How much faster is a road bike than a MTB on the road?

Hi,
I’ve recently taken up cycling in an effort to get fit and lose weight. At the turn of the year, I weighed in at over 17 stone; I’m now over 2 stone lighter through diet and exercise.
I bought a mountain bike and currently just ride on the road whilst I am building my fitness up, the plans is to go off-road when I think I am ready. I know that my fitness is improving as when I first started riding, I rode 2 local circuits in 1 hr 20. I now ride 3 circuits in 1 hr 25 (each circuit is just under 6 miles).
I’m considering cycling to work to get extra mileage in and am considering buying a road bike for the commute. How much faster is a road bike versus a mountain bike for the same rider (10%, 20%, 50%???). I know average speeds will always vary by rider and terrain but surely the speed increase for any given rider should be relative.
Another option maybe to fit slicks to my MTB but then when I do go off road I will need to mess about changing tyres each time, so that puts me off slightly.
Thoughts most welcome.
Thanks
I’ve recently taken up cycling in an effort to get fit and lose weight. At the turn of the year, I weighed in at over 17 stone; I’m now over 2 stone lighter through diet and exercise.
I bought a mountain bike and currently just ride on the road whilst I am building my fitness up, the plans is to go off-road when I think I am ready. I know that my fitness is improving as when I first started riding, I rode 2 local circuits in 1 hr 20. I now ride 3 circuits in 1 hr 25 (each circuit is just under 6 miles).
I’m considering cycling to work to get extra mileage in and am considering buying a road bike for the commute. How much faster is a road bike versus a mountain bike for the same rider (10%, 20%, 50%???). I know average speeds will always vary by rider and terrain but surely the speed increase for any given rider should be relative.
Another option maybe to fit slicks to my MTB but then when I do go off road I will need to mess about changing tyres each time, so that puts me off slightly.
Thoughts most welcome.
Thanks
2010 Specialized Rockhopper Comp
2010 Trek 1.5 Compact
Now to diet, get fit and lose at least 3 stone!!! (2 of the 3 stone now lost...)

Diet started 1/1/2010
2010 Trek 1.5 Compact
Now to diet, get fit and lose at least 3 stone!!! (2 of the 3 stone now lost...)

Diet started 1/1/2010
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Do it anyway even if it's only 3%. You'll be pleased at yourself when you do and wonder what took you so long. Save the wondering and buy a road bike tomorrow, or this afternoon.
Why not go and try one out and see what you think.
I bought the Trek as nothing more than a training aid to my MTBing. And the result is a leaner, quicker, fitter and faster off-road rider.
Trek 1.5 (2009)
Specialized Stumpjumper FSR Comp (2007)
I have recently both added slicks to a MTB commuter and bought a road bike, everyones experiences will be different but mine are detailed below I hope they help.
When adding slicks (michelin country rocks £8 a pop at CRC) to my MTB I was able to trim a good 5 minutes off my 20 (ish) minute commute to work without really trying I was well chuffed with this.
When I bought my road bike my average speeds went from 12s and 13s up to around 16 mph the difference was amazing compared to my commuter. I also found that my average mph was higher on some of the longer distance rides I was doing 15-16 mph on a 50 mile route where as if I'd attempted the same route on my commuter the average would be way down.
Give both a whirl, you'll be going faster and further in no time
Ok the point about comfort, i think was that a road bike is more comfortable over a longer distance as the weight is spread out over saddle and bar rather than just the saddle on the MTB.
Create a chaingang
Make sure you don't break your chain
I used to do 15 miles each way every day into the City on a MTB with slicks and bar ends and had no trouble keeping up with roadies - and I was in my mid 50's.
To be honest the aerodynamic and comfort issues are the greater issue than the tyres; my cross bike with Racing Ralphs is a lot closer to my road bike than my MTB (same tyres.) The position on a MTB is just horrible from an aero point of view, doesn't matter one jot off road but certainly does on it.
Off road is great fun but far more spikey in terms of effort; it is probably easier to lose weight road cycling and indeed all serious mountain bikers get their base from training on road bikes.
This isn't to say that the road bike isn't going to be faster. It defintely will, but it won't be because of the aero benefits of a different riding position (which equally apply on and off road... same air after all).
Off road you need the position and handlebar width for the added manoeuvrability and speeds are lower so aero is not so critical. Air resistance increases by the square of velocity while rolling resistance is linear.
As you are not yet riding the bike offroad anyway though why not get a pair of slicks and pump them up to a good high pressure. It is a cheap fix, will transform the bike and you can revisit the question of whether to buy a road bike (answer: yes you should) when you start having to swap the tyres to go off road.
Also, lock out your fork if you can.
PS: a road bike may be easier on your wrists over long distances but it certainly isn't easier on your ar$e - that's why MTB'ers (usually) don't need padded shorts or chamois cream
I lost 4 stone and all of it by riding a mountain bike, and after loosing the weight I bought a Road bike.
I would say I can do an average of 17mph on the road but only about 13-14mph on the Rockhopper.
The mountain bike will shed the pounds more, but you will enjoy the road bike for its speed and fast cadence.
Flyer
realistically it wont be alot quicker, but it will be quicker
So the differences are position, tyres, weight and gearing.
My mtb has 2 sets of wheels, 1 set with slicks and the other with knobblies. As long as you swap regularly and renew the chain on a regular basis, you shouldn't have problems.
Dunedin
Not sure how relevant if any, but that is my impression so far, i can not take the
Second choice would be a spare set of wheels with narrow roadie type tyres
Third choice would be change the tyres as required but its a pain in the censored , been there done it and bought a road bike instead.
£1.25 for sign up http://www.quidco.com/user/491172/42301
Cashback on wiggle,CRC,evans follow the link
http://www.topcashback.co.uk/ref/MTBkarl
judge for yourself.
also hills ,my local hill i will ride up it at 4mph on the mountain bike with mtb tyres on and 6.5 on the road bike .
hope this helps make your mind up
but the road bike will take a good few rides to get used to as your in a more aggressive position but after that i ride my road bike 70% and 30% mountain bike ,which do i favour?
trek 4300
trek 1.2
i like trek..