Seibishi 3kyu › Calculation questions
Calculation questions in the Class 3 exam: formulas and examples
Calculations need only a few formulas and some multiplication and division, so they are easy points if you practise. Here are the common types, each with an example written by ViMai.
Last updated: 2026-10-19
1. Total displacement (総排気量)
One cylinder = π ÷ 4 × D² × S (D: bore, S: stroke). Total = one cylinder × number of cylinders.
Example: D = 80 mm = 8 cm, S = 90 mm = 9 cm, 4 cylinders, π = 3.14. One cylinder: 0.785 × 64 × 9 ≈ 452 cm³. Total ≈ 1,809 cm³.
2. Compression ratio (圧縮比)
Compression ratio = (one cylinder's displacement + combustion chamber volume) ÷ combustion chamber volume.
Example: 450 cm³ per cylinder, 50 cm³ chamber. (450 + 50) ÷ 50 = 10. The common trap is 450 ÷ 50 = 9.
3. Gear ratio (変速比, 減速比)
Gear ratio = driven teeth ÷ driving teeth. Overall ratio = gearbox ratio × final drive ratio.
Example: a 15-tooth driving gear and a 45-tooth driven gear: 45 ÷ 15 = 3. With a gearbox ratio of 2.0 and a final drive of 4.0 the overall ratio is 8.0. At 3,000 rpm the wheels turn at 3,000 ÷ 8 = 375 rpm.
4. Torque (トルク)
Torque = force × distance. Example: 200 N at the end of a 0.3 m wrench: 200 × 0.3 = 60 N·m.
5. Ohm's law and electrical power
- V = I × R. Example: 12 V across 4 Ω: I = 12 ÷ 4 = 3 A.
- P = V × I. Example: a 12 V bulb drawing 5 A: 12 × 5 = 60 W.
- Resistors in series add up. Two equal resistors in parallel give half the value.
Tips for calculation items
Write the unit next to every number. Estimate first (a four-cylinder passenger car engine is usually somewhere around 1,000 to 2,500 cm³) to rule out absurd options. The wrong options are often the result of typical slips: a single cylinder only, forgetting to add the chamber volume, a ratio turned upside down. See more examples in sample questions with answers.
VIP unlocks papers 4 to 20 in all three subjects (51 papers), the per-item tips and the detailed explanations for missed items. VIP is bought inside the iOS app with Apple payment, and Apple shows the price. The App Store version is coming soon. You can try the free exams on the web now.
FAQ
Can I bring a calculator?
Follow the instructions on your admission slip and in JASPA's guide. The numbers in Class 3 items are usually chosen so you can work by hand.
What unit is displacement given in?
Usually cm³ (cc). Convert bore and stroke from mm to cm before you calculate, so you do not lose a digit.
Is the value of π given?
Items usually say what to use for π (for example 3.14). Use exactly the value given.
Take a 30-item mock paper and see right away which items you missed and why.
Try it free nowPapers 1–3 of every subject are free, with furigana. Opens in your browser, no sign-up.
More topics
- Seibishi 3kyuClass 3 automotive mechanic exam practice (vi / ja / en)
- Class 3 exam format30 four-choice items, 60 min, pass at 21/30
- Registering for Class 3Eligibility, application, 2026 test dates
- Class 3 practice test3 subjects × 20 papers, papers 1–3 free
- Car mechanic vocabularyWork words and part names with readings
- Class 3 study tipsStudy by subject, read items carefully, review
- Class 3 chassisClutch, gearbox, brakes, steering, tyres
- Class 3 gasoline engineFour-stroke engine, fuel, ignition, electrics
- Class 3 diesel engineCompression ignition, injection, glow plugs, exhaust
- Measuring toolsVernier caliper, micrometer, dial gauge, tester
- Safety standardsSafety standards, periodic inspection, lamps, tyres
- Four-choice itemsCorrect vs incorrect items, elimination
- 30-day study planA 30-day plan for one Class 3 subject
- Sample questions6 sample four-choice items with answers
- Car parts in JapaneseEngine, chassis, electrical and body parts
- Pass mark and pass rate21/30 pass mark and the latest JASPA pass rates
- New system from 2027Class 3 (general) from 2027 and the old tests