PromptCritical 

I have bike fit challenges so due to my relatively short torso, production road bikes don't fit me.

My Lynskey gravel bike fits quite well, so I'm wondering if I really need a custom road bike.

Any thoughts on whether a road bike of similar weight is faster than a gravel bike?

Polaris OBark

All other things carefully controlled, a comparable-quality road bike will be faster. But not by a huge amount.

For most mortals, a gravel bike won't hold you back.

If you have sluggish tires, that could make a difference, so many of us who want one bike wind up with two wheel-sets, and you can get the best of both worlds.

Iride01 

To me a gravel bike is a road bike. Usually with some minor tweaks to the geometry or other things to make it better for riding gravel. One of them being the slower speed you'll have going in gravel. So to that particular end, I'll suggest that if you are a performance demanding cyclist and care about the tenths of a second for certain segments of a ride or care about a few seconds on the entire ride, then you might easily see the difference between two bikes of equal weight and one being strictly for the road and the other made for gravel.

Though not all road bikes and gravel bikes are perfect in every respect. So differences and exceptions will abound.

Eric F 

If weight is similar, they have equal wheels and tires, and your position is the same, the difference in speed will be minimal, possibly nothing. Differences will show most when you're pushing performance limits. A racy road bike might accelerate a little quicker when jumping on it hard, for example.

mstateglfr 

I am part of the extremes when it comes to properly fitting production bikes and even I recognize that there are plenty of options I could choose from.
Having a short torso is hardly reason to declare production road bikes dont fit you, seeing as how a Lynskey GR300 frame fits you fine.

GR300 geometry-
S - 550mm stack and 375mm reach
M - 570mm stack and 380mm reach
L - 615mm stack and 390mm reach
XL - 635mm stack and 400mm reach

This type of geometry can be found on countless road bikes. Heck, there are plenty of endurance road bikes that will have even less aggressive stack/reach numbers.

As for if a road bike will be faster than your gravel bike- thats been answered, but history shows repeating helps solidify the correct answer so...
- If your position is the same and you have the same equipment on both, you really wont be any faster on your road bike.
- If you have faster rolling tires and your position and equipment are the same, then you will be slightly faster on your road bike.
And in the end, your road bike may feel faster simply because some geometry is different- HTA, Chainstay length, trail number, wheelbase, etc all impact how fast a bike feels. You might be the same speed, but it feels faster because of geometry differences.

Based on your comments in your gravel frame thread and how you set up your old Colnago road bike, I really suggest you work with a bike fitter to understand geometry and how various changes in bike angles impact fitment.
There will be a ton of bog common road bikes available that will meet your fitment needs, based on the fact that you fit a stock GR300 frame.

Koyote

Are you racing?

If not, then why does your speed matter, especially since any difference will be rather slight?

TMonk

Maybe a better question is "how much faster" is the road bike. The short answer is: not that much. The more techy/punchy the road course is, the more it would be an advantage.

RChung

You'll hear quite often that the difference in speed is small, and that's true.

However, speed is a tough metric to compare because the power needed to overcome aerodynamic drag increases with the 3rd power of speed. That means to go 1% faster requires about 3% more power. That also means that you can reduce your power by 3% with only 1% loss in speed. So if you ride alone, speed isn't that consequential. If there's a 2% difference in speed between road and gravel bikes when you ride alone, that's only a bit more than a 1 minute difference per hour. However, the difference is magnified when you ride with friends. If you're riding with friends, to close down a 2% difference in speed requires about 6% more power.

I would suggest that the more important question is, do you ride alone, or do you ride with friends? If you're a solitary rider, the difference won't matter much. If you ride with friends, a 6% difference in effort level adds up.

asgelle

Same weight, same position, same wheels and tires? There's no way to answer which is faster. It would depend on the particular road bike and gravel bike. Frankly, given those things in common, I don't really see there being any difference other than the label people put on them.

Polaris OBark

This is exactly why I only ride with my bitter enemies.

bbbean 

"production road bikes"? You realize there are dozens of reputable bike manufacturers making bikes for people of all shapes and sizes, and hundreds of millions of people ride them without issue.

All that said, road bikes are generally faster on the road.

Of course, if you just want a custom road bike, you don't need to justify it to us. If you can afford it, get to the custom maker of your choice and have them create the bike of your dreams already!

PromptCritical 

Thanks; that's what I suspected!

Eric F 

Yes. Late the hate flow through you. Feel the POWER of the dark side.

PromptCritical 

What do you mean by "techy/punchy?

Since you're from San Diego, you understand about the roads. Riding south of Miramar on a road bike with <35mm tires is not fun. I have a "South of Miramar" and a "North of Miramar" bike.

I love my gravel bike ("South of Miramar" bike) and based on the advice here, I guess there is no need for an (another) N+1.

PromptCritical 

When riding with friends, I can grab a wheel and demonstrate why my nickname while racing in college was "Glue" (sometimes "fxxg Glue" or the like).

Eric F 

That kind of talk will get you banned.

bbbean 

Worth noting that we all have different notions of "minimal". To a performance or competition oriented cyclist, a .5% difference could be huge. To a coffee shop cyclist, a 10% difference could be minimal. I suspect that if you're thinking of having a custom bike made, you fall into the first category.

TMonk

Anytime a course (or the riders) dictate a lot of acceleration. Think corners or frequent attacks/counter attacks. TBH it's more of a race or race-y ride application. For most steady state cruising, I'd think the weight and aero penalties of an otherwise equivalent gravel bike would be very. very small. Heck, I ride small knobby XC tires on my FS MTB and with it locked out and with higher than normal PSI I can ride it around the road at roadie speed no problem.

And I live in Kearney Mesa (Spectrum) so I am well aware of how crappy the roads are around here

Eric F 

Very possibly, but I know quite a few exceptions to that within my circle of cycling friends. Mostly older, ex-racers who still prefer high end gear, including custom stuff.

RChung

If there are more than two of you, you'd still be using 6% more power than the other guys who're also drafting.

PromptCritical 

Well, there is a difference between "need", "want" and "bought"........

Koyote

I see very few custom bikes in races.

Eric F 

While I agree with you, I think his point is that someone getting a custom bike built is very likely to be a serious rider who is concerned with performance, and rides at a high level.

PromptCritical 

In my case, it is about a good fit to deal with age and a knee injury.

I'm going to attend Dave Levy's next framebuilding class and am wondering out of several options what I should build.

PromptCritical 

That's pretty much what I figured, but thought I'd ask the BF experts. I was using a Gary Fisher Piranha MTB and loved it, but after ~20 years, the aluminum frame cracked.