There's a question for mountain bike tyres but I can't seem to find anything on here for road bikes. At the moment I'm running at around 90psi. Is that too much? too little? Also, am I more likely to get punctures running at higher pressure? I seem to be getting a lot lately.
All other things being equal, run them as high as the tyre manufacturer recommends (there will be an advisory notice on the tyre wall). For most tyres of even medium quality, this should be at least 100psi, if not 110; higher end tyres might go up to 130+. (I generally inflate both tyres to similar pressures, but I think that there are some theories that if either were to be higher, that should be the rear, because it carries the majority of the weight.)
Indeed, the higher the pressure and the harder the tyre, the fewer punctures you should expect as the surface should be better at repelling objects.
You should keep tyres topped up, natural leakage occurs and at the higher pressures you might expect 10-20 psi drift within a week.
When re-inflating, you should also take the opportunity to inspect the tyres. Any deformity or wear will be sure to be far more obvious and damaging when the tyre is inflated to the limits of its recommended pressure.
The only caveat is in poor weather, where a reduction in pressure increases the contact area and introduces give, which should make for enhanced traction.
I do not agree that running at the max pressure is a good idea. Most manufacturers express the recommended pressure as a range and there is a reason for that (as has been pointed out in various answers above). There is a "sweet spot" that tends to be affected by a variety of factors such as your weight, bike weight and road conditions. You'll probably need to experiment to find what feels comfortable without feeling "squirmy" or squishy.
There are some arguments that running full pressure actually wastes more energy than the increased rolling resistance from running at slightly lower pressures. At full pressure, the tires tend to "bounce" off road imperfections rather than absorb/conform to them. And of course the increased road shock will add fatigue.
One trick I'll add is to run your front tire about 5-10 psi lower than your rear. The rear tire has more weight over it and so it needs the higher pressure. The front tire has more bearing on perceived handling and comfort IMO, so running lower pressure there has more benefit.
As an example, I use Gatorskins on my road bike (23c) and my commuter bikes (28c). I run the rear tire 10 psi below the max and the front tire 15-20 psi below the max. When I made that transition (from running at max) I noticed a lot more comfort and no loss of speed or efficiency; that includes fairly objective measurements with a power meter. I also have not had any increase in the rate I flat--which is incredibly low by the way, I highly recommend Gatorskins! I am 160 lbs FWIW.
Most of the above observations are discussed in a Bicycle Quarterly article.
Without knowing what type of tire you are running, that question is hard to answer. For instanct, a 700x23 will be higher (like 120psi) than a 700x28 which will be lower (like 90psi). Generally tires give a range, the lower number will have more cushion but run slower and pinch-flat easier. Higher end will roll better and resist pinch-flats, but can cause a harsh ride.
I tend to inflate 10% shy of max because I'm light (150lbs) and I like my ride to not be jarring yet roll fast. For me its a compromise.There is a sweet spot you find after riding a particular tire for a while. Also if its wet or rough or sandy you may wish to run lower pressure resulting in a larger contact patch=more grip but also more rolling resistance. Ultimately, with experience you will feel whats best for you. It just takes a little time.We all start out not knowing!
On tires with tubes (which is what most of us have) you should always ride with the maximum recommended pressure but be sure to check both the tires and tubes max PSI. This is mainly to avoid pinch flats but also to decrease rolling resistance. It also will kick debris out from under the tire quicker to help avoid punctures. If you are on tubulars its less of a factor. If you are on nice roads on tubular tires i would always want to max them out as the are usually able to inflate to higher pressue thus reducing rolling resistance. On the other side with tubulars it is still safe to run them lower for nastier surfaces. For more information on tubulars and low pressure google Paris Roubaix + tires and you'll see why.
All things are not equal.
In particular, the load on the rear tire of a road bike is about 60% and on the front it is about 40%. Nor do all riders+bikes weigh the same amount. A 250 lb ultra-Clydesdale might not even want to ride on 21mm tire, let alone ride on it and inflate it up to the sidewall maximum.
If you do overinflate, you will get a rock hard ride with uneven tread wear and be more susceptible to flats. An overinflated tire will also have less cornering ability and greater rolling resistance (at least according to Sheldon Brown and he's smart).
