Slow leak tire repair in Denver

One tire is always the low one. You top it up, it is fine for a week, and then the light is back on a cold morning. Before you pay anyone to hunt for a hole, rule out the two things Denver does to every tire in the city.

First, the temperature, because Denver swings hard

Air pressure follows air temperature. The US Tire Manufacturers Association puts the rate at 1 to 2 psi for every 10 F, and where a tire falls in that range depends on how hard it is inflated: a normal passenger car near the bottom of it, a high-pressure light truck tire near the top. Denver's daily temperature range is unusually wide and it stays wide all year.

From the NOAA 1991-2020 normals at Denver Central Park, the gap between the normal daily high and the normal daily low runs from 26.5 F in February to 29.7 F in September, and averages 27.8 F across the year. At the passenger-car end of the USTMA range, 1 psi per 10 F, that is close to 2.8 psi of movement in a tire between dawn and mid-afternoon, every day, in July as much as in January. A pickup running higher pressures moves more.

The same station records 156.5 days a year with a low at or below 32 F, but only 21.0 days a year when the temperature never rises above freezing, against a January normal high of 44.5 F. Denver freezes at night and thaws by lunchtime for most of the winter.

So check the pressure cold, before the car has been driven, and compare it against the placard rather than against yesterday afternoon. The tire pressure placard covers where to find the number and what it means.

Second, altitude, which does not do what people think

This is the Denver question, and the answer is not the intuitive one.

A tire gauge reads gauge pressure, which is the pressure inside the tire above the surrounding air. Atmospheric pressure falls as you climb, so the same air in the same tire reads higher on a gauge up here. Les Schwab puts the rate at 0.5 psi per 1,000 feet of elevation, which works out at about 2.6 psi between sea level and Denver.

What does not change is the number on your door placard. NHTSA defines correct pressure as the vehicle manufacturer's recommended cold inflation pressure from that label, measured after the car has stood at least three hours. You set the placard figure with a gauge, wherever you happen to be standing. Do not add air to compensate for altitude. A tire filled to the placard at the coast will simply read about 2.6 psi high when it gets here, and one filled here will read low at the coast.

And the differences inside Denver are nothing. We queried the centroid elevation of all 78 Denver statistical neighbourhoods against the USGS elevation service: the lowest is Globeville at 5,152 feet and the highest Southmoor Park at 5,633, a spread of 481 feet. That spread is worth about 0.24 psi, which is inside the error of a forecourt gauge. Where you park in Denver does not change your pressure. What time you check it does.

What is usually leaking

Once the weather is ruled out, a tire that loses more than a few psi a month has something wrong with it. In rough order of how often we find them:

The valve stem. Rubber stems perish and metal ones corrode at the base. Cheap to fix, invisible without soapy water.

The bead seal. Corrosion on the rim flange stops the tire sealing against the wheel. Common on older alloys and after a winter of chloride de-icers.

The wheel itself. A cracked or bent alloy after a pothole leaks steadily with nothing showing on the tire at all.

Tire losing air with no nail in it goes through the full list.

What we can and cannot do about it

We come to where the car is parked, take the wheel off, get the tire off the rim and inspect the inner liner. That is the only way to find a liner cut or a bead issue, and it is a step the standard requires before any repair anyway.

If it is a repairable tread puncture, we plug and patch it, rebalance, reset the TPMS where the vehicle supports it and set the placard pressure. If it is a valve, we say so. If it is the rim, we say so, and a wheel repair is not something we do at the kerb. If the tire is unrepairable, you pay for the inspection and not for a repair that did not happen.

One limit from Colorado law worth knowing before you spend on a repair: C.R.S. 42-4-228(5)(c)(III) deems a tire unsafe below two thirty-seconds of an inch of tread. A leak in a tire already at the wear bars is not worth chasing. Repair or replace a tire covers the call and our Denver service area covers where we come.

Our price: $145

A mobile puncture repair on one tire is $145. That is a price, not a range and not a starting point. It buys the trip out, the wheel off, the tire off the rim, an inspection of the inner liner, a plug and patch fitted to the tire industry standard, remounting, rebalancing, a TPMS reset where supported, and the pressure set to the placard. Nothing is added afterwards for the call-out.

Questions people ask

• Should I put more air in because Denver is high up? No. The placard number is a cold inflation pressure and it does not change with elevation. A gauge simply reads about 2.6 psi higher here than at sea level for the same air inside the tire.
• Why is the light on every cold morning and off by lunch? Because Denver's normal daily temperature range averages 27.8 F across the year. USTMA puts pressure movement at 1 to 2 psi per 10 F; at the passenger-car end of that range it is close to 2.8 psi of swing in the tire between dawn and afternoon.
• How much loss is a real leak? A tire that needs topping up every week or two is leaking. Seasonal drift as the nights get colder is normal and shows on all four tires at once, not on one.
• Does it matter which Denver neighbourhood I park in? No. The elevation spread across all 78 Denver neighbourhoods is 481 feet, worth about a quarter of a psi. That is noise.
• Can you find a leak without taking the tire off? Not reliably. Bead and liner leaks are invisible with the tire mounted, which is one reason the repair standard requires it off the wheel.