Field Guides

The rules behind the calculators — with the disagreements left in.

Tire Size Difference Safety: The 3% vs 5% Rule

Is a 2% or 4% tire size difference safe? The community splits between 3% and 5% — here is both schools, the math, and how to decide.

AWD Mismatched Tires: How Much Difference Is Too Much?

Why full-time AWD destroys itself over mismatched tires — the 0.5" shudder and 0.75" destruction thresholds, with a 6.7% case study.

Mismatched Gear Ratios: How 2% Destroys a 4x4

Front and rear axle ratios differing by ~2% bind 4WD, throw faults, and destroy drivetrains. How to check before buying, and repair costs.

Speedometer Accuracy: How Wrong Is It, and What You Can Do

Speedometers read high from the factory and drift further with tire size. Legal limits, the +2–3 mph factory bias, GPS checking, and fixes.

Hand-Calculated MPG: The Number the Pump Actually Knows

Dashboard MPG estimates run 3–8% optimistic. The fill-up method for your true fuel economy, how to convert to L/100km, and when apps like Fuelly help.