Commercial DrivingFederal certification87 free questions
CDL Study Guide
Everything you need to know before sitting the Commercial Driver's License (CDL) Knowledge Tests exam, in one place.
Numbers you must memorize
| Topic | Value |
|---|---|
| General Knowledge pass | 80% (typically 40 of 50) |
| Air Brakes / Combination / endorsements pass | 80% on each test |
| Class A | GCWR ≥ 26,001 lb and towed unit ≥ 10,001 lb |
| Class B | Single vehicle ≥ 26,001 lb GVWR |
| Class C | 16+ including driver, or placarded hazmat, and not A/B |
| CMV BAC | 0.04% |
| Interstate minimum age | 21 |
| Following distance (under 40 mph) | 1 second per 10 feet of length |
| First cargo recheck | 50 miles, then every 3 hours or 150 miles |
| Railroad stop | 15–50 feet from the nearest rail |
| Driving time after 10 hours off | 11 hours inside a 14-hour window |
| Break | 30 minutes after 8 hours driving |
| Low-air warning | At or before 60 psi |
| Applied leakage | 2 psi/min single, 3 psi/min combination |
| Triangle spacing (two-way) | About 10 ft / 100 ft / 100 ft |
Which tests to book
- Every CDL: General Knowledge
- Air-brake truck (almost all Class A, many B): Air Brakes
- Class A trailer: Combination Vehicles
- Placarded freight: Hazmat
- Liquid/gas tank: Tanker
- Bus 16+: Passenger
Class comparison: Class A vs Class B. Scoring: 80% passing score. Permit wait: CDL permit.
Study plan (two weeks)
- Days 1–4 — General Knowledge. Classes, space, cargo, HOS, railroad, alcohol. Drill until 85%+.
- Days 5–7 — Air Brakes. Dual system, 60 psi, spring brakes, lag, slack adjusters, leakage.
- Days 8–10 — Combination (Class A) and any endorsement you need.
- Days 11–14 — Timed mocks. 50 questions / 50 minutes. Review every miss on the dashboard.
State booking: California · Texas · Florida · Georgia.