The slow leak, and the thirty-degree Boise day that hides it

A slow leak is the tire problem that gets ignored the longest, because it keeps appearing to fix itself. In this city it has help: Boise's daily temperature swing is big enough to mask a real leak for weeks.

The swing, in the normals

Tire pressure moves with temperature. Here is how much temperature Boise moves in a day, from the NOAA 1991 to 2020 monthly normals at Boise Air Terminal:

Month	Normal daily maximum	Normal daily minimum	Normal daily range
July	91.2 F	60.4 F	30.8 F (DERIVED)
August	89.7 F	59.6 F	30.1 F (DERIVED)
June	81.3 F	53.7 F	27.6 F (DERIVED)
January	37.8 F	24.7 F	13.1 F (DERIVED)

Each range is the month's normal daily maximum minus its normal daily minimum, and both inputs are in the table. A normal July day in Boise swings more than twice as far as a normal January day.

That is the high desert doing what the high desert does. The station sits at 857.7 metres, which is about 2,814 feet (DERIVED: 857.7 x 3.28084), and thin dry air loses heat fast after sunset. Note the airport is not the highest ground in the city — the foothills ZIPs rise above it, by how much we do not say, because we have not measured it.

We are not going to convert that swing into a psi figure. The one-psi-per-ten-degrees rule of thumb you will see quoted everywhere is not something we have sourced to a manufacturer or to NHTSA, and a number we cannot cite is a number we do not print.

What the swing does mean is practical: if you set pressures at a 60 F dawn you are driving on them at a 91 F afternoon, and a tire that reads low at breakfast and normal at lunch has not repaired itself. It has warmed up.

Idaho's own environment agency on the same point

This is a genuinely odd citation and it is the best one available, because a state agency put a number in writing. The Idaho Department of Environmental Quality, under "How You Can Help Reduce Vehicle Emissions":

"Keep tires properly inflated and wheels aligned to reduce tire drag on the road. Gas mileage drops 1% for every pound below the recommended pressure level."

It is a fuel-economy claim rather than a safety one, and it is the state of Idaho saying it rather than a tire company. Take it for what it is.

What actually leaks

Four things, roughly in order of how often we find them.

The valve stem. Rubber stems perish and crack, and the crack is usually at the base where nobody looks. A stem is a cheap part attached to an expensive inconvenience.

The bead seat, where the tire's edge seals against the rim. A wheel that has been kerbed, or simply has a rough seat, will weep air slowly and show nothing at all in a bucket of water unless the wheel is turned.

A puncture that sealed itself, more or less. A screw picked up months ago that has been holding well enough to hide.

The TPMS sensor's own seal, on a direct system. The sensor is bolted through the rim and its grommet ages like any other rubber part.

Only the first and third are visible from outside, which is why the tire comes off the wheel. A tread puncture no larger than a quarter of an inch is repairable under the US Tire Manufacturers Association's standard; the shoulder and the sidewall are not, whatever the leak rate.

One thing we will not tell you: what Boise's winter road treatment does to a wheel or a sensor. ACHD's published winter strategy uses "magnesium chloride, salt, and/or sand", which is a fact. The effect of any of that on a bead seat or a TPMS sensor in this valley is not something anyone has published, so we are not going to claim it.

Stems and sensors are where published prices stop being all-in

This is worth knowing before you compare anyone's number with anyone else's, and it is specific to the Boise field.

The one mobile seller in this metro that publishes an itemised out-the-door total sells stem valve services and TPMS installation on its services page — the list reads "Flat tire repair services; Tire rotation; Tire balancing; Stem valve services; Air pressure check; Tire Pressure Monitoring System (TPMS) installation" — and there is no valve stem line and no TPMS line anywhere on its out-the-door screen, at either quantity we read on 2026-09-20. The only stem price attached to that business anywhere is $2.50 a tire on its Tire Rack rate card, and that is not included in the $201.00 four-tire quote on the same page.

So its published total is out-the-door only if your stems and sensors are reused untouched. That is not a criticism, it is a scope statement, and it is the kind of line that turns a quoted number into a different number in a driveway. For contrast, the Tire Choice storefront at 1404 S Orchard St in Boise publishes rubber valve stems free and metal valve stems at $4.99 on its own rate card, with a shop fee of 9.99 percent of the install cost on top.

Our price has no such line. A TPMS reset is included where the vehicle supports it at the roadside, and if a sensor fails during fitting we tell you before anything is charged.

What we do about a leak we cannot find

Sometimes the honest answer at the end of an inspection is that the tire is sound and the wheel is not, or that the leak is at a sensor rather than in the rubber. We tell you that, we tell you what it would take to fix, and we charge the inspection rather than a repair that did not happen. We would rather do that than fit a patch over a leak that is somewhere else.

Where to go next

Mobile tire repair in Boise covers the visit and the local field. A nail covers the case with an obvious cause, a flat the urgent one, what it costs the money and who is open today the hours.

On the law: Idaho has no inspection and no tread rule, so a pressure warning light is the only routine check anything in this state performs on your tires. Seasonal: the studded tire window and the winter changeover. On access: hillside addresses.

Nationally: a tire losing air with no nail, a leaking valve stem, the tire pressure placard, plug versus patch, repair or replace and what tire repair costs. Our Boise service area covers where we come.

Our price: $145 for the first tire

A mobile puncture repair on one tire is $145, and $50 for each additional tire in the same visit. No separate call-out fee and no stem or sensor surcharge added afterwards. It covers getting to the car, the wheel off, the tire demounted and inspected from the inside, a plug and patch fitted the way the tire industry requires, remounting, rebalancing, a TPMS reset where the vehicle supports it, the wheel back on and the pressure set to the door placard.

If the leak turns out not to be repairable — a sidewall, a shoulder, a cracked rim — the inspection is charged and the repair is not. If the space is not safe to work in when we arrive, nothing is charged.

Questions people ask

• My tire is low every morning and fine by lunch. Is that a leak? Almost certainly. Air does not come back. A normal July day in Boise runs 60.4 F at its low and 91.2 F at its high in the NOAA normals, a range of 30.8 F, and warming a tire raises its pressure. The morning reading is the true one.
• How much psi does that swing cost me? We are not going to give you a number. The rule of thumb everyone quotes is not something we have sourced to a manufacturer, so we would rather you measured cold at the placard figure.
• When should I check pressures? Cold, before the car has been driven, against the figure on the door placard rather than the number moulded into the tire.
• Can a slow leak be repaired? If it is a tread puncture no larger than a quarter of an inch, yes. If it is the shoulder, the sidewall, a cracked rim or a failing sensor, no, and the fix is a different job.
• Do you charge if you cannot find it? We charge the inspection, because taking the wheel off, demounting the tire and checking the inner liner is the work. We do not charge for a repair that did not happen.
• Is a TPMS reset extra? No. It is included where the vehicle supports it at the roadside. If a sensor fails during fitting we tell you before anything is charged.