Tire Selection for 1973-1987 GM Square Body Trucks

Overview

Proper tire selection is crucial for maximizing the performance, safety, and appearance of your 1973-1987 GM Square Body truck. Whether you're restoring a C10 to factory specifications or upgrading a K20 for off-road capability, understanding the original equipment specifications, available upgrades, and compatibility considerations will ensure optimal results.

This comprehensive guide covers factory tire specifications, load ratings, sizing considerations, and modern replacement options for all Square Body variants including C10/C20 (2WD), K10/K20 (4WD), K5 Blazer, and Suburban models. Proper tire selection affects ride quality, fuel economy, load capacity, traction, and overall vehicle safety.

Key considerations include:
- Load capacity requirements based on truck model and intended use
- Wheel diameter compatibility (14", 15", 16.5", and aftermarket options)
- Sidewall construction differences between radial and bias-ply tires
- Speed ratings and their impact on highway driving
- Tread patterns for specific applications (highway, all-terrain, mud-terrain)

Common Issues

Factory Tire Availability Problems

16.5" wheel diameter shortage: Many Square Body trucks came with 16.5" wheels, but tire selection is extremely limited in 2024
Load Range confusion: Mixing different load ranges can cause uneven wear and handling issues
Radial vs. bias-ply mixing: Never mix radial and bias-ply tires on the same vehicle

Sizing and Fitment Issues

Rubbing on suspension components: Oversized tires may contact A-arms, tie rods, or frame components during steering lock
Speedometer calibration: Changing tire diameter affects speedometer accuracy
Clearance problems: Larger tires may rub fender wells, especially with lowered trucks or during suspension compression

Performance-Related Problems

Load capacity inadequacy: Using passenger car tires on trucks can lead to dangerous overloading
Uneven wear patterns: Incorrect tire pressure or mismatched tires cause premature wear
Handling degradation: Wrong tire size or construction affects steering response and stability

Parts & Materials Needed

Tools Required

Tire pressure gauge (0-80 PSI range recommended)
Valve stem tool for pressure adjustments
Measuring tape for clearance verification
Jack and jack stands for wheel removal and inspection

Essential Information to Gather

Current wheel size and bolt pattern
2WD trucks: 5x5" (5x127mm) bolt pattern
4WD trucks: 6x5.5" (6x139.7mm) bolt pattern
Load requirements based on vehicle GVWR
Suspension modifications that may affect clearance
Intended use (daily driver, towing, off-road, show)

Recommended Tire Specifications by Model

Light Duty Models (C10, K10, K5 Blazer)

Factory sizes:
- G78-15 (bias-ply) = 235/75R15 (radial equivalent)
- H78-15 (bias-ply) = 255/70R15 (radial equivalent)
- 31x10.50R15 (light truck radial)

Heavy Duty Models (C20, K20, Suburban)

Factory sizes:
- 8.75R16.5LT (Load Range C or D)
- 9.50R16.5LT (Load Range D)
- 235/85R16 (modern 16" equivalent)

Step-by-Step Guide

Step 1: Determine Current Specifications

Locate the tire placard on the driver's door jamb or glove box
Record current tire size from sidewall markings
Measure wheel diameter and width if upgrading wheels
Check vehicle GVWR on door jamb sticker for load requirements

Step 2: Assess Vehicle Condition and Modifications

Inspect suspension components for wear or modifications
Measure ground clearance at front and rear
Check for body modifications (lifts, lowering, fender modifications)
Verify wheel condition and proper backspacing

Step 3: Select Appropriate Tire Category

For Stock Restoration

1973-1980 models: Consider bias-ply reproductions for authenticity
1981-1987 models: Radial tires were standard, easier to source
Maintain original load range (typically Load Range C or D for trucks)

For Daily Driver Use

Prioritize radial construction for improved fuel economy and ride quality
Select appropriate speed rating (S-rated minimum for highway use)
Consider all-season tread for year-round capability

For Performance Applications

Upgrade to modern compounds for improved traction
Consider lower profile options if wheels are upgraded
Match tire width to wheel width (typically 1.4:1 ratio)

Step 4: Calculate Load Requirements

Determine vehicle weight (curb weight + cargo + passengers)
Apply safety factor (typically 1.2x calculated weight)
Verify load index meets or exceeds requirements
Consider weight distribution (60/40 front/rear typical)

Step 5: Verify Clearance and Compatibility

Test fit with suspension compressed (simulate loaded condition)
Check steering lock clearance at full turn both directions
Verify speedometer accuracy using online calculators
Confirm TPMS compatibility if equipped (1987 models only)

Year/Model Differences

1973-1976 Early Square Body

Standard wheel size: 15x6" or 15x7"
Common factory sizes: G78-15, H78-15 (bias-ply)
Load rating: Most used Load Range B tires
Special considerations: Some early trucks used 14" wheels

Recommended modern equivalents:

G78-15 → 235/75R15
H78-15 → 255/70R15

1977-1980 Mid Square Body

Wheel options expanded: 15x7", 15x8", 16.5x6.75"
Radial tires introduced: Optional on some models
Heavy duty packages: 16.5" wheels became common on 3/4-ton models
K5 Blazer: Typically used 15x8" wheels with wider tires

