I ride a Trek Domane with the stock Bontrager race tires. As far as tubes, I've use standard 20-25 Contis, Bontragers, and others.

In a typical week, I do 3-4 rides, 200 miles total on average paved roads. I normally inflate to 110psi before each ride, and find that my pressure is always around 80psi before inflation.

Is this loss of pressure fairly normal? I know some is expected.

That seems a bit faster than normal, but some air loss is normal.

Hi end of normal I would guess. Mine go from 100 to 85 or so in 2 days.

I ride Michelin Krylion 23's at 110-115psi with an approx loss of 10-15psi per week.

I ride at 100, but every time I air up before I ride it's around 80.

Where is it going?

Tubes are slightly porous to air

No, the main loss of air is to fill up the pump hose between the tube valve and the gauge. Even if you reconnect the pump right after filling the tire, you will see 10-20 psi loss depending upon (drum roll please ) how long your hose is. I guess that means some guys will see more air loss than others.

I don't think that is correct. The pressure in the tire is the same as the pressure in the hose which is the same as the pressure in the pump. That's the way the gauge works. Glorious Bernoulli's principle.

You may lose a couple PSI when you remove the nozzle from the stem, but not because of the air in the hose.

Beat me to it. I don't go more than two days between rides unless I fill up the tires anyway.

Oh! If you fill up tires with a CO2 cartridge, expect the tires to actually deflate faster. There some scientific mumbo-jumbo about the permeability of tubes and CO2 that I really don't understand, but it seems to be very true.

OK, but tubes are still air porous and will slowly deflate. I only fill them up when I can tell from pushing on them that they're too low. That's before any hoses are connected.

Same/similar bike here and I have the same issue. The other day I lost 15 psi in a day. I worked on the bike at 9am (inside the house where it is stored) and inflated the tires to 100 psi. At 5:30pm just as I was leaving for the weekly group ride, it was down to 85 psi. I can not find a leak in the tube nor a bad valve nor anything in the tire. This was a new tube. I topped it off, went to the LBS before the ride and had them install a new tube for me after inspecting the wheel, rim tape, tube and tire. They found nothing. They set it to 100 psi (with air, not CO2) and after the 1.5 hours of riding it was at 90 psi when I got home.

I can't afford to buy new tires or wheels to test, so i just keep pumping up as needed. I am a clyde but it seems excessive.

This sounds right to me. Some air in the tire will go back in to the hose when I reconnect it for inflation.

But I will say that out of habit, I press against the tires with my thumb before I get on the bike to ensure that they feel properly inflated. There is a noticeable loss between rides even with this method.

I can pick up a Presta tire gauge with no hose for about $10, so that will give me a better indication. I'll report back with the more accurate number.

I should add that I'm a 6'1" 180lbs. Perhaps a little heavy for a cyclist, but not excessively so.

Do you pump your hose to 100 psi before you attach it to the tube? Unlikely. Your tire is pressurizing the hose before you start pumping, ergo the pressure will drop a little.

It's also worth noting that some innertubes retain air better than others. I've had good luck with michelin airstop tubes... if you are a weight weenie with latex tubes, you'll be pumping more often.

Losing some air every day is not a big deal, just inflate before you ride. (daily is probably more than adequate unless you have a slow leak)

I am willing to bet most of it is going into the pump's reservoir.

Test: Pump up, detach pump, and repeat....I bet it goes from 100 down to 85ish. Check and get back to us.

Well, this thread solves that mystery for me. I've been wondering the same thing for years.

Thanks internet!

I am still curious to hear whether the presta tire gauge proves this out.

Of course permeability of the tube is also a source of lost pressure. I would say that with fairly substantial butyl tube you might see about 5-10 lb loss per day. Just a guess. Then another 10 lb to fill up the hose when you reconnect the pump, and voila, you have your apparent 15-20 lb loss overnight.

This is normal. When you remove the chuck from the presta valve, the loud hiss you hear is the air being released from the pump's hose. When you reattach it to check pressure before the next ride, air from the tire has to refill the pump hose and the gauge bellows.