I'm currently averaging 15 mph on a decent mountain bike, been riding for almost 2 months 2 to 3 x a week, want to improve speed , would a road bike make a lot if difference? And which one would be a nice transitioning bike?

Where/how are you riding?

I'm really not much faster on my road bike than on my rigid MTB commuter, unless it's a group ride.

What's your goal?
Higher speed is no guarantee for a better workout. If you're riding solo, no one cares how fast you are.

Recommending a bike w/o knowing your budget just won't work.

Can you get a second set of wheels with smooth (if not skinny) tread? Cheaper than a new bike and might get you most of the speed you desire.

Now if you are thinking about becoming a fast roadie - speed thrills - then YES you need a new bike. Try a few before you buy, the drop h'bar, forward leaning position is different from mtb-ing.

Based on your info, no one can predict which bike you might like best. Check out cyclocross, as well endurance road bikes, and gravel grinders. Take your time.

Yup, it'll matter. At 15 mph you're hitting the wall of wind resistance. A more aerodynamic position will help crack that barrier.

I know I'm catching more wind resistance on my rigid mountain bike with riser bars. I can average 14 mph over 10-60 miles, but it takes a huge effort, disproportionate, to crack 15 mph unless I'm drafting a faster rider or group.

Lower and narrower flat bars on your existing bike might help, although it could change the handling and comfort. I can't handle lower flat bars on my bike but I could trim an inch off both ends, get a somewhat narrower bar and catch less wind resistance in my chest and arm pits.

Excellent advice above.

My question is, where do you ride?

it is a matter of efficiency. A mountain bike has a lot of heavy MTB-specific design elements---even with a rigid fork,. They are simply not designed to do anything so well as bask over rocks and roots, short bursts of high-intensity pedaling and frequent direction changes on sketchy surfaces.

If that's not the type of riding you do ... you a re driving a jacked-up four-wheel-drive pickup with full rollbars and a winch, two-foot tires and rock-crawler gearing on the highway. Obviously it won't work as well as a sports car or even a family sedan.

If your focus is riding on the road, And you can afford another bike, then ... well obviously. If you like a mix of on- and off-road, Then look at gravel-bikes of "cyclocross" bikes----or, have two bikes, which makes a lot more sense. (Each should be much better at its intended use.)

A road bike will gain you a little speed through aero and weight, and probably more efficient tires---but don't expect a lot. I can run the same short route on my skinny-tire CF Wunderbike and my ancient fat-rubber touring bike which weighs easily twice as much and gain maybe a few tenths, of half a mile-per-hour on a good day.

The rider is what makes a bike fast. If you keep pushing on the bike you have (if, for some reason, you couldn't get the bike you want right away) your effort will not be wasted.

I like the slower speeds of MTB's, I know I'm safer and can go places not practical on a racer, like up and down gutters and along footpaths to avoid busy intersections. I never resent having to slow down or stop either, it's just a good excuse to speed up again. I also know if I was on a bike that could cruse 10MPH faster I would get used to that speed and would be right back where I started but with less margin of safety.

You know... I've never actually ridden a mountain bike. Nor have I ever ridden the [mountain] bike trails either. I ride road bikes... so I ride the MUP's and roads (and streets and side streets).

If your riding trails... you need a mountain bike (or maybe a cross or gravel bike). But if your terrain is more road bike-like... get a road bike.

Either way... get a cycling app or a bike computer and track those stats.

Forget a bike ... they are expensive and need maintenance. Just download and app and punch in numbers. I can generate 700 watts anytime I want to, while sitting at my keyboard, or ride 300 miles in a day, with about ten seconds worth of typing.

I track my info with an app ... I never look at it whole I ride, though, because unless I were training for a race, knowing some number doesn't improve my ride.

Others feel differently. But the idea that you have to "track those stats" to ride a bike is like saying you haven't gone somewhere unless you've taken a selfie there.

if you are any good at adjusting brakes and derailleurs, I would suggest some of the mail-order stores like Nashbar, Performance, and BikesDirect ... but definitely first go test-ride a bunch of bikes a local shops to find out what you would be getting into. There will be a lot of adaptation going from a more upright MTB to a road bike.

If you do decide to make the switch, you can get a lot of bike for $500 or $600 (new) if you shop around. if you are confident buying used .... whole different discussion.

And I thought larceny was limited to golf and fishing (and I both golf and fish).

I run "CycleMeter" on my phone.... which I keep in a ZipLoc baggie in my rear jersey pocket, when cycling. But I do find the stats helpful in my conditioning. I also use the app "LoseIt!". I believe the technology is helpful... particularly for some newbies... creating fresh cycling habits.