Factory specifications:
- C10: 235/75R15 or equivalent bias-ply
- K10: 31x10.50R15LT common
- C20/K20: 8.75R16.5LT or 9.50R16.5LT

1981-1987 Late Square Body

Radial tires standard across all models
Metric sizing adopted on many applications
16.5" wheels common on heavy duty trucks
Improved load ratings and speed ratings

Common factory sizes:

C10: 235/75R15
K10: 31x10.50R15LT
C20: 8.75R16.5LT
K20: 9.50R16.5LT
Suburban: 235/85R16 (if converted to 16")

Model-Specific Considerations

C10/C20 (2WD Models)

Lower ground clearance: Limits maximum tire diameter
5-lug bolt pattern: More wheel options available
Lighter suspension: Requires careful load rating selection

K10/K20 (4WD Models)

Higher ground clearance: Accommodates larger tire diameters
6-lug bolt pattern: Fewer aftermarket wheel options
Transfer case considerations: Affects approach angle with larger tires

K5 Blazer

Shorter wheelbase: More prone to tire rub during articulation
Removable top: Weight distribution changes affect tire selection
Recreational use: Often benefits from all-terrain tread patterns

Suburban

Heaviest model: Requires highest load ratings
Longest wheelbase: Less prone to tire clearance issues
Towing applications: May require Load Range E tires

Tips & Best Practices

Selection Best Practices

Always replace tires in pairs (minimum) or complete sets (preferred)
Match tread patterns on the same axle
Consider seasonal requirements when selecting tread compounds
Verify load index calculations rather than assuming adequacy

Installation Guidelines

Break-in period: Drive gently for first 500 miles on new tires
Initial pressure check: Verify pressure after first 100 miles
Rotation schedule: Establish rotation pattern based on tread design
Balancing requirements: Always balance new tires, consider road force balancing

Maintenance Recommendations

Monthly pressure checks: Use accurate gauge, check when cold
Visual inspections: Look for uneven wear, sidewall damage, embedded objects
Rotation intervals: Every 5,000-7,500 miles depending on use
Alignment verification: Check if uneven wear develops

Budget Considerations

Tire life expectancy: Factor replacement intervals into cost calculations
Performance vs. longevity: Softer compounds perform better but wear faster
Warranty coverage: Compare mileage warranties and road hazard protection
Installation costs: Factor mounting, balancing, and disposal fees

Vendor Recommendations

Coker Tire: Excellent for bias-ply reproductions and vintage sizing
BFGoodrich: Strong selection of light truck radials
Michelin: Premium options with excellent longevity
General Tire: Good value options for daily drivers

Troubleshooting

Clearance Issues

Symptoms: Tire rubbing during turns or suspension compression

Diagnosis steps:
1. Jack up vehicle and manually turn wheels to full lock
2. Compress suspension by hand to simulate loaded condition
3. Check for witness marks on tires, fenders, or suspension components

Solutions:
- Reduce tire diameter or width
- Adjust wheel backspacing with spacers or different wheels
- Trim fender wells carefully (irreversible modification)
- Modify suspension stops to limit compression travel

Handling Problems

Symptoms: Poor steering response, wandering, or instability

Diagnosis steps:
1. Verify tire pressures match manufacturer recommendations
2. Check for mismatched tires (different sizes, brands, or wear levels)
3. Inspect for uneven wear patterns indicating alignment issues

Solutions:
- Standardize tire specifications across axles
- Perform wheel alignment if wear patterns indicate problems
- Consider load range - too stiff may cause harsh ride, too soft may cause handling issues
- Verify speed rating adequacy for driving conditions

Speedometer Inaccuracy

Symptoms: GPS speed differs significantly from speedometer reading

Calculation method:

New Speed = Indicated Speed × (New Diameter ÷ Original Diameter)

Correction options:
- Speedometer gear change (mechanical speedometers)
- Electronic calibration (1987 models with electronic speedometers)
- Aftermarket calibrators available for some applications

Load Capacity Problems

Symptoms: Excessive tire wear, sidewall bulging, or handling issues when loaded

Diagnosis:
1. Calculate actual vehicle weight including cargo
2. Verify tire load index meets requirements
3. Check tire pressure - may need adjustment for load

Solutions:
- Upgrade to higher load range (C to D, or D to E)
- Increase tire pressure within sidewall limits for heavier loads
- Distribute load properly to avoid overloading individual tires

Premature Wear Issues

Common wear patterns and causes:

Center wear: Overinflation
Edge wear: Underinflation or overloading
One-sided wear: Alignment problems
Cupping/scalloping: Suspension problems or imbalance

Preventive measures:
- Maintain proper pressure checked monthly
- Regular rotation following appropriate pattern
- Address suspension issues promptly
- Professional alignment every 12,000 miles or when problems develop

16.5" Wheel Conversion

Problem: Limited tire selection for factory 16.5" wheels

Conversion options:
1. Convert to 16" wheels: Widely available tire selection
2. Upgrade to 17" wheels: Modern performance tire options
3. Custom wheel solutions: Maintain original appearance with modern sizing

Considerations:
- Bolt pattern compatibility (6x5.5" for 4WD models)
- Load rating requirements must be maintained
- Brake clearance verification required
- Speedometer recalibration may be necessary

Tire Selection - Square Body Trucks