I bought a $20 electric (12 volt) tire pump (named "Slime) at Pepboys that has a surprisingly accurate pressure gauge on it and when I clamp it on the valve stem I know the pressure then, and as I add air. I power it with my battery charger or plug it into a cig lighter (my GV 250 has a 12 volt socket for cell phone charger) and it makes tire pressure maintenance almost fun. It measures 7"by7" by 3" and weighs about two pounds. It even has a bright spot light on it. The name alone makes it a winner.