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Tire Size and Fitment Calculator - Square Body Trucks

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SquareBodyAI
• 8 min read • Updated 3 days, 18 hours ago

Tire Size and Fitment Calculator: 1973-1987 GM Square Body Trucks

Overview

Proper tire sizing and fitment is critical for Square Body truck performance, safety, and aesthetics. This comprehensive guide covers tire size calculations, fitment considerations, and clearance requirements for all 1973-1987 GM C/K series trucks and utilities. Understanding tire dimensions, load ratings, and physical clearances prevents costly mistakes and ensures optimal vehicle performance.

Key Topics Covered:
- Tire size calculation and conversion
- Stock fitment specifications by model and year
- Clearance requirements for different tire sizes
- Load rating and capacity considerations
- Speedometer correction calculations
- Wheel offset and backspacing effects on fitment

Why This Matters:
- Prevents tire rub on suspension components, fenders, or bed
- Maintains proper load capacity for truck applications
- Ensures accurate speedometer readings
- Optimizes performance and fuel economy
- Maintains safe handling characteristics

Common Issues

Tire Rub Problems

  • Front fender rub at full lock with oversized tires
  • Rear fender rub under load or suspension compression
  • Suspension component interference with wider tires
  • Bed clearance issues on short bed trucks with large tires

Speedometer Inaccuracy

  • Over-reading speedometer with smaller than stock tires
  • Under-reading speedometer with larger than stock tires
  • Odometer errors affecting maintenance intervals and resale value

Load Capacity Reduction

  • Insufficient load rating when switching to smaller sidewall tires
  • Reduced payload capacity with incorrect tire specifications
  • Handling degradation under load with improper tire sizing

Clearance Miscalculations

  • Insufficient suspension travel clearance
  • Steering lock limitations with wider tires
  • Chain clearance issues in snow conditions

Parts & Materials Needed

Essential Tools

  • Measuring tape (minimum 25 feet)
  • String line or straightedge for alignment checking
  • Tire pressure gauge (0-80 PSI range)
  • Jack and jack stands for suspension measurement
  • Wheel chocks for safety

Reference Materials

Stock Tire Size Reference Chart
Wheel Offset Measurement Guide
Load Index Chart
Speed Rating Chart

Measurement Supplies

  • Chalk or marking pen for tire contact patches
  • Ruler or caliper for precise measurements
  • Level for accurate ride height measurement
  • Flashlight for inspecting clearances

Step-by-Step Guide

Step 1: Determine Current Tire Specifications

Read Current Tire Sidewall:

Example: P235/75R15
P = Passenger vehicle
235 = Section width in millimeters
75 = Aspect ratio (sidewall height percentage)
R = Radial construction
15 = Wheel diameter in inches

Calculate Overall Diameter:
1. Convert section width to inches: 235mm ÷ 25.4 = 9.25"
2. Calculate sidewall height: 9.25" × 0.75 = 6.94"
3. Calculate overall diameter: (6.94" × 2) + 15" = 28.88"

Step 2: Measure Current Clearances

Front Clearance Points:
- Fender lip clearance: Measure at full steering lock
- A-arm clearance: Check at full compression and extension
- Tie rod clearance: Verify at all steering positions
- Frame clearance: Check inner sidewall to frame rail

Rear Clearance Points:
- Fender lip clearance: Measure under load conditions
- Leaf spring clearance: Check sidewall to spring clearance
- Differential clearance: Verify adequate space around pumpkin
- Bed clearance: Measure on short bed models

Step 3: Calculate New Tire Dimensions

Use Tire Size Calculator Formula:

Overall Diameter = (Section Width × Aspect Ratio ÷ 25.4 × 2) + Wheel Diameter

Example Calculations:

Stock 235/75R15: 28.88" diameter
Proposed 255/70R15: 29.06" diameter
Difference: +0.18" (acceptable)

Stock 235/75R15: 28.88" diameter  
Proposed 31x10.50R15: 31.00" diameter
Difference: +2.12" (requires clearance verification)

Step 4: Verify Load Rating Compatibility

Check Load Index Requirements:

