Short version. I have an old bike I make pretty ridable! It originally had 23mm tires and I wanted something bigger because the roads in the northeast are just awful. 25 will fit in the back and I can do a 28 in the front. I figure that may help with the impact of long stretches of road I cannot avoid.

Is that bad? I was going to run both at a slightly lower pressure since I am not too worries about speed.

No, it's not bad. You should run both at slightly lower pressure even if you were worried about speed.

That will work just fine. I am doing that now on my fix gear which has the same issue. (In fact, all of my bikes will take a larger front tire. There's no really good reason not to have clearance in front other than wanting a rim brake that will not allow it but in back, there can be compromises that have to be made: tire clearance at the seat tube, chainstay width, chainlines and inner chainring clearances, Q-factor, etc.)

In my racing days (about a million years ago) there word was that if the race was important, you put your best, fastest, lightest tire in back; that a slower front tire cost you almost nothing.

Now I have owned bikes with long chainstays that had (for my riding style) a lightly weighted rear wheel and cornering that was skittish in back. Those bikes did better for me with the bigger, softer tire in back. But the lesson for me was those were bikes that did not fit me very well.

Edit: I know everyone now is a fan of much lower pressures than we ran years ago. I still run close to my old pressures most of the time on pavement. I really don't like pinch flats and rim damage. At 155 pounds, I run 92 +-2psi with 28c and 99 +- with 25c. (I'm betting that in 10 years pressures will be back up some and the fat road tire craze will have died down.)

That's unlikely. The change didn't come about by some collective whim on the part of riders. We have data from lab and field tests showing the advantages of wider tires with lower pressure.

Narrower tires require less material, so they are lighter. Same with narrower rims. Narrower also decreases frontal area, reducing wind resistance so when narrower time/'rim aero combinations are made, they will be faster than wide ones.

Hand waving is nice, but I'll stick with the data. (and unless you're addressing impedance and pinch flats/rim damage, you're missing the point.)

Unfortunately, the wider/lowerPSI thing has been evangelized to an extent that many have gone I think too far.

650b will be all the rage just as soon as we've all changed bikes to fatish 700c tyres with disc brakes.

All of this depends on what "lower pressure" is relative to, and the manner in which the road is "bad."

Whatever you do, don't run the tires at below the manufacturer's minimum pressure! Proper pressure will always be weight sensitive. The less one weighs the lower the pressure and be and still not pinch flat. If you pinch flat at high speed, the chance of going down it pretty good, so pressure is a safety thing and safety in more important than perceived comfort.

Can you point us to the data to which you refer? I've seen some questionable data on this subject.

Narrower being lighter and narrower being less frontal area is handwaving? OK, I guess the world isn't what it seems and that what I learned engineering school is BS. (We learned that wind resistance equaled frontal area (for a rim/tire ~= height times width) times coefficient of drag and velocity squared. So a narrower rim/tire of the same coefficient of drag would yield a lower wind resistance. Guess I have to go back to school to learn what is really happening.

No, there are too many to keep track of at this point, but it always makes sense to start with the reliable sources Poertner, Chung, Anhalt, .... I would add, though, the presence of low quality data doesn’t negate the value of quality ones.

It’s hand waving to say an unquantified reduction in area or weight will offset an equally vague reduction in rolling resistance. On the other hand there’s lots of data for a wide range of specific cases where wider, lower pressure tires test out faster.

Your description of drag is incorrect. It’s left to the reader to find the error.

There’s no reason to believe changing the tire width will not affect Cd. In fact, the data show just to opposite.

You are right. But if you look back on my posts you will notice that I never mentioned rolling resistance once. I only talked about wind resistance and weight. It is you that is waving your arms about an argument only you are talking about.

When you say narrower, high pressure tires are faster than properly inflated wider ones, you imply the factors you mention offset the lower rolling resistance in the wide tires. If you really mean to consider only weight and drag neglecting rolling resistance, your analysis is fatally flawed.

How do you know that? The OP never said it. They could have meant relative to the pressure they had been running in their narrower tires, or they could have meant something else entirely.

Even if that is what the OP meant, your statement is ambiguous. There are a lot of systems out there for recommending tire pressure, and they sometimes give wildly different results!

Okay, but "bad" road can have other meanings as well. For example, there are a few counties around here where many roads are well-maintained in the sense of not having large defects, but use rough chipseal for their surfacing. Using fairly low tire pressures to achieve a smooth ride on these roads does not produce an unusually large pinch risk.

O000hh. That does not cut it! CFB is not happy. Assertions without data, Really? I think you mean that the data is equivocal so you best not get into a linking match or an argument over what data is reliable and thus trash the thread. I'd accept that argument.

To the OP - run the same width tires front and rear, run them somewhere in the tire's stated pressure range. I'd also say don't run a wide tire on a narrow rim, especially a front tire, and especially an underinflated front tire. Worry more about safety than bar vibration. If your hands are uncomfortable, you probably have too much weight on them and not enough reach. With a good fit, your hands go up and down a tiny bit on rough roads, but so what.

Personally, on a 20 y.o. carbon bike, aluminum bars, I run 23 mm front and back, 80 psi front, 100 psi rear, and weigh 146. I have 23 mm outside deep alu rims to almost match the tires and CX-Ray spokes. I don't have any particular problem riding rough roads and never wish I had wider tires. I don't ride gravel if I can help it, though smooth dirt and hardpack are not a problem. My setup is noticeably faster than most.

We shouldn't talk about tire width in isolation, though. All things being equal, a narrower tire might be more aero, but coupled with a wider rim and the smoother transition from tire to rim, might the wider tire have an aero advantage over a narrower tire? Not claiming one way or the other, just throwing it out there.

Somehow I’ll have to find a way to push on knowing you’re not happy.

Found this in one of the silca blog articles.. some additional tweaking adjustments you all need to know.
Actually these are supposedly a stab at I think equating comfort levels using PSI as the measuring stick, with some selected component changes. I don't think the recommendation is to actually adjust your inflation.

• 1 1/8 Steerer vs Tapered 1 1/8-1 1 ¼ steerer (same brand carbon fork): 1.2psi
• 24 vs 28 spokes Zipp 303: 1.8psi
• 3x vs radial spoke lacing, Zipp 303: 2psi
• Curved vs Straight seat stays, Carbon Frames (Model Year Change): 4psi
• Carbon Vs Steel Similar Geometry Custom Frames: 4psi
• Comfort/Cobble Frame design vs Full Aero frame design: 19psi
• Aluminum bar to Zipp SL: 7psi
• Aluminum bar to Zipp SLC: 2psi
• Zipp 27.2 Seatpost to Zipp 31.6 Seatpost: 4psi
• Zero Offset Zipp seatpost to 25mm offset Zipp seatpost: 3psi
• Thomson post to Canyon VCLS SeatPost: 24psi

I'm pretty unreliable, and an example of low quality data. Tom collects much better data than I, damn him, and Josh too. Better to start with this and also the Marginal Gains podcast episode #2, for the 4 minutes from about 25:55 to 29:50. [Edited to add:] The entire podcast is worth listening to but those 4 minutes kinda cover the "how do we know" bit.

Interrupting this fascinating discussion to respond to the OP:

That will work fine- go for it.

And this is why I included you in the first place.

Probably true. The pendulum may have swung from people running super skinny tires at ridiculously high pressures to running overly-wide tires at too low of pressure.