# How can I inflate a tire to the correct pressure?

I have a 700 x 38c tire with a Schrader valve I just did a puncture repair on. The recommended pressure for the tire is 75-100 psi (520-690 kPa). I would normally just inflate the tire until it felt about right, but since I had recently purchased a digital tire pressure gauge for my car I decided to use that.

I pumped it up to what I thought was about the correct pressure then checked it using the gauge and it was only at 25 psi (170 kPa). I continued inflating the tire with my hand pump until it became difficult to continue adding more air and it was still at only 37 psi (250 kPa).

I just don't see how it would be possible to inflate it to anywhere near the correct pressure.

According to the label, the gauge is also suitable for bicycles and has a range up to 100 psi (690 kPa).

• Note that if you are using your hand to judge tire pressure you're not getting above about 35. A bike tire at 50 psi or above will seem rock-hard when you squeeze it with your hand. Apr 28, 2020 at 0:06
• Can you please confirm the tyre and specs? 75-100 PSI feels a little high for a 38mm wide tyre. I would guess 60-65 PSI for road and 40-50 PSI for offroad, weight dependent.
– Criggie
Apr 28, 2020 at 0:19
• Oh, and that hand pump will struggle to get above 25. It's really just intended to help you limp home. A pump to achieve higher pressures would need to be about twice as long (and probably thinner). Apr 28, 2020 at 0:42
• Get a proper, large pump with pressure gauge for at home (I’ve never had any which couldn’t go to 7 bars with ease) and a road bike pump (something like a Lezyne Road Drive where a normal person can actually pump to 5 or 6 bars) for on the road. Apr 28, 2020 at 4:51
• With a Schrader valve, you can also use compressed air canisters at gas stations, but you have to be careful not to overfill. Apr 28, 2020 at 9:28

## 4 Answers

One term: Mechanical advantage.

You need a pump with the correct mechanical advantage. In the case of an air pump, the size of the piston defines how fast it does its job and how high a pressure you can put into a tire.

Let's say your pump has an effective piston area of 1 square inch (6.5 cm²). That would mean that in order to pump 100 psi (pounds per square inch; 690 kPa = 690 N/m²) you would need to exert 100 pounds of force (or in SI units, 690 kPa * 6.5 cm² = 450 N) onto the pump.

A pump with a smaller piston area will give you an advantage, requiring you to push less hard in order to achieve the same pressure inside the tire, but it also will be slower, as every piston stroke will push in a smaller air volume.

That's why there are some pumps better for low pressure, high volume (like fat MTB tires) and other are better for high pressure, low volume (like skinny road tires).

There are also some pumps with dual mode. They have a mechanism to switch from high volume/low pressure to low volume/high pressure.

The other parameter of a piston is its travel (length of the stroke). A pump with small piston can be "faster" if it has longer stroke, as each stroke will push in more air.

A 700x38c tire is usually wide enough that can be used at lower pressures (I use a set of those and keep them between 40-60 psi (280-410 kPa). Remember most tires have a marking stating the maximum pressure, not necessarily meaning that you must use that pressure. You have to consider your weight, the desired comfort, and the quality of the roads you use.

If you have been setting your tire pressure by hand, and have got good riding performance/ride quality/comfort, and have not had pinch flats (snake bites), then most surely, that pressure is the right one for you on that bike.

If you seriously need to be able to pump your tire higher, consider investing in a pump better suited for higher pressures.

A track pump is the best option to have at home or carry in the car, as it is big and allows you to apply all of your weight to it, they are more comfortable to use and may have an integrated pressure dial. A hand pump is more oriented to be an emergency device to get you out of trouble in case of a flat.

Just in case: Have you calibrated the gauge? If not, compare it with another measuring device. Take several readings to check whether it is giving you the correct reading.