C10 (½ ton): Minimum Load Index 104 (1984 lbs per tire)
C20 (¾ ton): Minimum Load Index 110 (2337 lbs per tire)  
K10 (½ ton 4WD): Minimum Load Index 104 (1984 lbs per tire)
K20 (¾ ton 4WD): Minimum Load Index 112 (2469 lbs per tire)

Load Index Conversion Chart:

100 = 1764 lbs    105 = 2028 lbs    110 = 2337 lbs
101 = 1819 lbs    106 = 2094 lbs    111 = 2403 lbs
102 = 1874 lbs    107 = 2149 lbs    112 = 2469 lbs
103 = 1929 lbs    108 = 2205 lbs    113 = 2535 lbs
104 = 1984 lbs    109 = 2271 lbs    114 = 2601 lbs

Step 5: Calculate Speedometer Error

Speedometer Error Formula:

Error % = ((New Diameter - Stock Diameter) ÷ Stock Diameter) × 100

Example:
Stock: 28.88" diameter
New: 31.00" diameter
Error: ((31.00 - 28.88) ÷ 28.88) × 100 = +7.3%

At indicated 60 MPH, actual speed = 64.4 MPH

Acceptable Error Range: ±3% for minimal impact on performance and legality

Year/Model Differences

1973-1980 Models

C10/K10 Stock Specifications:

Standard: 8.75x16.5 Bias Ply (28.7" diameter)
Optional: H78-15 Bias Ply (28.4" diameter)
Load Range: C (6 ply rating)
Wheel Size: 15" x 6" or 16.5" x 6"

C20/K20 Stock Specifications:

Standard: 8.75x16.5 Bias Ply (28.7" diameter)
Heavy Duty: 9.50x16.5 Bias Ply (30.1" diameter)
Load Range: D (8 ply rating)
Wheel Size: 16.5" x 6.5"

K5 Blazer/Suburban:

Standard: 8.75x16.5 Bias Ply
Optional: L78-15 Bias Ply (29.2" diameter)
Wheel Size: 15" x 7" (1975-1980) or 16.5" x 6"

1981-1987 Models

C10/K10 Stock Specifications:

Standard: P235/75R15 (28.9" diameter)
Optional: 31x10.50R15 (31.0" diameter)
Load Range: B or C
Wheel Size: 15" x 7" or 15" x 8"

C20/K20 Stock Specifications:

Standard: LT235/85R16 (31.7" diameter)
Heavy Duty: LT245/75R16 (30.5" diameter)
Load Range: D or E
Wheel Size: 16" x 6.5"

K5 Blazer/Suburban:

Standard: P235/75R15 (28.9" diameter)
Optional: 31x10.50R15 (31.0" diameter)
Wheel Size: 15" x 7" or 15" x 8"

Wheel Offset Differences

1973-1980 Wheel Specifications:

15" x 6": +3.75" offset (6" backspacing)
15" x 7": +4.25" offset (6.5" backspacing)
16.5" x 6": +3.75" offset (6" backspacing)

1981-1987 Wheel Specifications:

15" x 7": +4.25" offset (6.5" backspacing)
15" x 8": +4.75" offset (7" backspacing)
16" x 6.5": +4.00" offset (6.75" backspacing)

Tips & Best Practices

Sizing Recommendations

Conservative Upgrades (No Modifications Required):

1973-1980 Models:
- P235/75R15 (28.9" diameter) - Direct upgrade from bias ply
- 30x9.50R15 (30.0" diameter) - Moderate increase

1981-1987 Models:
- 255/70R15 (29.1" diameter) - Wider contact patch
- 235/85R15 (29.7" diameter) - Taller sidewall for load capacity

Performance Upgrades:

All Models:
- 31x10.50R15 (31.0" diameter) - Popular upgrade size
- 255/85R15 (31.1" diameter) - Metric equivalent
- LT265/75R15 (30.6" diameter) - Load range D/E available

Clearance Guidelines

Minimum Clearance Requirements:
- Fender lip: 0.75" minimum at full lock
- Suspension components: 0.50" minimum clearance
- Frame rails: 0.25" minimum inner sidewall clearance
- Leaf springs: 0.50" minimum sidewall clearance

