Inspecting the tires is the easiest part of the safety inspection. Using a tread depth gauge, check the depth of the tires tread across the full width of the tire. The majority of passenger tires brand new have a tread depth of 10/32nds. A tread depth of under 5/32nds should be replaced for traction reasons but does not fail. If the tread depth of a tire is at 2/32nds on 2 adjacent treads it fails an inspection. Sidewall bubbles from impacts and punctures will cause a vehivle to fail too. Check for edgewear on the tire. This can indicate an alignment problem and you can upsell that to the customer. If there is abnormal wear on a tire and it goes into the secondary layer of rubber this fails. The term for this is secondary wear. This layer of rubber comes right before the metal cords in the tire and is extremely dangerous to drive on. If a customer refuses to install tires with secondary wear, be sure to note down that the customer refused to install tires so in case a blowout does happen, it doesn't fall back on you.