Two parts:

a) I inflate to 120lb. (8.3 bar). I usually pump the tires up until the dial on the pump reads slightly over 8.3 (because I know that when I remove the pump a small amount of air will escape. I think I am being anal, but any idea how much air can escape from a tire inflated to 8.5 bar in the split second it takes to pull the pump from the valve stem?

b) If I check my tires on Monday and I ride Tuesday, I usually do not recheck, but what about Wed.? Thurs.? How quickly do they lose pressure? Does it vary widely depending on the type of tire/tube?

In my experience, tubes lose pressure at different rates, from tube to tube, manufacturer to manufacturer, on a daily basis. Because of that, I check and adjust before every ride. Anal? I don't know. I figure I have two tiny contact patches on the ground when I ride. When descending on those tiny patches at 40 - 50mph, I feel more comfortable knowing they're at the psi I want, which was checked right before the ride.

Just MHO.

Two parts:

a) I inflate to 120lb. (8.3 bar). I usually pump the tires up until the dial on the pump reads slightly over 8.3 (because I know that when I remove the pump a small amount of air will escape. I think I am being anal, but any idea how much air can escape from a tire inflated to 8.5 bar in the split second it takes to pull the pump from the valve stem?

b) If I check my tires on Monday and I ride Tuesday, I usually do not recheck, but what about Wed.? Thurs.? How quickly do they lose pressure? Does it vary widely depending on the type of tire/tube?

a)Almost all of the air that is coming out when the pump head is removed is air from the pump house. When you put the pump back on the valve, the pressure reads lower because air has to fill the pump hose and part of the chamber, which lowers the overall pressure. Pump till the guage reads what you like and leave it. FWIW, many guages have an accuracy of +/-5 PSI.

b) Most butyl tubes will hold rideable pressure for a week or so in 700x23-28 tire sizes. During stints where I'd actually ride everyday for a period of time (hard with work and school) I'd top off my tires every 2 rides or so. I considered that pretty anal. If you're really worried about your tires having the "optimal" pressure in them before each ride, just top them off before every ride. A quick test can be done by simply squeezing the sidewalls of the tire before a ride. They should feel pretty firm (or fill a tire to the pressure you like and get a feel for the firmness and remember it in the future).

Though I've never used them, I've heard of latex tubes losing most of their pressure in a few days. But those are fairly rare these days.

Two parts:

a) I inflate to 120lb. (8.3 bar). I usually pump the tires up until the dial on the pump reads slightly over 8.3 (because I know that when I remove the pump a small amount of air will escape. I think I am being anal, but any idea how much air can escape from a tire inflated to 8.5 bar in the split second it takes to pull the pump from the valve stem?

b) If I check my tires on Monday and I ride Tuesday, I usually do not recheck, but what about Wed.? Thurs.? How quickly do they lose pressure? Does it vary widely depending on the type of tire/tube?
I pump every ride. If they are normally down 10-15 psig and this time they are down 30, either find the problem before you ride or be prepared to fix a flat out on the road. - TF

I generally ride about once a week. I inflate to around 110-115. I find after a week the pressure is down about 10-15 pounds. So that would equate to 1 1/2-2 pounds per day. It is possible as the pressure gets lower it may lose at a lower rate. And as mentioned different tubes probably lose at different rates.

If you really want to know how much air is lost when taking the pump head off, just pump it to the desired pressure and use a hand gauge after.

Though I've never used them, I've heard of latex tubes losing most of their pressure in a few days. But those are fairly rare these days.[/QUOTE]

Oh I can tell you that is pretty true. Well, over the course of TWO days you would not lose
most of the pressure but enough to make you want to top off the tire pressure before another ride.

I generally ride about once a week. I inflate to around 110-115. I find after a week the pressure is down about 10-15 pounds. So that would equate to 1 1/2-2 pounds per day. It is possible as the pressure gets lower it may lose at a lower rate. And as mentioned different tubes probably lose at different rates.

My (unscientific) experience is that my tubes lose from 100 to 80 in a day or two and then lose pressure more slowly after that.

Who's got a middle-schooler who needs a science fair project??

My (unscientific) experience is that my tubes lose from 100 to 80 in a day or two and then lose pressure more slowly after that.

It's quite simple, really. Pressure loss is a function of the difference in pressure between the tire and the atmosphere. Since the atmospheric pressure is pretty much fixed, your tire will lose pressure twice as fast at 100 psi as at 50 psi. How fast it will lose pressure is largely a function of your inner tube and valve (some valves leak slowly). IME, a good tube (Michelin A1) will lose about 2 psi per day, hardly worth pumping up IMO.

I pump 'em up before every ride, because it takes 30 seconds, and I've already spent 10 minutes putting on shorts, jersey, shoes, helmet, filling bottles, etc. It probably doesn't "need it" every ride, but for me it's better just to make that part of my pre-ride habit. No pinch flats is a good thing.

In my limited experience, 2 psi a day for decent tubes seems about right.