Numbers and units on the test

A US length unit, about a third of a meter.

1 foot = 0.3 m (12 inches). 10 feet is about 3 m; 100 feet is about 30 m. Your vehicle's length in feet sets your following distance, and warning triangles and railroad stops are placed in feet.

The US unit for road distance, longer than a kilometer.

1 mile = 1.6 km; 50 miles is about 80 km. Road signs, the odometer, and the manual all use miles.

The US speed unit: miles traveled in one hour.

1 mph = 1.6 km/h. 55 mph is about 89 km/h; 65 mph is about 105 km/h. US speedometers show mph first; some add km/h in smaller print.

Pounds per square inch, the US pressure unit for air brakes and tires.

1 psi = 6.9 kPa, or about 0.07 bar. 100 psi is about 6.9 bar, roughly three times the pressure in a car tire. Air brake gauges and tire ratings in the US read in psi.

The US weight unit used for vehicle ratings, cargo, and hazmat quantities.

1 pound = 0.45 kg; 1,000 pounds is about 450 kg. On labels and signs you will see 'lbs'. A loaded tractor-trailer weighs tens of thousands of pounds.

The US unit of liquid volume, a bit less than four liters.

1 gallon = 3.8 L. A typical truck fuel tank holds over 100 gallons, more than 380 L. Fuel economy is given in miles per gallon (mpg).

A small US length unit, about two and a half centimeters.

1 inch = 2.54 cm. The manual often uses fractions of an inch, such as 4/32 or 1/4 inch; 1/32 inch is under one millimeter.

The GVWR or GCWR at which a vehicle needs a CDL.

A single vehicle rated 26,001 pounds or more (about 11,794 kg) needs at least a Class B CDL. A combination rated 26,001 pounds or more needs Class A when the towed unit is rated over 10,000 pounds. Some manuals say 'more than 26,000 pounds'; it means the same threshold.

Not the same as: It is a rating threshold, not a scale reading; see the state note for the states where actual weight also counts.

The towed unit's GVWR that makes a heavy combination Class A.

A trailer rated 10,001 pounds or more (about 4,536 kg) behind a combination rated 26,001 pounds or more means Class A. If the trailer is rated 10,000 pounds or less, the power unit's own rating decides the class. Most manuals write it as 'more than 10,000 pounds'.

Not asked directly in our bank; it appears inside Class A definitions, so questions on which class you need depend on it.

The blood alcohol concentration (BAC) at or above which a CMV driver is legally intoxicated.

0.04 is half the limit most states use for car drivers. Driving a CMV at 0.04 or more costs your CDL for at least one year, and for life on a second offense. Alcohol below 0.04 still puts you out of service for 24 hours.

Not the same as: It is not a 'safe limit': alcohol below 0.04 still takes a CMV driver off the road.

The least tread allowed: 4/32 inch on front tires, 2/32 inch elsewhere.

Measure in every major groove with a tread depth gauge. 4/32 inch is about 3.2 mm; 2/32 inch is about 1.6 mm. Steer tires need more because they steer the truck.

Not the same as: No fabric showing and no tread separation are separate tire checks, not tread depth.

The tank pressure below which the low air warning must be on.

Most state manuals print 55 psi, some print 60 psi, and federal rules require the warning at no less than 55 psi. Either way, it comes on well above the pressure where the spring brakes apply on their own. When it comes on, stop safely while you still have brakes.

Not the same as: Not the pressure at which spring brakes pop out (that range is lower) and not the governor cut-in.

The pressure range (20 to 45 psi) at which spring brakes apply on their own.

Spring brakes are held off by air. When pressure falls into this range, usually nearer 20 to 30 psi, the springs put the brakes on by themselves. The tractor protection valve closes in the same range to save the tractor's air.

Not the same as: Not the low air warning value; the warning comes on much earlier, so you have time to stop.