Originally Posted by Bekologist
..my thumb has a hard time remembering what 65 in a 26x2.125 or the 45 in the balloon tires is versus 120 in a 700x23 and the 75 in the 700x37s so I use the gauge to make sure my thumb is still calibrated for roadside checks!
With slick tires, your thumb will be surprisely accurate with ANY size tire. With knobby tires, it is more difficult to get a feel of tire deflection.
The reason thumb pressure can work well is that the ideal pressure for any tire is the pressure that will very slightly deflect under the rider's weight. When a 200 pound rider gets onto the saddle, that puts about 110 pounds of weight onto the rear tires. The "perfect" PSI is the PSI level that just allows his rim to "sag" about 1/16th of an inch, or 1/8th inch toward the pavement under load.
When you place your finders around the rim, place the widest part of your thumb on the tire, and press with ALL of your strength, you are placing around 90 to 120 pounds of pressure on the tire (depending on your hand size and hand strength). So, when there is a barely detectable amount of deflection, the PSI is in the "ideal" range.
I have a bike where the "measured" ideal pressure is just 30 PSI. A mountain bike were the "ideal" is 60 PSI. Road bikes where the "ideal" is around 100 PSI. The thumb test works on all three types of tires. When the super fat beach cruiser tire just SLIGHTLY deflects under thumb pressure, it is at 30 PSI. When the 25mm road tire just slightly deflects, it is at 100 PSI.
The ultimate test is when you put your weight on the saddle. If the rims deflect about 1/16th of an inch down toward the pavement, your PSI is perfect. No gauge needed.