• And if you're going to buy a new pump, get one with a built-in pressure gauge. Apr 28, 2020 at 0:04
• For portability, a minipump with a hose (e.g. a Topeak Morph) is a good option for reaching higher pressures. You can brace one end between the ground and your foot like attack pump, so you’re not having to supply muscle force from both ends or wrestle with keeping it from moving while attached to the valve.
– RLH
Apr 28, 2020 at 0:38
• Regarding pressure range for the tire: I'm used to MTB tires marked 40-65 PSI, but at 40 they are already rock hard, and using them at 35 psi is both comfortable and safe (a too low pressure tire is prone to pinch flats and slipping out of the rim). If you haven't had such issues, it may be fine to ride them at lower pressure, especially if you are not a heavy rider. Apr 28, 2020 at 0:58
• @RLH has it right. I use a Topeak Road Morph for these tires and it works quite well. I can pump them up pretty easily to 85psi; it just takes a while. I'm seriously thinking about experimenting with electric pumps now. Apr 28, 2020 at 14:04
• The length of the piston stroke is not only for speed of pumping, it also limits the maximum pressure you can get if it is too short: the air in the "dead" volume outside the piston needs to be compressed as well. If that is 1 : 9 with the piston volume, the maximum pressure will be 10 bar (10 volume units compressed to 1) - not very efficient to pump even to 5 bar, and likely impossible if pressure is needed (and lost!) to open the Schrader valve (i.e. if the valve isn't depressed by a mechanical "thorn" in the pump front) Apr 28, 2020 at 18:14

If you want to achieve 75psi (5 bar) with a pump you can carry on the bike, look for one with the following features:

• A hose, so when you push hard you're not pushing against the valve
• An end that can be rested against the ground, ideally with:
• A stirrup to hold it down with your foot.

I use Topeak's Road Morph, which has all of these. It's good but not perfect: The gauge has been known to get stuck when wet and dirty. That led to me overinflating a tyre significantly and blowing out the sidewall, but I checked against a couple of other gauges and 100psi (7 bar) is a little boring but doable.

If you don't want a big pump to keep at home as well as a smaller one to keep on the bike, this type is a good option. While significantly slower than a track pump it will do the job in the end.

My typical use case though isn't just limping home, it's finishing a ride of over 200km, even if I get a puncture at the start, so I have to be able to get up to proper pressure. In my commuting bag I carry the similar but smaller mini morph, which lacks a gauge and takes longer to pump up the tyre.

• I had my Road Morph mounted on my downtube. It took quite a while before I realized this was not a good idea. It did eventually accumulate enough dirt and dust inside to fail catastrophically. Now I keep its replacement in my pannier. As for the gauge, I usually just tap it a few times if it seems to be stuck, and it appears to loosen up. Apr 28, 2020 at 14:06
• @MichaelHampton I keep mine on the seat tube because I sometimes carry a lot elsewhere. But after having one stolen off the bike I no longer keep it there full time (hence a decent one in my commuting bag) Apr 28, 2020 at 14:34
• In the olden days bikes often had "pump pegs" on the headset and the seat tube, just below the top (horizontal) tube. Pumps were then designed with holes on both ends to lock into these pegs. And a pump that was about as long as the top tube was about right in term of achieving max pressure needed. Apr 28, 2020 at 18:42
• I have a similar Lifeline brand one - works well enough but lacks the gauge. Price is good though.
– Criggie
Apr 28, 2020 at 20:54
• Thanks for the edit @Gerrit. I've further tweaked it as the metric units on actual pumps are bar rather than Pa (also suitable rounding) Apr 29, 2020 at 13:29

I'd recommend a track pump for pumping to a high pressure. A guage is a must at those pressures and regularly check your tyres for any cracks and holes as if it explodes it's at best a huge bang at worst can do some damage!

Happy riding!

Key take-away: that pump doesn't meet your needs. I'm 2XL, I wouldn't ride any significant distance on 250 kPa (37 psi) tires. Even if the tire and tube survived, it could mess-up the rim.

If you can walk to a gas station with compressed air, or a friend with a more capable pump, take the gauge with you and go.

I have and recommend a plastic bodied floor pump with a guage, for home. Bought at the bike store du-jur 25-30 years ago. Marked to 125 psi. Also have a white painted, metal, hand pump, carried on the bike, with my patch kit, about the same length as the one in your photo. 25-30mm diameter, suitable for getting mountain bike tires up to 75-100 psi (520-690 kPa). For example, after patching a puncture at the side of the road while commuting.

I could never tell by pressing on a tire what pressure it was. Bike or Car.

Gas station air sources often have a slide-out inidcator that sticks out further for higher pressure, with the measurement value engraved on the moving bit. Some people poor-mouth them, but unless they're visibly damaged, I've had good results using them since the 1960s. Check with your real, separate, gauge. And start saving for your new pump.

• Hi, welcome to bicycles. The question indicates that the OP has a pressure gauge and used it; the question is about how to actually put >500kPa in the tire when the pump barely gets it to 250kPa. Apr 29, 2020 at 13:58
• Thank you. I caught the insufficient pump and judging by hand feel. A second read to confirm they did have a separage gauge would have been a good investment. I'll edit down to match the OP's question. May 1, 2020 at 7:56