Hydroplaning and tread depth in Atlanta

Two facts about Atlanta sit in different files and mean nothing apart.

The first is legal. Georgia sets a tread minimum of 2/32 of an inch, measurable in all major grooves, and it binds every vehicle rather than only commercial ones. The second is meteorological. Atlanta receives 49.71 inches of rain a year and delivers it in a way that most American cities do not.

Put them together and you have the only tire argument this city really needs.

What the grooves are actually for

A tire does not grip wet tarmac. It grips the tarmac under the water, and the grooves are the plumbing that gets the water out of the way fast enough for the rubber to reach it. When more water arrives under the contact patch than the grooves can move, the tread floats and the steering goes light. That is hydroplaning, and it is a drainage failure before it is anything else.

Groove depth is the size of the pipe. It is also the one variable in the whole system you control, because you cannot choose the rain and you cannot choose the road.

The Georgia number, verbatim

O.C.G.A. Section 40-8-74(e)(1), read 2026-09-20:

All tires: (1) Shall have not less than 2/32 inch tread measurable in all major grooves except that school buses and commercial vehicles shall have not less than 4/32 inch tread measurable in all major grooves on the front tires … such measurements shall not be made where tie bars, humps, or fillets are located

Two phrases deserve attention.

"In all major grooves." Not the best one. A tire worn on its inner shoulder — the wear pattern an alignment problem produces — can read comfortably in the centre groove and be under the line in the groove you have to turn the wheel to see. Under this statute, all of them count.

"Such measurements shall not be made where tie bars, humps, or fillets are located." Those are the raised strips moulded across the floor of a groove. They sit higher than the groove, so a gauge on one reads deeper than the tire really is. The statute itself rules them out as a measuring point. Measure the groove, not the bump inside it.

And 2/32 of an inch is a floor rather than a target. It is the depth below which Georgia says the tire is no longer legal, not the depth at which it stops draining well.

Nobody in Georgia is scheduled to check it

This is the part that makes Atlanta different from a state with an annual inspection.

Title 40 Chapter 8 is titled "Equipment and Inspection of Motor Vehicles" and contains exactly five articles: Equipment Generally, Control of Vehicle Emissions, Inspections by Officers of the Department of Public Safety, Inspection of Public School Buses, and the Vehicle Equipment Safety Compact. None of them creates a periodic safety inspection for an ordinary passenger car. Article 3 is roadside inspection by Department of Public Safety officers, which is not an appointment you attend.

The one appointment you do attend is the annual emissions test, and Georgia's Clean Air Force describes it in full as "An OBD test to check the vehicle's emissions control performance history" plus "A visual inspection of the catalytic converter to check for tampering or removal."

So the number exists, it applies to your car, and no part of your year brings a gauge near it. The Georgia tread law page sets that out at length.

Now the rain, ranked and dated

All from the NOAA 1991 to 2020 normals at Atlanta Hartsfield-Jackson, and all ranked over the same population: the primary weather station of each of the ten metros HopTire serves.

• 49.71 inches of rain a year. Third of those ten, behind Chattanooga at 52.48 and New York Central Park at 49.94.
• 113.3 days a year with measurable rain, of which 75.4 bring a tenth of an inch or more, 33.0 bring half an inch or more and 14.6 bring an inch or more. On that last count Atlanta is second of the ten, behind Chattanooga's 15.7 and ahead of Savannah, New York and Dallas.

The interesting figure is not the annual total. It is how the total is delivered, and the cleanest way to see it is against Pittsburgh, a city with a reputation for being wet.

	Atlanta	Pittsburgh
Annual rainfall normal	49.71 in	38.19 in
Days a year with measurable rain	113.3	151.2
Inches per rain day (derived from those two rows)	0.439	0.253
Days a year with an inch or more	14.6	5.9

Atlanta gets 30.2 percent more rain than Pittsburgh across 37.9 fewer rain days. That works out at 1.73 times as much water per rain day, and 2.5 times as many days that deliver an inch or more. Both columns are published normals from the same 1991 to 2020 series and the same NCEI request; the per-rain-day row is our own division of one by the other.

Pittsburgh is damp. Atlanta is not damp. Atlanta is dry for a while and then a lot of water arrives at once, which is exactly the condition a groove has to cope with.

Over the full daily record at the same station — 35,323 days from 1930 to 2026 — the wettest calendar day on record is 6.68 inches, on 4 October 1995, and over 1991 to 2020 the station averaged 3.07 days a year of two inches or more.

There is no season to wait out

Atlanta's monthly rainfall normals, in inches: January 4.20, February 4.67, March 4.81, April 3.36, May 3.67, June 3.95, July 5.27, August 3.90, September 4.47, October 3.41, November 4.10, December 3.90.

