I have a Wrench Force pump that is about 15 years old, maybe older (I think it was sold by Trek). I always pump up the tires on my 3 Speed to what I thought was 70PSI. The other day I came across an cheapo electronic tire pressure gauge that was a freebie but never used. After pumping up my tires to 70PSI I tried the gauge and it read 59PSI. I tossed it on the bench thinking it was junk but paused and pulled out a high pressure truck gauge and it read 60PSI. I connected the pump back to the tire and it read 60PSI.... Thinking I was going nuts, I pumped it back to 70, checked with the two other gauges and the read about 62PSI. Tried the same on the front tire same result.. Obviously 60PSI is what I must be using and happy with and the pump is wrong. Why the pump reads correctly when its reconnected is a mystery to me unless there is that much air pressure built up in the hose. Any thoughts on a fix or am I going to have to buy a new one? I could just keep using 70 as 60, but I have no idea what the spread is at other pressures.

What kind of tire valve do you have? Presta, Schrader or ?

Schrader. Schwalbe tubes.

Everything is working, your pump's gauge isn't that accurate, your cheapy electronic gauge works pretty well.

Here's what's going on:
When you reattached the pump, the tyre's air is under pressure and it fills up the pump's hose and pressure gauge. Some of the tyre's pressure is lost when it fills up the additional volume of the pump's hose and pressure gauge.

It's just a coincidence that the pump gauge reads 60PSI after you attach the pump.

Several years ago I had to test air pressure gauges for the company I was working for to find a reliable one for our technician's tool cases. I evaluated several types: Electronic, Dial and Stick. The most discrepancy in readings was among the various electronic gauges. Some were accurate to only +/-10%. Others varied with battery voltage. The imported sensors used on many of them were very poor.

The better quality electronic qauges were +/- 1% or better. The better quality stick gauges were about +/- 2% or better and good quality dial gauges were +/- 1%. These were all tested against a pressure standard using an NIST certified gauge that was accurate to +/- 0.25%.

We ended up supplying our regular techs with 2% accuracy stick gauges the field engineers with 1% accuracy dial gauges. If you want a good gauge, buy from an industrial supplier such as McMaster-Carr. They have a good selection of gauges. You may have to supply your own hose and connector. They are all rated as to accuracy on their web site...remember that most ratings are for full scale...so if you have a gauge that maxes out at 200 psi and a +/- 2% accuracy, that would be plus or minus 4 psi...at 100 psi that would be plus or minus 4 psi. However we found in our testing that good quality gauges kept their rating across the range from about 25% to 100% of the scale.

Thanks for all of the input. I guess I am going to have to pop for a new pump.

You can keep the pump and just check with a good gauge to get it exact.

By now, 15 years of doing the same thing , you should be able to feel the tire is pumped up enough
By squeezing it with your hand, pressing thumb in the side..
& for 75 psi be able to judge 'good enough',,,

Ie, + or minus 5 psi




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Yes, that is true. In fact I use the squeeze method to determine when I need some more air.
The eye opener is that what seemed like the right pressure at 70psi was actually 10 psi less. I need to pump to 60 psi going forward!! The bike in question is a 73 Raleigh Sports with original steel rims. The tires are Michelin World Sports which I think are rated to 80PSI. I would never have pumped that high on those rims but thought 70 would give me the firm ride I wanted as I am a Clydesdale....

With out the internal hook edge of the rim, common to modern rims, less so with old steel rims

dont push your PSI to max...

look at the pictures of cyclists in Amsterdam, they carry their friends
seated on the rear rack, so the weight compresses the tire , but then down the street they go..




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