You probably already know that having under-inflated tires on your car will cause you to burn more gasoline (i.e., use more energy). The same thing is true for bicycle tires—if the tires are under-inflated it will take more effort (i.e., use more energy) to peddle the bike. On the sidewall of every bicycle tire you will find both the minimum and maximum pressure it is tire is designed to hold (usually measured in PSI, pound per square inch). If you are a heavy cyclist you should probably keep your tire pressure at the maximum PSI for your tires, while lightweight cyclists can often run their tires down to the minimum pressure (though this is not always advisable). While low tire pressure will force you to use more energy as you ride, if the tire pressure is too high it usually results in a very bumpy ride. One of the best ways to accurately measure your tire pressure is with the Topeak SmartGauge D2 tire pressure gauge.
The Topeak SmartGauge D2 is a digital tire pressure gauge that works on both Presta and Schrader valves. This precision instrument is also useful for suspension forks, rear shock units, and even your car tires. The easy-to-read LCD display can show pressure in PSI, Bar, or kg/cm2 (it takes just a second or two to switch settings). This unit runs on a single CR2032 battery and weighs a bit over two ounces. The swivel head (Topeak calls it a SmartHead) rotates 180 degrees so you can easily read the gauge regardless of the position of the valve stem. This unit can measure a maximum tire pressure of 250 PSI (17 bar).
When I say this gauge is accurate, I mean that you can measure your tire pressure six times in a row and get the same reading each time. One of the problems with the cheap gauges found on most tire pumps is that they are not very reliable.
While the Topeak SmartGauge D2 is perfect for about 99.9% of cyclists, there is one small group that might have trouble with it, i.e., those of us who ride Fat Bikes in temperatures well below zero (Fahrenheit). The piston-plunger gauge on the SmartGauge and and the gauges on most bicycle pumps are affected by changes in temperature and humidity, but gauges with a Bourdon tube are not. In the winter most Fat Bikes run at 6 to 10 PSI in the snow and are extremely sensitive to changes in tire pressure—even a difference of one-half PSI can be felt by the rider. So, if you are riding your Fat Bike in extreme winter conditions I would suggest you try a low pressure tire gauge with a bronze Bourdon tube, like the Accu-Gage.
The Topeak SmartGauge D2 retails for around $32 and I highly recommend it. You should be able to find this at your local bike shop—if that fails you can find it on Amazon.com.