There are some very handsome prices on bikes right now. I like Performance (and there is a local one near me) but even the othe LBS seem to have good deals as well. And some prices on Craigslist.... makes it hard for me to not own more bikes.

I always question opening comments like this one from a new member. My thought is that most people would have a hard time averaging 15 mph after 2 months of riding on any bike. Now add to the claim that the OP can do this on a heavy mountain bike with a poor aero dynamic posture with non-aero wide tires and heavy wheels, and you have to ask yourself why people come here to troll our site.
Statistically, the claims made are highly unlikely.

That is nonsense. I can do that and I think more than half the people on this forum can.

If the person is young, healthy and athletic by nature, I might agree with you. I am an older guy in very good shape for my age and very athletic in my youth. I can't imagine a 15 mph average on a MTB. However, I live in a very hilly area.

I'll ask the obvious question: what tires are on there? If you are still on knobbies, get some skinnier slicks. But, I agree with [MENTION=114951]dabac[/MENTION], I'm not that much faster on a road bike than a MTB.

On an average to good day, I'm at 15MPH on a MTB on my local MUPs, it isn't that difficult. I get passed like I'm standing still by folks on all sorts of MTB and fat bikes, regularly enough even when on my road bike. It wouldn't really surprise me to find an otherwise fit, but new to biking, person doing 15MPH average after two months.

This is a biking forum, comprised of cyclist. My comment was directed at a new rider, a beginner and what they are capable of after 2 short months.
I'll bet that 90% of us could not hold 15 mph on a normal road course over 30 miles today, never mind when we have only been riding for 2 months!
I'm talking about your local big box store mountain bike, which is the bike most beginners would buy. Post a Strava link showing you can do 15 mph now for 2 hours...

When I first started cycling regularly as an adult I was stronger, fitter, and faster than I am now. I see no reason any reasonably active teen couldn't do 15 mph average on an MTB, distance and terrain dependent of course.

I hear over and over again that a little weight really doesn't matter, only matters if you climb a lot or stop and start a lot, and even then, not much. If he pumps the tires up hard and rides say, half-an-hour or maybe an hour on flat paved roads ....

You could consider a recumbent.

My speed went up a good bit (IMO) when I switched from my mountain bike to my road bike. My mountain bike had 26x1.5" road type tires, but there was still a nice increase in speed and overall ease once I switched over to a road bike.

I was able to maintain that average speed on a full-suspension MTB past age 50. You must live in a REALLY hilly area!

My commute has a one mile hill at 12 degrees one direction, then the return trip is a 1/2 mile hill with almost the same gain, so much steeper (I live on a ridge, ride into a valley, then up a ridge to work). And, there is virtually no flat land, anywhere.

Yep... that qualifies. My worst on the commute, any of four routes, is on the way home, about a half-mile of 5-6%. I haven't gone that way but 1-2x since the shoulder injury.

Thank you everyone for your help all advise taking in consideration. I'm averaging 15mph but on a flat road, Santa Ana River Trail, started with 10 to 12mph depending on wind conditions. I'm 5.6" 190 lbs and 44 years old. Used to be an avid bike rider in my 20's, doing 10 miles one way on the same course in about 30 min or less. No way closer to that now, at that time I was riding and old Bianchi rode bike 10 speed that I bought used for 50 bucks, don't even new the model type. Now I am riding a giant yukon full suspension mtb, would like to transition to a rode bike that would not break the bank. I like colnago but it's way out of my budget which is about $500 used or new would love your input, and any tips on improving technique and speed, thank you so much

2 mph for knobbies to slicks on the MTB, and another 2 mph for MTB-with-slicks to road bike, at least that was my experience. One road bike will not be an awful lot faster than another as long as the bearings are all in good shape and you have decent tires.

OK , I'll give some in put . First though what is your budget ? I'll guess 250-300 ? There are some awesome mid 80s' steel bikes that can be had for that price. My flavor is the Centurion Ironman , but there are many . Panasonic , Nishiki , Fuji . I love the Italians but that stuff is way outa' my $s' . Here is a bike I think would be good , what is your size . (not my bike) https://orangecounty.craigslist.org/bik/6018978486.html here is one more https://orangecounty.craigslist.org/bik/6020689134.html this one is prolly your size https://orangecounty.craigslist.org/bik/6037333788.html

Why do you want to improve speed? More cooling? Home sooner for dinner?

I was under the impression that if I'm 5.6" I would have to get a 50 to 51 cm frame size?