I had trouble getting a good seal the first few times I used it (in the dark); no problems in the daylight. The reading was consistent when measuring repeatedly the same tyre. I had bought it to measure the pressure in mountain bike forks since the gauge on the pump was not very precise. Unfortunately the small chamber inside the fork means that the act of measuring lowers the pressure significantly (in my case by about 10psi) even though there was no hiss of escaping air.
Superb quality item very easy to use, bought as one of my M bikes has 90 deg valves so old one wouldnt fit this one does as just push on. On my other bikes with vertical valves this doesnt fit as not enough room, will be fine for cars so beware if buying for m bikes.
I have used this type with different vehicles over several years, they are pretty accurate and reliable. You do have to keep them square on to the valve but this isn't difficult. The only downside is that the rubber seal will eventually perish and you'll have to replace the unit, but having said that they are the best small type I have come across in my 34 years driving.