Testing Procedures:
1. Full steering lock test with vehicle unloaded
2. Suspension compression test with maximum payload
3. Articulation test on uneven surfaces (4WD models)
4. Chain clearance test for snow chain compatibility

Load Rating Selection

Upgrade Recommendations:
- C10/K10: Use Load Range C minimum for improved durability
- C20/K20: Use Load Range D or E for maximum capacity
- Towing applications: Upgrade one load range above minimum
- Off-road use: Prioritize sidewall strength (Load Range C minimum)

Speedometer Correction Options

Mechanical Speedometer (1973-1981):

Speedometer gear changes available from:
- GM Performance Parts
- Classic Instruments  
- LMC Truck Part #12-1436 (gear set)

Electronic Speedometer (1982-1987):

ECM recalibration options:
- PROM chip replacement
- Aftermarket speedometer corrector
- VSS (Vehicle Speed Sensor) modification

Troubleshooting

Tire Rub Diagnosis

Front Tire Rub:

Symptoms:
- Scraping noise during turns
- Tire wear on outer shoulder
- Fender liner damage

Solutions:

Minor rub (0.25-0.50"):
- Trim fender lip with cutting wheel
- Install fender flares (LMC Truck #20-1436)
- Reduce tire pressure 2-3 PSI

Major rub (>0.50"):
- Install wheel spacers (0.25-0.50" maximum)
- Change to lower profile tire
- Modify suspension geometry

Rear Tire Rub:

Symptoms:
- Tire wear on inner shoulder
- Leaf spring contact marks
- Fender well damage under load

Solutions:

Load-related rub:
- Install overload springs (Part #465-1108)
- Upgrade to helper springs
- Reduce payload or tongue weight

Constant rub:
- Install wheel spacers (0.25" maximum recommended)
- Narrow tire selection
- Modify fender wells

Speedometer Correction Issues

Over-Reading Speedometer:

Cause: Tire diameter smaller than stock

Correction Methods:

1973-1981 Models:
- Install speedometer gear with fewer teeth
- Available ratios: 17, 18, 19, 20, 21, 22 teeth

1982-1987 Models:  
- Install corrector module (Dakota Digital SGI-5)
- ECM PROM replacement
- VSS gear ratio change

Under-Reading Speedometer:

Cause: Tire diameter larger than stock

Correction Methods:

1973-1981 Models:
- Install speedometer gear with more teeth  
- Custom gears available for extreme size changes

1982-1987 Models:
- Electronic correction module required
- Professional ECM programming recommended

Load Capacity Problems

Insufficient Load Rating:

Symptoms:
- Excessive tire deflection under load
- Poor handling when loaded
- Premature tire wear

Solutions:
- Upgrade to higher load index tires
- Increase tire pressure within manufacturer limits
- Distribute load more evenly
- Consider dual rear wheel conversion for extreme loads

Load Distribution Issues:

Diagnosis:
- Measure tire pressure after loading
- Check for uneven tire wear patterns
- Verify proper tongue weight (10-15% of trailer weight)

Corrections:
- Adjust load placement in bed
- Install helper springs or air bags
- Upgrade to Load Range E tires
- Consider weight distribution hitch for towing

Clearance Verification Failures

Insufficient Testing:

Common Oversights:
- Testing only at normal ride height
- Ignoring suspension articulation
- Not considering loaded conditions
- Forgetting chain clearance requirements

Comprehensive Testing Protocol:
1. Static clearance: All positions, normal load
2. Dynamic clearance: Full steering lock, various speeds
3. Loaded clearance: Maximum payload conditions
4. Articulation clearance: Uneven surfaces, parking lots
5. Chain clearance: Snow chain installation simulation

Professional Verification Services:
- Local tire shops with alignment equipment
- 4WD specialty shops for lift/tire combinations
- Performance shops for racing applications

This guide provides general information for 1973-1987 GM Square Body trucks. Always verify specific applications with professional installers and refer to manufacturer specifications for safety-critical applications.