Aftermarket Wheel Options for GM Square Body Trucks (1973-1987)
Overview
Selecting appropriate aftermarket wheels for 1973-1987 GM Square Body trucks requires understanding the specific bolt patterns, offsets, and load ratings required for these vehicles. Square Body trucks came with various factory wheel configurations, and aftermarket options range from period-correct reproductions to modern performance wheels that enhance both appearance and functionality.
This guide covers wheel specifications, compatibility considerations, and installation requirements for C10, C20, K10, K20, K5 Blazer, and Suburban models. Proper wheel selection affects not only aesthetics but also handling, braking performance, and drivetrain stress.
Key considerations include:
- Bolt pattern compatibility (6x5.5" standard)
- Load rating requirements for different truck models
- Offset and backspacing effects on suspension geometry
- Tire size compatibility and clearance issues
- Impact on speedometer calibration
Common Issues
Incorrect Offset/Backspacing
Many aftermarket wheels use modern offset specifications that can cause:
- Interference with suspension components, brake lines, or steering linkage
- Altered scrub radius affecting steering feel and tire wear
- Increased stress on wheel bearings and ball joints
- Clearance issues with fender wells during suspension travel
Inadequate Load Rating
Critical for C20/K20 models: Many passenger car wheels lack sufficient load capacity for 3/4-ton applications, leading to:
- Wheel failure under load
- Premature bearing wear
- Unsafe handling characteristics when loaded
Hub-Centric vs. Lug-Centric Mounting
Most aftermarket wheels are lug-centric, while factory wheels are hub-centric, potentially causing:
- Wheel vibration at highway speeds
- Uneven lug nut wear
- Difficulty achieving proper wheel mounting
Speedometer Calibration Issues
Significant changes in overall tire diameter affect:
- Speedometer accuracy
- Odometer readings
- Transmission shift points (on later models with electronic controls)
Parts & Materials Needed
Essential Components
Wheels (per axle set):
- Front (2WD): Standard passenger load rating acceptable
- Front (4WD): Must clear brake components and steering linkage
- Rear: Load rating must match vehicle GVWR requirements
Hardware:
- Lug nuts: 12mm x 1.5 thread pitch (standard)
- Hub-centric rings: Required for most aftermarket wheels
- Valve stems: Standard rubber or TPMS-compatible (if applicable)
Tools Required:
- Torque wrench (80-100 ft-lbs capability)
- Jack and jack stands rated for vehicle weight
- Lug wrench or impact gun
- Wheel alignment tools (recommended)
Recommended Suppliers
- LMC Truck: Period-correct reproductions and modern options
- Classic Performance Products (CPP): Performance-oriented wheels
- Summit Racing: Wide selection of aftermarket brands
- Wheel Vintiques: Steel wheel reproductions
- American Racing: Classic and modern aluminum wheels
Step-by-Step Guide
Pre-Purchase Verification
-
Measure Current Setup:
Bolt Pattern: 6 x 5.5" (6 x 139.7mm) Hub Bore: 78.1mm (standard) Current Offset: Measure from mounting surface to wheel centerline -
Determine Load Requirements:
- C10/K10: 1,500+ lbs per wheel minimum
- C20/K20: 2,000+ lbs per wheel minimum
- Suburban/Blazer: 1,750+ lbs per wheel minimum -
Check Clearance Requirements:
- Measure brake caliper clearance (minimum 1/4" recommended)
- Verify suspension component clearance
- Check fender well clearance at full lock and compression
Installation Process
-
Preparation:
- Park on level surface and engage parking brake
- Loosen lug nuts 1/2 turn before lifting vehicle
- Use proper jack points (frame rails or axle housing) -
Wheel Removal:
- Raise vehicle and secure with jack stands
- Remove lug nuts in star pattern
- Pull wheel straight off hub to avoid damaging studs -
Hub Preparation:
- Clean hub surface with wire brush
- Apply thin coat of anti-seize compound to hub face
- Install hub-centric rings if required -
New Wheel Installation:
- Align wheel with studs and push fully onto hub
- Install lug nuts hand-tight in star pattern
- Lower vehicle until wheel contacts ground (maintains alignment)
- Torque lug nuts to specification in star pattern:
Initial: 50 ft-lbs Final: 80-100 ft-lbs (verify with vehicle manual) -
Final Checks:
- Re-torque lug nuts after 50-100 miles
- Check for vibration or unusual handling
- Verify speedometer accuracy with GPS comparison
Year/Model Differences
1973-1980 Models
- Standard bolt pattern: 6 x 5.5"
- Hub bore: 78.1mm
- Factory offset: Typically +12mm to +25mm
