my frequent rides through some farmland, there is a 12 mile, mostly flat stretch. With a tailwind, I'm a pro. A headwind, I'm a novice. With no wind, I'm somewhere in between, depending on the day.

Another anecdote... In my area I frequently use a particular paved bike path on both a road and mtn bike, mainly because it leads to a park full of single-track, and beyond that to nice countryside for road cycling. Anyway, I have 8 years of ride data for that route. Along that path there is a 6 mile, flat open, stretch that was repaved 2 years ago; originally it was very rough, chunky asphalt and upon repaving it was converted to very smooth asphalt. Lo and behold, after the repaving, my average speed on both bikes jumped up ~4 mph.

The biggest difference between a mountain bike and a road bike is gear ratio.

Any other difference can be overcome. You can change tires, increase pressure, lock out your suspension, and use clipless pedals, but if you are on a typical mountain bike, your gearing will still limit you to a speed which is roughly 30% slower than the same effort will produce on a mountain bike.

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. See the chart below for the speed of each gear combination with a typical MTB gearing.

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, with all else equal.

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 9 mph 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. (See the Charts for complete and accurate differences, with crank lengths, tire size, and gear spread accounted for)

The difference in wheel size alone, assuming high pressure slicks, is worth the 2 mph difference that @Gary.Ray claims in his answer above. This last chart is identical to the MTB chart above, except I've changed the wheel and tire size to match a road bike. RPMs, gearing, and pressure are identical to the first MTB chart. The difference in gearing from the wheel size change is 2 MPH.

By the way, the app I used to do the calculations and the charts here is called Gear Head, it's on the app store for iPhone and iPad, and it's the best tool I've found for these kinds of calculations. I like it, but I don't have any other connection to it, just for the record.

Data points for your comparison: I switched from a 26" MTB with 3x7 speed to a 700 road bike, also with 3x7.

http://strava.com/ tells me that my ride speeds have increased by about 5 km/h on pretty much every segment.

My climbing times have dropped by 10%-20% but it wasn't any easier.

The aluminium road bike is 11 KG vs the 17 KG of the steel MTB.

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Climb https://www.strava.com/segments/3265928

Rank Date           Speed       Time
1    Dec 7, 2015    13.2km/h    1:11   <-- Road bike
2    Jul 18, 2015   9.7km/h     1:37   <-- Steel MTB
3    Jun 13, 2015   8.9km/h     1:45   <-- Steel MTB
4    Nov 23, 2015   8.3km/h     1:53   <-- Road bike-I stopped for some reason.

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Flat bit https://www.strava.com/segments/9200895

Rank Date           Speed       Time
1    Sep 24, 2015   37.3km/h    1:27   <-- Road bike
2    Dec 24, 2015   35.7km/h    1:31   <-- Road bike
3    Dec 22, 2015   35.3km/h    1:32   <-- Road bike
4    Mar 14, 2015   30.6km/h    1:46   <-- Steel MTB
5    Apr 18, 2015   29.0km/h    1:52   <-- Steel MTB
6    Jul 19, 2014   28.2km/h    1:55   <-- Steel MTB

EDIT Additionally - the MTB's bottom gear was 26/38 whereas the road bike is 26/27. Both are triples. So in the MTB its easier to drop another gear when the going gets tough, but on the road bike you have to keep going but at a lower cadence.

It strongly depends on what bikes we are talking about. Modern "gravel bikes" are an excellent choice for many cyclists just looking to have fun on the open roads, but may be noticeably slower than a racier road bike. With mountain bikes, it becomes even more varied. Are we talking about a <25 pound XC hardtail? An older MTB with 60mm of suspension and relatively narrow 26" tires? A 40+ pound downhill bike that you slapped some stickers onto for +10% speed? You will get wildly different results depending on what bikes we are looking at, so it's hard to just say "oh yea, a MTB will be exactly 20.5748% faster than a road bike at STP".

Personally, I can hold ~24km/h on my MTB (modern trail hardtail with intermediately aggressive tires), and ~33km/h on my gravel bike (with road tires on).