Understanding Tire Size Markings

• P - Passenger vehicle tire designation
• 265 - Section width in millimeters
• 70 - Aspect ratio (sidewall height as percentage of width)
• R - Radial construction
• 17 - Wheel diameter in inches
• 113 - Load index (max load capacity)
• S - Speed rating (max safe speed)

Common Tire Prefixes

• P - Passenger car tire
• LT - Light truck tire (higher load capacity)
• ST - Special trailer tire
• T - Temporary spare tire

Key Wheel Measurements

• Diameter - Size of the wheel from bead seat to bead seat (e.g., 17", 18", 20")
• Width - Distance between inner and outer bead seats (e.g., 8", 9", 10")
• Offset - Distance from wheel centerline to mounting surface (measured in mm)
• Backspacing - Distance from mounting surface to inner wheel edge (measured in inches)
• Bolt Pattern - Number of lugs and diameter of bolt circle (e.g., 6x139.7)

Offset Types

• Positive Offset - Mounting surface is toward the street side (wheels sit inward)
• Zero Offset - Mounting surface is at the centerline
• Negative Offset - Mounting surface is toward the brake side (wheels stick out)

Proper Fitment Principles

• Tire width should be within manufacturer's recommended range for wheel width
• Wider wheels require wider tires for proper bead seating and safety
• Narrower wheels can cause tire sidewall bulging and poor handling
• Always verify clearance with suspension, fenders, and brake components

Recommended Tire Width Ranges

• 8" wheel - 225-255mm tire width
• 9" wheel - 245-275mm tire width
• 10" wheel - 265-295mm tire width
• 12" wheel - 305-345mm tire width

Common Fitment Mistakes

• Installing too wide of a tire on a narrow wheel (unsafe bead seating)
• Not accounting for offset changes when upgrading wheel width
• Ignoring backspacing and causing brake caliper interference
• Exceeding vehicle load rating with improper tire selection
• Not checking fender clearance at full suspension compression

How Offset Affects Your Vehicle

• More Positive Offset - Wheels sit further inward, may cause inner fender contact
• More Negative Offset - Wheels stick out further, wider stance but may rub fenders
• Changing offset affects scrub radius and steering feel
• Extreme offset changes can stress wheel bearings and suspension

Calculating Offset from Backspacing

• Formula: Offset (mm) = (Backspacing - Wheel Width / 2) × 25.4
• Example: 10" wheel with 5.5" backspacing = (5.5 - 5) × 25.4 = +12.7mm offset

Backspacing Considerations

• More backspacing moves wheel inward (toward brakes and suspension)
• Less backspacing moves wheel outward (toward fender)
• Critical for brake caliper clearance on lifted trucks
• Affects track width and vehicle stability

P-Metric Sizing (Passenger)

• Format: P265/70R17
• Width in millimeters, aspect ratio as percentage, wheel diameter in inches
• Most common for cars, crossovers, and light-duty trucks

LT-Metric Sizing (Light Truck)

• Format: LT285/75R18
• Higher load capacity and reinforced construction
• Required for heavy-duty trucks and towing applications
• Often has load range rating (C, D, E) indicating ply rating

Flotation Sizing

• Format: 35x12.50R20
• Overall diameter x section width R wheel diameter
• Common for off-road and lifted truck applications
• Easier to visualize actual tire dimensions

Euro-Metric Sizing

• Format: 265/70R17 (no P prefix)
• Similar to P-metric but different load standards
• Common on European vehicles and some imports