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SAE J966-2019: Test Procedure for Measuring Passenger Car Tire Revolutions Per Mile
🛠️ This SAE Recommended Practice defines a consistent test procedure for determining passenger car tire revolutions per mile. The method supplements SAE J678 and ensures repeatable results across different tires and vehicles. Below we break down the essential requirements and steps from the standard.
1. Scope and Key Objectives
The standard specifies road conditions, vehicle preparation, break-in, test speeds, and measurement techniques. It is intended for passenger car tires and provides a basis for comparing revolutions per mile at different speeds. The primary reference is SAE J678, which covers speedometers and tachometers.
2. Test Conditions and Preparation
Consistent environmental and vehicle conditions are critical. The following table summarizes the key requirements:
| Parameter | Requirement |
|---|---|
| Road Surface | Dry, level, straight, consistent pavement (concrete or asphalt) – 2 miles minimum |
| Ambient Temperature | 40–80 °F (4–27 °C) |
| Tire Position | Drive wheels (same brand, matched set, similar wear) |
| Test Load per Tire | 88 % of maximum load rating per Tire & Rim Association |
| Break-in | 100 miles ± 20 miles at 60 mph (97 km/h) |
| Tire Pressure | 29 psi (200 kPa) after warm-up |
| Test Speeds | 25, 45, 65 mph (40, 72, 105 km/h) |
| Distance per Run | 2 miles (3 km) in each direction |
| Revolutions Counted | Using fifth wheel or calibrated speedometer |
🔍 Design Insight: The break-in schedule of 100 miles at 60 mph stabilizes the tire construction and tread compound, ensuring that subsequent measurements reflect steady‑state behavior rather than initial wear effects.
3. Measurement Method and Data Interpretation
After the break-in, tires are warmed up with a 10‑mile run at 45 mph (72 km/h). Immediately after warm-up, each tire’s pressure is adjusted to 29 psi (200 kPa). The test then proceeds at the three specified speeds in the following order: 45 mph (standard), then 25 mph, then 65 mph.
For each speed, the vehicle must traverse a measured 2‑mile course in both directions at a constant speed. Using a fifth wheel or calibrated speedometer, the number of tire revolutions is recorded. The revolutions per mile at 45 mph are derived from the average of four runs (two in each direction) that agree within 1 % of each other. The same procedure is repeated for 25 and 65 mph, and the results are reported to show the influence of speed on revolutions per mile.
🔍 Engineering Design Insight: Averaging runs in both directions compensates for road grade, wind, and other directional biases. The use of a fifth wheel eliminates errors from speedometer inaccuracies and tire growth effects, providing a true distance reference.
⚠️ Important: If the test is interrupted and the tires cool, the warm-up cycle and pressure adjustment must be repeated. Always verify tire pressure immediately before each new set of runs.
Frequently Asked Questions
Q: Why is 45 mph (72 km/h) chosen as the standard speed?
A: This speed provides a stable and representative measurement for passenger car tires, balancing low‑speed and high‑speed behavior. It is the reference value from which deviations at other speeds are reported.
Q: What load should be applied to the tires during testing?
A: Each tire is loaded to 88 % of its maximum rating as published in the current Tire and Rim Association Year Book. This approximates typical vehicle loading while remaining safely below the tire’s maximum capacity.
Q: How many test runs are required and how are they averaged?
A: For each speed, four runs (two per direction) are required. The average revolutions per mile at 45 mph is taken from these four runs, provided they are within 1 % of each other. The same applies to the other speeds.
Q: What should be done if the test is interrupted?
A: If the tires cool significantly, the warm‑up and pressure adjustment steps must be repeated. This ensures that all measurements are taken under identical thermal and pressure conditions.
Adhering to these guidelines yields repeatable, comparable data for tire revolution rates, assisting engineers in vehicle calibration and performance prediction.
