How much faster is a good road bike than a mediocre mountain bike?

A much smaller investment would be to replace the knobby tires on your mountain bike with something a bit slicker. Reducing the rolling resistance will make a noticeable difference.

Also - most reputable bike shops are going to let you take something out for a test ride. Try out a couple road bikes and see if you can tell a difference.

You're not going to be able to keep up with them on a mountain bike. It's not about your relative physiques, it's about the bike. This is sort of a "right tools for the job" situation. Would you debone a chicken with a butter knife? Nthing that you don't need a high end carbon fiber road bike necessarily, just any old road bike will probably do.

I had a nice high end mountain bike with slick tires(early 90s handmade marin for those curious). It was pretty light as far as mountain bikes go. The slicks were a huge improvement, but more in the sense that an actual fried hot dog is infinitely better than a microwaved one. The crappiest road bikes i've ever had would not only pull away from that thing like a ferrari away from a vespa, but were also way less tiring to ride. I will note that i've never quantified this as whenever i get a bike with a cyclecomputer thrown in i just sell it or throw it in a drawer. But i've ridden a road bike around with friends who had slick-equipped MTBs(or even just knobby tire ones), cruisers, and other road bikes and even the guys who are in way better shape with me are really working to keep up on the mountain bikes. And inversely, what bike i was riding even if it was the MTB with slicks made a huge difference in keeping up with people. If i was on an MTB, i'd be falling behind or working really hard. You don't have to spend a lot of money to get a good road bike. $300-4 is about the magic price point at which you can get something that will honestly be fine. Look at older trek's(1000, 1200, 1400 models are a good place to look), Cannondale's(basically any road bike model. Not an "H" number series, something like an r3/4/6/8/900 or a "2.8/3.0"), a lower end old bianchi. Avoid walmart brands and you should be fine. You'll also get infinitely more dollar value from a midrange(or even high end sometimes pops up for cheap!) older bike than a bottom end new bike. I've gotten bikes that were close to 2k new in the 90s for $200 before. I also think going to a shop and going on some test rides is a good idea to get a feel for what kind of improvement it would make. But for me, it was just going out and buying something cheap but decent and riding the crap out of it that made me realize the difference. And it's also hard to quantify how much more effort you'll subconsciously put in, and how much stronger and better you'll actually get when you're just able to see and feel how much more speed you're getting for the effort you put in. The first time a hill that was previously a bear feels like nothing you'll go :O it may also be worthwhile for you to check out my answer in http://ask.metafilter.com/243390/Should-I-upgrade-my-bikes-wheels#3534391, and the whole thread really

For what it's worth, I gained about 1.5 mph instantly, and 2.5 mph over 3-4 months going from a heavy hybrid to a carbon fiber road bike. Averaging 18-20 mph over multiple hour rides requires some effort. That said, so does averaging 16 mph on a mountain bike, even on flat terrain! I think you definitely have some good base to work off of, but I think what's more important than your physical condition is your familiarity with longer rides in a group. Two hours is a reasonable time for anyone, but it's not a great idea to go from no group riding to a group of 18-20 mph riders who are probably pacelining without any practice. Is there an organized bike group in your area? Try some slower rides first - not for the physical effort, but for the group riding experience. Once you're comfortable riding in a group, ride in progressively faster groups, emphasizing safety and pacelining technique. Once you've done that, you'll have a great idea whether or not you can handle the group you're interested in.

Another factor is that longer weekend rides are less stop-and-go than your typical commute so that brings the average up a bit. But you can spend a lot less than $2,000 and be a good amount faster or at least enjoy your ride more! Absolute cheapest - go to your bike shop and get smooth road tires as skinny as your current rims will allow. This will cost about $100-$150 and you'll notice an enormous difference. Likewise, you don't have to jump right to a carbon bike. You can get a nice intro-level road bike in steel or aluminum for about $1,000 and be a lot faster. I went from a "hybrid" bike after a couple of years to a touring bike (~$750 previous year model on sale) and the difference was astounding! Go on some test rides and see how it feels!

Is it remotely possible to quantify this? Time trial your mountain bike on a known, predictable short course (say, a mile or two), record your time. Take a break for a little bit to recover. Borrow one of your friends' nice road bikes and ride the same course again, record your time. There you go, a concrete estimate.

I own and ride regularly an old Schwinn hybrid. Heavy steel frame, platform pedals, flat handlebars, geared for climbing and city riding. (as others have suggested here, I have narrow-ish slick tires installed, which made a huge difference over the knobby mountain tires I had on there.) On my long rides of 30+ miles, I usually average between 14-15 miles per hour. Last summer, though, as a treat, I rented a nice carbon-fiber road machine for a day. HOLY SHIT. On that bike, which wasn't even close to fit for me - the seat was sort-of-kind-of at the right height, but that's about it - I was already in the 16-17mph range, and was able to ride 45 miles and could have gone longer if I didn't have to get it back to the shop. It was almost TOO easy to ride. That having been said, though, when I returned the rental bike I asked the guy at the LBS how big of a difference there was between that $3500 carbon bike and aluminum road bikes like the ones I was considering buying for somewhere between half and a third of the price. He told me "less than you think - especially when you're upgrading from an old hybrid." So, the moral of this story: yes, a $2000 road bike will blow the pedals off your old mountain bike, but a $1200 bike would probably get you 95 percent of the way there.

i switched from a mountain bike to an older, metal road bike a few years ago and noticed the speed change immediately. my bike is old, as i said, but still a good bike and i don't think it's possible to really ride it slowly. i once had the pleasure of riding a super high-end carbon-fiber racing bike and couldn't believe how amazing it was. it was a pleasure to ride and i never even though about changing gears till about 2.5 miles later when i was almost to my destination. i looked it up afterwards and it was a $9,000.00 bike! while i would never pay that much for a bike i can appreciate the fact that i've experienced both ends of the bicycle quality spectrum and can personally attest to the difference. so, short-story-long, a different style of bike will definitely make a big change in your speed. as will new tires or even a new crank. you don't need to spend a lot to get a really great bike either. a little judicious Googling should give you an idea of what to go for. i'd think finding the stats on your current bike and comparing weight, parts etc. should allow you to decide on what best fits your needs. you can look at eBay and Craigslist or even used bike shops for deals. or visit the people at your local bike shop (or a couple different bike shops). tell them what you're looking for and how much you're willing to spend. don't be intimidated or afraid to look stupid with your questions and don't feel pressured to buy. if you already commute by bicycle you might want treat this as you'd treat buying a car. p.s. i'm not really a bike person. i'm just a guy with the day off from work who's lately been researching ways to update my bike on the cheap.