I am riding more on gravel these days, so I have tried to increase my tire width for more comfort. I went up to 28mm on my Panasonic Team Japan, and while the ride is good, I'm was hoping for a little better.

I was wondering about going up to 30mm, but I may already be at my limit. I've attached photos of the current brake clearance. Could I go to 30mm here?

I would be confirming wheel dish and fork alignment.
I would not attempt a bigger tire.

I have read there is an ISO ( Internal Organization for Standardization ) standard of 6mm, I see lots of 4mm on frames/forks/brakes and in my retirement community where the roads are kept clean I can get away with slightly less but no way in gravel/wet.

I think I'm more OK with running things close to the clearance limit than perhaps most people, and that's about as tight as I'd go. You could take some measurements and see how much clearance a 650b conversion would get you, but that'd involve new wheels/tires/brakes. Or use this as an excuse to get another bike that can run fatter tires for gravel rides!

Agreed. Wheel in the last photo does not seem to be centered (can't really say about the one in the first photo). Clearance there is also already fairly tight IMO.

I suggest getting some calipers on those tires to measure actual width. You might already be over 28mm.

I'd say that both front and rear wheels do not appear to be centered. Something's bent and/or the wheel dish is way off. Also note that the clearance shown is as low as I would go. Furthermore, I've found that the tire width limiting factor is usually clearance at the chain stays, not the brakes.

Thanks. I'll look into the alignment issues.

My previous road bike had little clearance at the fork crown. It was fine for road riding. Until I rode over a road with newly laid chipseal -- there was loose gravel dust and small stone particles that were damp that day, and stuck to my tire. I was hearing "zing" as it scraped the paint on the fork, yikes!

So, I'd want a lot more clearance for gravel road riding than for riding on paved roads.

I'd be thinking about a 650b conversion assuming that this measures out OK for that conversion or a different bike with more generous tire clearance. Finding a vintage bike that can take a 35c tire isn't hard; it's a little harder to find one with 38c clearance if we're talking about road bikes. If you really want an all roads bike, an old school mountain bike or hybrid can work well. I did a drop bar conversion on a 90s Specialized Stumpjumper for gravel riding and I'm running 26 x 1.95 tires on it.

Since nobody else has mentioned it, this sounds like there perfect excuse for another bike.

Ha! That's why I got this bike. I was limited to 26mm on my DeRosa.

That sounds like my Bianchi. Works fine with Continental 700Cx28s, which run a bit small, but I could never use Specialized or other 700Cx28s that run large. That was the one big advantage my same-vintage Peugeot PKN-10 had -- larger tire clearance.

There ya go. n+1 until you get it right!

Which rims do you have the 28’s mounted on? I haven’t made actual measurements with wider tubeless type modern rims But I’m thinking you might eke out a bit more width with something with more internal width like a HED Belgium +, a Pacenti Brevet or a Velocity Quill. My theory is that a 28 would fit more like a low profile car tire and possibly ride a few mm lower.

The older standard Mavic Open Pro type aluminum clincher rims are like 14mm internal width which leads to the “lightbulb” tire profile with certain wider clincher tires.

It appears from the photos that you have adequate side clearance for a bit more tire width, it is the height under the brake caliper that is the limiting factor.

They're FiRs. I may try to experiment with some other rims. Thanks!