Bicycle Mechanics - Tire Pressure Question

I use a Specialized floor pump in the morning to get 100psi in a set of Schwalbe marathons.
If I check the pressure with a Topeak Smart Gauge i get less than the 100. Which gauge should I disregard? On the road I use a Topeak Road Morph with the built in gauge. Is this gauge accurate? Most of my riding is 20-50 mile day trips with the yearly 7 day fully loaded camping trip.

If it's plus or minus a few psi it does not matter. Choose whichever one you want or average them. If you really care I would guess the hand guage is more likely to be accurate, or you can have it calibrated.

Putting the gauge on the valve allows air from the tire to enter the gauge. On high pressure/low volumne tires this can effect the pressure slightly so if it was 100 psi before putting the gauge on it will be slightly less afterwards.

My guess is that the Smart Gauge is more accurate but the only way to tell would be to test it against a gauge that is properly calibrated. As to the accuracy of the Road Morph gauge, pump the tires to a certain pressure on it and check it with the gauge.

Putting the gauge on the valve allows air from the tire to enter the gauge. On high pressure/low volumne tires this can effect the pressure slightly so if it was 100 psi before putting the gauge on it will be slightly less afterwards.
Of course a good test for this is to pump up the tire, check it with a gauge, then check it again with the same gauge.

For the cost of the guages used and the acceptable tolerance there is no way to tell which, if any, are dead on accurate. It's unlikely but the one that is the farthest out and away from the grouping of the others just might be the most accurate. And as mentioned unless there's a 5 psi or greater difference they would all be considered to be OK by their makers because of the low cost of these items.

If you're feeling like you just have to know for sure you'd need to obtain a lab grade guage that comes with a calibration chart and that has a certified tolerance of something under 1%. But such things are not going to be cheap. Then your other guages would be tested against this certified guage and you'd print out a difference card that says what the offset is for each 5 or 10 psi jump. In use you'd go with the cheap guage but add or subtract the difference based on the chart for that guage.

Such a bother is the only way you can be certain that any guage you have is spot on or if it needs to carry a difference fudging chart.

I use a Specialized floor pump in the morning to get 100psi in a set of Schwalbe marathons.
If I check the pressure with a Topeak Smart Gauge i get less than the 100. Which gauge should I disregard? On the road I use a Topeak Road Morph with the built in gauge. Is this gauge accurate? Most of my riding is 20-50 mile day trips with the yearly 7 day fully loaded camping trip.

Since you probably don't want to be checking with the smart gauge every time, use the gauge on the floor pump. What's important is not that it's exactly 100 psig, but that you get consistent readings every time you use it. So go ahead and pump your tires up to 105 or 110 psig and see if you like it the ride better and if so - then that's what you should do every time, and if not, go back to using 100 psig every time.

Of course a good test for this is to pump up the tire, check it with a gauge, then check it again with the same gauge.

Or, if you have a gauge with a bleed valve, just overinflate and bleed it until it's right where you want it.

use one gauge all the time. be sure it is "patented". each time you check the pressure you let some air out. if you were to keep checking the pressure repeatedly the pressure would slowly go down with each check.

from one obsessive compulsive tire pressure checker ... try to let go ... :)

hope that helps