I just bought a Serfas FP-200 floor pump and I tested it out on my mountain bike just to see how accurate the pressure gage on it was. I pumped the tire up to 40 psi (according to the pump) and check the pressure with 3 different tire gauges. All three gauges read 45 psi. I wanted to use the pump on my road bike but I'm a little worried it will not be as accurate as I want. Should I just return the pump, buy a cheaper one, get a presta pressure gauge, and call it a day? Or is there a way to adjust the pressure reading on the gauge itself?

Thanks

Don't worry about it. There are way too many other things in life to obsess about. A few psi isn't going to ruin your day, or anything else for that matter.

woah! having the same problem with a joe blow air pump.

I'm trying to get my road bike up to 110-120 psi and it's only reading 40?

Why not "calibrate" the pump gauge using your gauge. You know 40 is 45 already. Once you know what to make the gauge read to agree with your tire gauge, you are golden. You probably cannot adjust the reading of the on pump gauge, but you can figure out the deviation at your desired tire pressure.

Your gauges may read entirely opposite at a different pressure range.

When you buy a 160 PSI pump, you don't really expect stellar accuracy @ 40 PSI.

what about pinch blowouts?

You really won't know which pressure will prevent pinch flats for you and your riding conditions until you experiment with pressures - and this would be the same whether your gauge is perfectly accurate or not. In other words, there's no magical formula to guarantee no pinch flats other than "pump it harder than he**". All formulas and charts are estimates from which you have to figure how low you can go and still avoid pinch flats, even if your pump is perfectly accurate. And how of us really know anyway? I've got two floor pumps and several frame pumps. I've never checked any pump or gauge for accuracy as I always figured tire pressure is an educated estimate anyway.

I don't think that the number on the gauge matters.

What makes you so sure that exactly 40psi is optimum? Why not 45psi or 35psi? My thinking is the smart thing to do is to experimentally reduce air pressure until you start to get a rash of pinch flats or until you subjectively feel that "rolling resistance" has reached the unacceptable zone.

Whatever pressure number your pump says at that point is the right one. It's an analog thing.

Make a graph at several pressure ranges Pump gauge vs an accurate "borrowed" pressure gauge. Plot the readings out over a range from 30 psi to 130psi and you will have an Accurate pump gauge offset. Also, call customer service at the company that makes the pump...they will probably be very helpful if there is an adjustment to be made.

a difference of 5 psi wont cause pinch flats or blowouts. Think about the minimum/maximum range on a tire, it's usually about 20 psi right? as long as you are within that range you are golden, so inflate to 10 or 15 psi above the min. if you are really concerned about it.

When the pump is disconnected from everything and vented, is the pointer at 0psi. If not, there are gauge pointer pullers available (an instrumentation tech will have one) that allow you to remove the pointer, reorient it to 0 and then press it back on.

After 0 is properly set, you will have to get a cal standard for whatever the max pressure of your gauge is and use it to monitor your reading at the max level. Adjusting this reading requires the disassembly of the gauge and making an adjustment to the full span adjustment link (This may not be present on guages costing less than hundreds of dollars) of your bordon tube inside the gauge. If there is no link, it may be more practical to use the pointer puller to put the gauge at 45 when your standard reads 45. This will result in a reasonably accurate reading at 45 psi but probably and inaccurate reading at 0. Since it is rare to fill a tire to 0psi this may proove to be an acceptable solution. Did I mention that all of this must be done in a container kept at standard temperature and external pressure (easier than doing all of the math to correct the readings.)

On the other hand, you might consider that 5% accuracy for a gauge in this price range is pretty good. 5% of 160psi being +/_ 8psi would point toward your gauge being ok.

And they said that bicycles weren't rocket science..............................

So basically there is no point to having this floor pump with this lame gauge that doesn't really work?

I bought it so i'd know my tire is at 110.
But now 40 is 110?
If im just pumping it until it's full what's the point?

If you think you're around 100 and your pump is showing 40 - that's a MASSIVE inaccuracy and your gauge should do better than that.

But if you are at 40 and your pump is showing 45 it's still doing an acceptable job. Other factors will then be more influential for your ride than the precision in the gauge.

A note to Capolover: I have run into one malfunctioning JoeBlow myself - the built in gauge will start on 0, no matter what the pressure in the tire.

Can anyone say "kaboom?" That's what happened the first time I used it

-Kurt

Perhaps someone can explain to me why a tire has to be inflated to max pressure? Every rider has a different weight. Isn't it more important that you consider the deflection of the tire when you sit on the bike? Fill it until there is little deflection.

Why would a 100 lb. rider need the same air pressure as a 200 lb. rider? Obviously the 200 lber is going to need more pressure than the lighter rider to achieve the same results.

I was under the impression that your pump read 40 psi and tire gauges were reading 45, if its that big of a difference, return the pump.

Ding ding ... we have a winner! Don't get hung up on numbers. Experiment with different tire pressures with the pump you now have. Either that, or just check it with a known good guage.

... Brad

I think most posts here were talking about the original poster's question, not yours. His inaccuracy was 5 lbs in 40-45.

Your pump's inaccuracy is horrible and makes the pressure gauge worthless. I would either return it or get a new one.

They don't. You're exactly right: different weight, different tire size = different optimal pressure. Just like a car or truck.

[QUOTE=Camilo;6658953]I think most posts here were talking about the original poster's question, not yours. His inaccuracy was 5 lbs in 40-45.[QUOTE]

Yeah, my pump is only off by 5 psi at 40 psi. I have no clue how far off it is when I pump my road tires to 110 psi, but I've been playing with it and I think I found a comfortable pressure.

Well the tire is full, if i push any more it will explode and all it's showing is 40 on the gauge.

I did take it back and the new one is doing the same thing.

This may be a silly question, but are you sure that you're reading from the right scale?

yup.