1. What is Taxable Horsepower?

Taxable Horsepower (Tax HP) is a historical tax rating system based on engine bore and number of cylinders, not actual power output. It was used primarily in the UK and other countries for vehicle taxation purposes before modern power measurement systems became standard.

2. How Does the Calculator Work?

The calculator uses the RAC (Royal Automobile Club) formula:

Where:

• \( Bore \) — Cylinder bore diameter in inches
• \( Number\ of\ Cylinders \) — Total cylinders in the engine
• \( 2.5 \) — Constant divisor in the RAC formula

Explanation: This formula was designed as a simple calculation for tax purposes, focusing only on engine dimensions rather than actual power output or performance characteristics.

3. Historical Context of Tax HP

Details: The Tax HP system was developed in the early 20th century when accurate power measurement was difficult. It provided a standardized way to categorize vehicles for taxation based on engine size, making administration simpler for governments and manufacturers.

4. Using the Calculator

Tips: Enter bore diameter in inches and the number of cylinders. Both values must be positive numbers (bore > 0, cylinders ≥ 1).

5. Frequently Asked Questions (FAQ)

Q1: Why was Tax HP used instead of actual horsepower?
A: In the early automotive era, accurate power measurement was complex and expensive. The Tax HP system provided a simple, consistent formula that could be easily calculated from basic engine specifications.

Q2: How does Tax HP compare to modern horsepower measurements?
A: Tax HP typically underestimates actual power output, especially in modern high-efficiency engines. It doesn't account for stroke length, RPM, or technological advancements.

Q3: Is Tax HP still used today?
A: Most countries have moved to more accurate taxation systems based on CO2 emissions, engine displacement, or actual power output. However, it remains relevant for historical vehicle classification and restoration.

Q4: Why doesn't the formula include stroke length?
A: The RAC formula was intentionally simplified for administrative ease. Including stroke length would have made calculations more complex without significant benefit for tax classification purposes.

Q5: Can Tax HP be converted to modern horsepower?
A: There's no direct conversion formula as the relationship varies significantly between different engine designs and technologies. Tax HP should be considered a separate classification system rather than a power measurement.