What should the depth of tread on a tire be?

Answer 1

1/32 inch (according to driversed.com)

From the California Vehicle Code online:

(1) One thirty-second (1/32) of an inch tread depth in any two adjacent grooves at any location of the tire, except as provided in paragraphs (2) and (3).

(2) Four thirty-second (4/32) of an inch tread depth at all points in all major grooves on a tire on the steering axle of any motor vehicle specified in Section 34500, and two thirty-second (2/32) of an inch tread depth at all points in all major grooves on all other tires on the axles of these vehicles.

(3) Six thirty-second (6/32) of an inch tread depth at all points in all major grooves on snow tires used in lieu of tire traction devices in posted tire traction device control areas.
Stick a penny into the tread with Lincon's head upside down, and if the top of his head is visible it is time to repace the tire.

Always replace both tires on the front or both tires on the rear at the same time, and all 4 tires if you have 4 wheel drive or all wheel drive.
In the UK, 1.6mm across 75% of the width of the tread.

There is no national law in the U.S. on tread depth. However 42 states consider 2/32 the minimum. California & Idaho consider 1/32 the minimum. Arkansas, Montana, New Mexico, North Dakota, South Carolina, and West Virginia have no minimum. A tire below 3/32 cannot pass the Texas safety inspection. In Canada the minimum is 2/32. Some experts including Consumer Reports consider 4/32 the minimum. My opinion is that when the tire has less than 3/32 it should be replaced.
Minimum is 2/32 inch

the above answer is correct. most people replace before that around maybe 3 to 4/32nds. 2/32's is by law and is actually a dangerous tread depth in adverse weather. you might be okay around town on dry pavement, but don't stray too far. most people use a penny to measure to the top of Lincolns head is 2/32nds but if you turn your wheel out all the way and get out there is a little bar in the first tread line parallel to the sidewall about every 6 inches or so. That bar is exactly 2/32nds. If your tire is worn in the middle you need to run your tires with less air pressure for longer life. it they are worn on the shoulders, maybe you ran too low air pressure and didn't rotate every 6,000?
The minimum depth of tread depends on the class of vehicle

Passenger vehicles, Goods vehicles not exceeding 3,500kg max gross weight and Light trailers not exceeding 3,500kg max gross weight - At least 1.6mm throughout a continuous band in the centre 3/4 of the tread and around the entire circumference

Most vehicles larger than those listed above - At least 1.0mm throughout a continuous band across at least 3/4 of the breadth of the tread and around the entire circumference

It depends where you are . -I would never drive on less than 1/8 inch. - I value my safety.
Dangerous - especially going round curves in wet weather

Depends on which type. There are wear bars on most if not all tires, that will show a gap or an interruption where the wear limit has been reached. It's easy to see. Having your tires rotated regularly as is in the tires' warranty and in the Owner's Manual will help to get the most out out your tires and any potential warranty claim. If the tires are under or over-inflated most of the time, the wear will show this very clearly as well. Your Owner's Manual will have a page or two on this topic.

2/32 inch
Minimum tire tread on a passenger vehicle tire is at least 1/32nd of an inch, and truck tires should have a tread of at least 1/8th of an inch on the drive axles and the rest of the tires 1/16th of an inch.

Answer 2

It depends where you are . -I would never drive on less than 1/8 inch. - I value my safety.