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.

Example: A question describes a 40-foot vehicle; you picture about 12 meters, a tractor with a short trailer.

Also called: ft, foot

• following distance
• inches
• miles
• stopping distance
• warning devices

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.

Example: The sign says the next rest area is 30 miles away; that is about 48 km, roughly half an hour at highway speed.

Also called: mile

• cargo securement
• feet
• miles per hour
• pre-trip inspection

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.

Example: The limit drops to 45 mph in a work zone; that is about 72 km/h, so you slow well before the sign.

Also called: mph

• following distance
• miles
• safe speed
• speed limit
• total stopping distance

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.

Example: Your dash gauge reads about 110 psi, roughly 7.6 bar, as the compressor builds air before you move.

Also called: pounds per square inch

• air leakage rate
• air pressure gauge
• cut-out pressure
• low air pressure warning
• spring brakes

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.

Example: A pallet is marked 1,200 lbs; you note that is about 540 kg when planning where to place it in the trailer.

Also called: lbs, lb

• 10,001 pounds
• 26,001 pounds
• gross vehicle weight rating
• placard
• reportable quantity

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).

Example: You pump 120 gallons of diesel, about 450 liters, into the two saddle tanks.

Also called: gallon

• fuel tank
• miles
• tank vehicle
• tank vehicle endorsement

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.

Example: You slide a tread depth gauge into a steer tire groove and read the depth in 32nds of an inch.

Also called: inch

• fifth wheel gap
• pushrod
• steering wheel play
• tread depth
• tread depth minimums

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.

Differs by state: Texas adds its own passenger rule: a vehicle built for 24 or more passengers is Class B regardless of weight. California's Class B also covers any three-axle vehicle over 6,000 pounds and farm labor vehicles. California applies the thresholds to GVWR or actual weight (GVW). North Dakota, Rhode Island and Wisconsin use the greater of the rating and the actual or registered weight.

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

Example: A rental box truck is rated 26,000 pounds; one pound more on the label and it would need a CDL.

• 10,001 pounds
• Class A
• Class B
• gross combination weight rating
• gross vehicle weight rating

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'.

Differs by state: California prints 'a towed GVWR of 10,001 pounds or more' and defines commercial Class A as any legal combination, with no GCWR floor. Texas and New York write 'more than 10,000 pounds', the same threshold.

Example: Your rig's GCWR is 30,000 pounds and the trailer's GVWR is 20,000 pounds, so you need Class A.

• 26,001 pounds
• Class A
• Class B
• gross combination weight rating
• semitrailer

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.

Differs by state: The 0.04 limit itself is the same in every state manual. Michigan sets a numeric floor of 0.015 for the out-of-service rule instead of 'any detectable amount'. Kentucky prints the band 0.01 to 0.039.

Does not mean: It is not a 'safe limit': alcohol below 0.04 still takes a CMV driver off the road.

Example: You had several drinks late at night; in the morning you may still be at or above 0.04, so you do not drive.

• blood alcohol concentration
• disqualification
• DUI
• implied consent
• out of service

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.

Does not mean: No fabric showing and no tread separation are separate tire checks, not tread depth.

Example: Your gauge reads 3/32 inch in a steer tire groove; that tire fails inspection and must be replaced.

• inches
• pre-trip inspection
• tires
• 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.

Differs by state: 55 psi in 29 to 30 state manuals (including Texas, Florida, Colorado, Virginia and Pennsylvania). 60 psi in 12 to 13 manuals, among them Georgia, Michigan, New Jersey, North Carolina, Washington and Wisconsin. New York's current manual also prints 60 psi. California prints a range, 'between 55 and 75 psi'. 49 CFR 393.51 requires 'not less than 55 psi', so both printings comply: this is a manual-edition split, not a difference in law. Ohio and Maine print both values in different sections.

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

Example: Pumping the brake pedal with the engine off, you watch the gauge and expect the light and buzzer before the needle reaches the warning value.

• air pressure gauge
• low air pressure warning
• psi
• spring brake pop-out range
• wig wag

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.

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

Example: A trailer air line bursts; as the gauge drops into this range the red knob pops out and the trailer brakes lock.

• low air warning pressure
• spring brake pop-out test
• spring brakes
• tractor protection valve
• trailer air supply control