- Brake clearance: Generally more forgiving due to smaller brake components
- Load ratings: C10 models often had lighter-duty specifications
1981-1987 Models
- Bolt pattern: Unchanged (6 x 5.5")
- Brake systems: Larger front discs on many models require careful clearance checking
- Suburban/Blazer: Often came with wider factory wheels
- K-models: 4WD variants may have different offset requirements
Model-Specific Considerations
C10 (1/2-ton 2WD):
- Most flexible for aftermarket options
- Standard car-type load ratings often sufficient
- Wider range of offset compatibility
K10 (1/2-ton 4WD):
- Must clear front differential and CV joints
- Limited low-offset options due to suspension components
- Verify clearance with steering at full lock
C20/K20 (3/4-ton):
- Critical: Require commercial/truck load ratings
- Heavier-duty wheel bearings can handle wider wheels
- Often came with 8-lug pattern (8 x 6.5") - verify before ordering
K5 Blazer:
- Similar to K10 requirements
- Shorter wheelbase affects handling with wider wheels
- Popular for larger diameter wheel upgrades
Suburban:
- Longer wheelbase provides stability for larger wheels
- Higher load requirements due to passenger capacity
- Consider ride quality impact with low-profile tires
Tips & Best Practices
Offset and Backspacing Guidelines
- Conservative approach: Stay within 1" of factory backspacing
- Performance builds: Wider track width improves stability but increases stress
- Calculation formula:
Backspacing = (Wheel Width ÷ 2) + Offset + Lip Width
Load Rating Selection
- Always exceed requirements: Use next higher load rating for safety margin
- Commercial ratings: Look for "C" or "D" load ratings on 3/4-ton applications
- Verify with manufacturer: Confirm load capacity before purchase
Aesthetic Considerations
- Period-correct builds: Rally wheels, slots, or steel wheels with trim rings
- Modern performance: Consider brake upgrade clearance for future modifications
- Finish durability: Powder coating superior to paint for longevity
Common Mistakes to Avoid
- Ignoring load ratings: Especially critical on work trucks
- Excessive offset changes: Can cause premature component wear
- Cheap lug nuts: Invest in quality hardware for safety
- Skipping hub-centric rings: Causes vibration and uneven wear
- Improper torque: Both under and over-torquing cause problems
Professional vs. DIY Installation
DIY Appropriate:
- Direct bolt-on replacements with similar specifications
- Basic wheel swaps with proper tools and knowledge
Professional Recommended:
- Significant offset changes requiring suspension adjustment
- Custom applications requiring modification
- When wheel alignment is needed post-installation
Troubleshooting
Vibration Issues
Symptoms: Steering wheel shake, seat vibration at highway speeds
Diagnosis Steps:
1. Re-torque lug nuts to specification
2. Check for bent wheels or damaged tires
3. Verify hub-centric rings are properly installed
4. Balance wheels and tires
5. Check for worn suspension components
Solutions:
- Install proper hub-centric rings
- Professional wheel balancing
- Replace damaged components
Clearance Problems
Symptoms: Rubbing noises, tire wear patterns, steering interference
Diagnosis:
1. Check clearance at full steering lock
2. Inspect at maximum suspension compression
3. Look for contact marks on components
4. Measure actual vs. specified dimensions
Solutions:
- Different offset wheels
- Suspension modifications
- Fender modifications (last resort)
Load-Related Failures
Symptoms: Cracked wheels, premature bearing wear, handling issues
Immediate Actions:
1. Stop driving immediately if cracks are visible
2. Inspect all wheels for damage
3. Check load capacity specifications
Prevention:
- Verify load ratings before purchase
- Regular inspection of heavily loaded vehicles
- Upgrade to commercial-grade wheels if needed
Speedometer Inaccuracy
Calculation:
Actual Speed = Indicated Speed × (New Diameter ÷ Original Diameter)
Correction Options:
- Speedometer gear change (mechanical speedometers)
- Electronic correction module (later models)
- Tire size adjustment to maintain original diameter
Handling Changes
Symptoms: Different steering feel, increased effort, unusual tire wear
Evaluation:
- Professional alignment check
- Suspension component inspection
- Tire pressure verification
Corrections:
- Wheel alignment adjustment
- Return to closer-to-stock specifications
- Suspension modifications to compensate
This article represents established practices for Square Body truck wheel modifications. Always consult vehicle-specific service manuals and qualified professionals for critical safety systems.