The wettest month is only 1.57 times the driest. Compare that with Denver, whose wettest month is roughly six times its driest. There is no dry half of the Atlanta year in which worn tires are a smaller problem.

And the wettest month, July at 5.27 inches, is also the hottest: 13.0 normal days at or above 90F, more than any other month. Atlanta's heaviest rain and its hottest days are the same four weeks. If you are going to check your tread once a year, June is a better month than January to do it.

What a Georgia court did with this exact combination

The annotations to 40-8-74 carry Ga. DOT v. Heller, 285 Ga. 262, 674 S.E.2d 914 (2009), affirming Heller v. City of Atlanta, 290 Ga. App. 345, 659 S.E.2d 617 (2008). The annotation opens, verbatim: "Decedent was killed when the taxi in which the decedent was riding spun out of control on a rain-slick road and hit a tree." The city employee who had cleared the taxi for the roads lost the protection of official immunity because, verbatim, "the employee did nothing to verify whether the taxi's badly worn tires had the legally required minimum amount of 2/32 inch of tread on the tires under O.C.G.A. Sec. 40-8-74(e)(1)".

A rain-slick road, badly worn tires and the 2/32 inch figure, in a City of Atlanta case that reached the Supreme Court of Georgia.

Be exact about what that is. It is one wrongful-death case, decided on whether a municipal inspector could be sued. It creates no duty on a private driver that 40-8-74 does not already create, and one case is not a rate.

What this page will not tell you

We have not sourced any Atlanta crash figure. We do not know what share of Atlanta collisions happen on wet roads, or how many involve worn tires, and we are not going to estimate it from rainfall or from one court case. Georgia crash data was not queried for this work, and rainfall normals are climate figures that say nothing at all about crashes.

Everything above is what the statute says, what the weather station recorded and what one court wrote. That is enough of an argument without a number we do not have.

What to do about it, in five minutes

1. Buy a tread depth gauge. It costs less than the fuel to drive to a shop that would use one.
2. Measure every major groove on all four tires — inner shoulder, centre, outer shoulder — not just the deepest one.
3. Keep the gauge off the tie bars, humps and fillets. The statute says measurements there do not count, and it is right: they read deep.
4. Write the numbers down so next time you can see which way they are moving and how fast.
5. While you are there, look for the other two faults 40-8-74(e) names: cuts, breaks or snags on tread or sidewall deep enough to expose body cord, and any bump, bulge or separation.

If you find something near the line, when to replace your tires covers the decision, find your tire size covers what you would be buying, replace two or four covers how many, and what mobile tire replacement costs covers the money. We supply the tires we fit, we do not mount tires bought elsewhere, and we do not sell used tires.

Our price: $145, and $50 for each extra tire

A mobile puncture repair is $145 for the first tire and $50 for each additional tire in the same visit. That is the price, not an estimate and not a starting figure. It covers getting to you, 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, and the wheel back on. There is no separate call-out fee. If the damage turns out not to be repairable, we tell you on the spot: the inspection is charged and the repair is not. We measure every major groove on all four tires at the same visit and give you the numbers.

Tire repair in the rain in Atlanta covers what a downpour does to the visit itself, pothole tire damage in Atlanta covers impact damage, flat tire repair, mobile tire repair and what tire repair costs in Atlanta cover the jobs and the money, and our Atlanta service area covers where we come.

Questions people ask

• What tread depth is legal in Georgia? 2/32 of an inch, measurable in all major grooves, under O.C.G.A. 40-8-74(e)(1). It applies to all tires; the 4/32 inch front figure in the same subsection is an exception written for school buses and commercial vehicles.
• Is 2/32 of an inch enough for Atlanta rain? It is the legal floor, not a recommendation. Grooves are what move water out from under the tire, and a shallower groove moves less. We give you the measured numbers and let you decide.
• Where should I not put the gauge? On tie bars, humps or fillets — the raised strips inside a groove. The statute states that measurements "shall not be made" there, and they read deeper than the tire is.
• Does Atlanta really get more rain than Pittsburgh? Yes, on the 1991 to 2020 normals: 49.71 inches against 38.19, delivered over 113.3 rain days against 151.2. Atlanta is third of the ten metros we serve on annual rainfall and second of ten on days with an inch or more.
• When is the wet season? There is not one. Monthly normals run from 3.36 inches in April to 5.27 in July, a spread of 1.57 times. July is both the wettest month and the month with the most days at or above 90F.
• Does worn tread cause crashes in Atlanta? We do not have a figure for that and will not invent one. What we have is the statute, the rainfall normals, and one Supreme Court of Georgia case, Ga. DOT v. Heller (2009), involving a taxi on badly worn tires that spun out on a rain-slick road — a decision about a municipal inspector's immunity, not a statistic.