I just purchased a new bicycle pump with a pressure gauge and after checking my tyres, inflated them to the top end of interval printed on the tyres (4.5-6.5 bar).
I'm wondering when the tyres should be in the lower range of the interval, and when the higher end is appropriate. Is it a question of weight? Type of terrain I'm going to ride?
The tyres in question are Schwalbe Kojaks, mounted on a light urban hybrid bike.
My general rule of thumb is 10-15% lower than the posted maximum for normal road riding. Higher if you're carrying a load or are a larger rider, lower if you're a lighter rider. There's an article in Bicycle Quarterly that's a pretty good guide on tire pressure for road riding depending on weight and width of tire.
A higher pressure will be faster and 'harder' (less suspension); a lower pressure will be slower (more rolling resistance) and smoother (it'll ooze over cracks and bumps).
I was told that the 'max pressure' is just a guideline and that in practice you can go a little higher. I think my tires say 85 psi maximum, and I inflate them to 95 (not because I'm especially heavy but because I'd prefer to go faster with less effort than smoother).
Well, I am a large rider and I was advised (and seen) that I need to keep my tires to the max before each ride. If I do a long ride - over 2 hours - I have to make sure to check and inflate accordingly. I ride 700cc road tires and with my weight a tire that starts to slack gets a "pinch flat". Basically the tube pinches inside the tire. If you are a very light rider, you can get away with a little less.
If you are riding a bike with "knobbie" tires like a mountain bike, you can get away with less too.
