My road bike currently has Specialized All Condition tires that are 23s and hold 125 PSI. I'm considering Hutchinson Excels that are 20s but only hold 100 PSI. I'm only interested in changing if I gain speed since the Specialized tires are only about a year or two old. If there is no speed/resistance gain, I'd rather put the money elsewhere. Any thoughts?

Just because they hold that much psi doesnt mean you have to use that much and i wouldnt use 20's for everyday riding.

Most people don't think 20s are any faster than 23s... at least not the experts on rec.bicycles.tech Measuring rolling resistance accurately is quite difference and can depend on a wide variety of conditions.

Rolling resistance depends on many things besides tire WIDTH, such as the flexibility of the tire sidewall and rubber, the density and type of the threads, etc etc. A high quality 28 mm tire could easily have better rolling resistance than a crappy 23 mm tire.

I've only tried a 20 mm tire once, on the front wheel of my road bike. It sucked, it made the handling very poor. When I tried to turn, the tire felt like it would briefly lose traction. A scary feeling that caused me nearly to crash. I went back to 23 mm on the front tire, and 25 mm on the rear, which I feel is about right for me at 165 pounds.

Higher pressure will give you more efficient ride than lower pressure.
For any given pressure, wider tyres have lower rolling resistance than narrow tyres
BUT
narrow tyres are more aerodynamic.
RR is proportional to velocity but aerodynamic drag is proportional to V2.
This means that up to about 12mph, RR is the dominant resistance. At higher speeds, air resistance becomes the limiting factor.

If you ride fast and solo, thin tyres are more of an advantage.

Nonetheless, comparing a random 20 mm tire to a random 23 mm tire is pretty useless... a good 23 mm can easily have a much lower RR than a crummy 20 mm tire. You'd need have to have specific data on the RR of the tires, which is hard to measure.

TPI - threads per inch - in the casing has a big effect on rolling resistance and preceived tire "comfort". In general, the higher the TPI the less rolling resistance and the silkier the road feel (and the more expensive the tire!!) Of course, as others have mentioned above, there are a whole host of other considerations that need to be factored in.

I prefer higher count 23c racing-type tires, but that might not work for your application. I choose to give up longevity for a comfortable, fast ride, and only get about 1300 miles out of my tires before having to swap out at least the rear.

According to this calculator, https://www.kreuzotter.de/english/espeed.htm :

going from 1.75" to "narrow racing tire (say 21mm. 1/2 the knobby tire) will cut the overall air drag 20%, and cut 8x the area difference (700 x 21mm) of the tires.

Going from 23mm to 20mm, you are cutting 4% air drag in the flatbar position. Probably 6% in drops.

Many tire manufacturers have found that thinner tires at the same pressure increase rolling resistance (RR). So thinner and lower pressure would likely increase RR.
Even if narrow tires increase RR, they can be faster if you are going fast. (Air drag slows you down proportional to speed squared, wheras RR is only linear with speed)

Thats sounds great to someone racing and means less to an everyday rider.

Thanks for all the great responses. You've collectively answered my question. It does not appear there is any distinct advantage for a recreational rider, like myself, to make the proposed change. So I'm staying with the Specialized tires. I feel like printing out this entire thread because all of you provided me with excellent information.
Thanks again.

One more thing to take a look at - a nice summation of the things to consider and how they interoperate with each other:

https://www.rouesartisanales.com/article-1503651.html

FWIW, the right set of tires can make a HUGE difference. Even if you don't race.

Dude, how do you figure wider tires have lower rolling resistance? That is wrong. Thinner tires have lower rolling resistance and are more aerodynamic. If they werent made out of rubber the wider tires would have less resistance, but since they are rubber... wider = more resistance.

Only up to a point! Maybe "in theory" 20 mm tires should have less rolling resistance than 23 mm tires, but in practice they don't (not sure about ultra-high-pressure track tires). Search the rec.bicycles.tech newsgroup, or read the book Bicycling Science and you'll see some hard data on this...

Nope - see:
https://www.rouesartisanales.com/article-1503651.html

BigBossman, Great post, thanks for the reference. I'm printing that one out for future reference. I'm really glad I asked this question since the responses I recieved provided more about tire choice than I was aware of. Thanks to all the responders.

Wow! That's a great site... I wish more people knew about it. I am still slightly confused, because I have heard from knowledgeable people that rolling resistance DOES decrease between, say, 35 mm and 25 mm. But I guess that site only considers the difference between 23 mm and narrower tires.

That's my conclusion, as well....and notice the top three tires in regards to least rolling resistance are racing tires, and of those three, only one has decent puncture resistance (Michelin Pro Race 2). That Michelin Pro Race 2 tire is a fast tire that rides very smooth - this I know from personal experience. I only get about 1300 miles out of them, but I glady put up with that for the smooth, fast ride. Plus, they're a little spendy - about $48/ea at the LBS, or $27/ea on line.

Bottom line - if you want to get the most bang for your buck in performance, evaluate your tire requirements and spend money on a top quality tire.

You're welcome. I've only just learned a lot of this recently, both by asking questions in the road forum as well as doing extensive forum and web searches. In addition, I've experienced different ride characteristics on different tires, and have formed my own opinions on what is best suited for my needs.

You'll need to do the same to get a complete picture and make the most informed decision for your requirements - but take it from me, all tires are not created equal. I used to think that the differences were there, but not really noticeable. I was Very, Very wrong.

I just learned a $40 lesson two weeks ago - instead of buying another set of the tires I had just worn out, I got cheap and tried to save a few bucks by buying a different set that looked fairly comperable but were less expensive.

After fighting them for 2 weeks, I gave up and bought a new set of the tires I was used to. Yes, it was that big of a difference for me.

There are two reasons why a 25mm tire might have lower rolling resistance than a 25mm tire.

1. The 25mm tire may have a higher maximum pressure rating that results in a smaller contact patch than the wider tire.

2. The 35mm tire may have tread features or other construction details that are lossier than the 25mm tire.

Keep in mind that the general rule of wider tires having lower rolling resistance applies only for two tires of the same construction and pressure. The reason the wider tire would have lower rolling resistance is because it does not have to flex as much to make the contact patch. Tire features such as tread, stiff sidewalls, puncture resistance (kevlar belts) all generate losses as the tire flexes to create the contact patch.

Again,you need to think about this stuff for racing vs everyday riding.

Absolutely - you must define your requirements in order to pick the appropriate tire for your application.

For my requirements, a racing type tire gives me many desireable ride qualities, even though I don't race. The suppleness and low rolling resistance come in very handy for riding long distance ( I ride a lot of centurys). I do commute to work on them as well, but my ride is short on well maintained and relatively debris-free roads.

I glady accept the relative accelerated wear of the ones I choose, as a trade-off for the qualities I prize.