I squeeze the tire with my fingers.

Trial and error, this is what works for me:
Road bike, 700 x 23: 100 R, 90 F
MTB (used mostly on dirt roads), 26 x 2.0: 40 R, 35 F
Hybrid city bike, 700 x 38: 60 R, 50 F
My weight is 68/70 kgs BTW.

I look at how much the tires pooch out under my weight, and if it looks like too much, then I add air.

Part of the pressure rating has to do with the casing strength. For larger tires to be rated at higher pressures, they need tougher casings than skinnier tires. This adds weight and increases rolling resistance. Of course, there are so many factors going into these pressure ratings that you can't really infer much about tire construction from them. A tire might be rated at an extra-low pressure just for liability reasons.

Thankfully, the message is starting to get out there that max pressure isn't always best. But there's still a lot of momentum behind the old school of airing up one's tires to the max. Back when people were riding 700x20C tires on their road bikes, they didn't have much choice if they wanted to avoid pinch flats.

Tires with stiff sidewalls ride poorly when underinflated. Good tires don't suffer as much. But of course, you need stiff sidewalls if you're going to carry a lot of weight.

When inflating presta valves, removing the pump causes air to make a sound, but that's all coming from the pump. With schrader valves, it might be somewhat from the inner tube, but I think most of it comes from the pump. In either case, I don't worry about it, as it's not important to be exact.

The question ended yielding lots of good answers, so once we changed the question to how to determine ideal pressure (rather than correct pressure), we brought up a lot of useful information.

I have some big tires (45mm Protek) w a max pressure of 90psi, but I can ride them at 45psi w/o the sides bulging & I weight 225lbs. the width changes tho, with less pressure it gets narrower & fits in the frame better (other benefits too). I think they might be considered "touring" tires

Sorry, but not true.

Stiffer sidewalls do ride worse because they resist the flexing that happens at the pavement. That increases rolling resistance, and may cause a bit more road shock than otherwise.

But stiffer walls don't help support weight. We ride on air, and the tire's function is limited to two things. The first is to contain that air, and the second is to provide traction.

Obviously higher pressures, and/or larger cross sections increase stress and call for stronger sidewalls, but they don't have to be stiffer to do that.

Consider the typical canvas fire hose. It lies perfectly and has as close to zero stiffness as one might imagine. But charged with water it acts like a steel tube. When you stand on a charged hose, you're standing on a canvas bag and water, neither of which could support you, but together can support 10 of you.

Thanks for the explanation, [MENTION=158672]FBinNY[/MENTION]. I think I confused strength with stiffness, which bike folks seem to do often. So tell me if I'm wrong here: To bear extra weight, a tire needs extra strength. To make a tire stronger, it may be necessary to add weight. Heavier tires give a very crappy ride when underinflated. Also, cheap tires, because they use inferior materials, must use extra material to have adequate strength and may not be suited for bearing a lot of weight. I think this is probably what my wife and I rode on cheap 3-speed bikes rented from a hotel. They didn't have an air pump, and we had a lousy ride as a result.

Yes, the lower the pressure the more that you'd feel the effects of a less supple sidewall. But I'm talking about a small difference equal to the difference you'd feel when squeezing an unmounted tire between your fingers. (note that I said, equal, not proportional.

You're also right that building better tires capable of handling higher pressures calls for more or better materials or both. But the materials are out there to build very supple thin walled tires that are also extremely strong. BITD I was importing tubulars from a small maker. Like others they ha cotton and silk bodies, plus one tire made of a polyester. That tire was the first of it's kind, and outperformed the silk. It was a beautiful tire offering silk performance at cotton prices. But it didn't have the cachet of silk, and remained something of a cult favorite.

However, discounting weight, as the pressure increases, the differences in the tires themselves become less obvious because the flex is reduced. It's like the hose analogy, empty there's a world of difference, charged you can't tell thm apart.

Thank you again. Panaracer sold some nylon tubulars in the early 80s, and they were pretty amazing for the prices, and they were tough, too.

[MENTION=134410]rumrunn6[/MENTION] mentioned Protek tires -- I've ridden a set of 700x40 Michelin Protek Cross Max for almost two years and am still pleasantly surprised by how well they defy conventional wisdom about heavy, puncture resistant tires with stiff sidewalls.

To me, they feel harsh at the recommended 75 psi for my weight (160 lbs), and not any faster than at my preferred 50-60 psi. Occasionally I'll let them drop to 40 psi unless I'm hauling a heavy load of groceries or other stuff -- it's my errand bike. At around 50 psi they feel cushy but not sluggish. Better than two other sets of lighter weight but similar design chevron tread 700x38 tires (Specialized and Innova).

It's difficult to be sure what the tires are capable of since they're on a heavy upright comfort hybrid with suspension fork. I can cruise all day at 12 mph. I'd need to swap 'em over to my lighter, rigid fork/frame Univega to see how the Michelins would compare with the Conti Speed Rides on the Univega, which are less than half the weight of the Protek Cross Max.

In comparison, the same felt effort on the lighter weight Univega with lighter Conti tires usually cruises around 14 mph. Not a whole lot faster all things considered.

And the same approximate felt effort on my much lighter weight road bike (23-24 lbs) with skinny featherweight slicks (205 g each) is good for 16 mph. Seems like I should be a whole lot faster than on a 35 lb upright comfort hybrid with tires that weigh 1,100 g each. Can't be the engine. I'm blaming the tires.

bear in mind, I've got the smooth ones, straight up Protek, no other wording in the model name

I adjust tire pressure by trial and error. I look for comfort first then rolling resistance. Once I find the optimum pressure for a given set of tires I inflate to that pressure each ride. Those pressures do vary from model and brand and bike I ride. My 32 mil Challenge Strada Bianca's and 27 mil Paris Roubaix's like 80 front and 85 rear on my Wabi fixed gear bikes. My Specialized 27 mil SWorks on my old Pinarello ride 90 front and 95 rear.


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Yeah, I think this this is pretty much dead on. When you pump the snot out of a tire, the differences in the casing become less important to ride quality. And at lower pressures, the difference in much more noticable.

Um, I just go to the mid-point of the range that is on the side of the tire.