If on the other hand you underinflate, you will get a mushy ride, again with more rolling resistance and with snake bite flats. Cornering will also suffer. Even going straight, your bike can shimmy uncontrollably if it hits something.
Getting the pressure RIGHT is worth the effort. Fast, more control and longer life, both yours and your tires'.
This chart is a good starting point:
Take this and add about 10% for the rear and subtract about 10% for the front. You have less load in the front and you DON'T want a rock hard ride working up through your bars.
Don't go over the sidewall maximum. If the number you get is considerably over the sidewall maximum then take that as a hint that you need a tire which can sustain a greater load.
A wider tire is going to have a lower rolling resistance and will last longer. It's going to weigh a leetle more (matters for acceleration and not climbing) and it's going to have a leetle more drag (not significantly until much faster).
You can stagger tires and ride a beefier tire on the rear for that 60% load. Its increased drag will be insignificant in the frame turbulence. It's also easier to fit that tire within the chainstays than in the fork, say 23 front, 25 rear or 25 front or 28 rear. What's important is that the PSI you calculate for front+rear is within their sidewall maximums.
Your floor pump is not to be trusted. Its gauge will have you progressively overinflating with time. That's another reason to go with Schrader tubes since you can use a better gauge with your car as well.
In summary, before you even contemplate inflation pressure for your tires, make sure you have the right tires. Just so you know, pro tour riders are riding wider tires these days than they did even 10 years ago.
Thin and rock hard to the sidewall maximum is NOT the right answer.
Be careful with overinflation, as while rolling resistance drops with increasing pressure, there comes a point when further increasing tyre pressure begins to dramatically increase rolling resistance.
Often this is around the point of the tyre's maximum rated pressure, although it can be less than that depending on a few factors.
Here is an article that discusses some of the physics of tyres and demonstrates a methodology to assess where the sweet spot for lowest rolling resistance is: http://www.slowtwitch.com/Tech/What_s_in_a_tube__1034.html
Tyres that are overinflated become more skittish when cornering, and are more likely to slip in wet road surface cornering.
Also many modern carbon rims have maximum pressure ratings that can be less than the tyres themselves, so be careful with that.
In general, it's better to be a little under inflated than over inflated.
Yep, generally run them at the sidewall rated pressure, or maybe 5psi under. If you want a softer ride (at the expense of higher rolling resistance) you can drop the pressure maybe 20% from it's sidewall rating -- no more (unless you're a fairly small person). Running 30-40% below rated pressure on road tires greatly increases the chance of "snake bite".
The wider the tire the lower the rated pressure, generally.
If you're worried about puncture resistance, buy Kevlar belted tires -- they're just about bullet-proof in normal road use. (I haven't had a puncture in maybe 5000 miles riding Kevlar tires.)
I run my 35mm tires at 95-100 psi -- they're rated 100.
There's always a safety factor built in to the engineering design. I'm fat and out of shape, and I've had pretty good luck running about 20% over the sidewall spec. Yes, you'll run a risk that the manufacturer never intended. Yes, you'll be more likely to pop your tires and cause injury to yourself and to others. (Yes, I have a BSME Mechanical Engineering with a fluid mechanics specialization and happen to have been doing this for years on old, worn-out bicycles without a problem.) I do want to warn you that a harsh ride is hard on your bicycle, especially the bearings. It's also going to be more important that the rim itself is free of burrs and that the backs of your spoke nuts aren't exposed (if applicable).
Within a minimum and maximum pressure range, bicycle tire pressure is largely subjective. If you want a smoother ride or are worried about dry grip, go low. If you want a harsher ride that's easier to pedal or are worried about wet grip, go high.
Apparently, the subject of tire pressure is much like that of chain maintenance (ask 10 people & you'll likely get 10 different answers). I like the Michelin chart as posted by svinto. I must admit, my personal method for determining the best pressure is quite subjective. I like the pressure to be on the low side of the min/max range. I also like to run about 10psi lower on the front than rear. Unfortunately, I don't believe anyone can tell another rider what their tire pressure should be. Study what has been posted here, inflate the tires to something that seems to make sense. Use that pressure for awhile, then, experiment with slightly lower/higher psi. Some riders are very exacting in the psi used while others simply inflate to something in the ballpark. Most riders still enjoy the ride.