Can I replace my 18 wheels with 20?
In short, you typically cannot simply swap 18 wheels for 20 on a standard tractor-trailer. Such a change requires substantial engineering, regulatory approvals, and cost. This article explains why and what you should consider before pursuing it.
Understanding whether a 20-wheel configuration is feasible hinges on how your current rig is built, where you operate, and what you’re hoping to achieve (payload, stability, or regulatory compliance). Below is a guide to the regulatory, mechanical, and practical factors involved in moving from an 18-wheel setup toward a 20-wheel configuration.
Regulatory and safety considerations
The following factors dictate whether a 20-wheel arrangement could ever be allowed, and under what conditions. A careful review with authorities and engineers is essential before attempting any changes.
- Regulatory approval and permits: Adding or reconfiguring axles typically requires state or national permits and may trigger registration changes for the vehicle.
- Federal and state weight rules: Weight limits depend on axle count and configuration (the Federal Bridge Formula in the U.S. governs how weight can be distributed across axles). A new axle arrangement often affects permissible gross vehicle weight and may necessitate a different permit class.
- Braking and safety systems: Additional axles require proportionate braking capacity, air lines, controllers, and ABS/ESC compatibility. Braking efficiency and synchronization must be verified.
- Steering and turning geometry: Extra axles change steering dynamics, turning radius, and potential tire scrubbing. This can affect maneuverability and road safety.
- tires, rims, and hubs: The added wheels demand compatible tires, wheels, hubs, and wheel-end components rated for the new load and speeds.
- Inspections and compliance: Any modification will likely trigger inspections by DOT or equivalent authorities to ensure roadworthiness and safety compliance.
- Insurance and liability: Modifications can affect insurance coverage and premiums; carriers often require updated risk assessments.
- Overwide/overlength considerations: In many jurisdictions, increased width or length comes with additional routing restrictions, signage, and limitations on road use.
Even when a jurisdiction permits heavier or more wheels for specialized freight, the change is not routine. Most operators pursue such configurations only for very specific heavy-haul applications, with approved equipment, routes, and schedules.
Mechanical and engineering hurdles
Beyond legality, the mechanical feasibility of moving from 18 to 20 wheels involves several substantial engineering challenges. Here are the main hurdles to evaluate with a qualified shop or OEM partner.
- Chassis and frame compatibility: The frame may need reinforcement or a different design to carry the altered load distribution and increased axle count.
- Axle and suspension changes: Adding an axle requires compatible front/rear suspension, possibly new leaf springs or air-ride components, and matching load ratings.
- Drive system adjustments: If the extra axle is on the tractor or trailer, drive shafts, transfer components, and articulation points may need redesign or redesign of the drivetrain.
- Tire and wheel considerations: Matching tire sizes, load ratings, speed ratings, and wheel hubs is essential to avoid imbalances or failures.
- Braking system integration: An additional axle demands more brake capacity, control modules, and proper brake balance across all axles.
- Aerodynamics and clearance: Longer or wider configurations affect aerodynamics, ground clearance, and curb/turn clearance, potentially requiring deratings or routing changes.
- Electrical and signaling: Extra lights, sensors, and air/brake lines must be integrated into the vehicle’s electrical system with proper fault tolerance.
- Maintenance and lifecycle costs: More components mean higher maintenance, inspection frequency, and potential downtime.
In practice, these mechanical hurdles mean a retrofit is rarely cost-effective for standard fleet operations. Any project to add wheels should be treated as a specialized, engineering-driven upgrade rather than a simple accessory change.
Alternatives to consider
If payload or stability is the goal, there are safer, more practical paths than retrofitting to 20 wheels. Consider these options before pursuing a major axle modification.
- Maximize payload within existing limits: Upgrade tires, wheels, and suspension components to support higher load ratings while staying within current axle counts.
- Adopt a compliant heavy-haul configuration: For specialized loads, use purpose-built heavy-haul trailers with approved axle lines and permits designed for the job.
- Optimize routing and scheduling: Improve load planning, freight consolidation, and route choices to maximize efficiency without modifying vehicle architecture.
- Consult OEM and engineering partners: If a 20-wheel configuration is truly necessary, work with the chassis manufacturer and a licensed engineer to explore a purpose-built, certifiable solution rather than a retrofit.
These alternatives emphasize safety, reliability, and regulatory compliance while still achieving higher payload or improved capabilities where necessary.
Practical steps if you’re pursuing a 20-wheel configuration
If you determine that pursuing a 20-wheel setup is still the best path for a legitimate operational need, follow these steps to proceed responsibly. Each step should involve qualified professionals and regulators to ensure compliance and safety.
- Engage a qualified engineer: Obtain a certified Engineering Analysis or approval that demonstrates structural integrity, braking capacity, and safe operation with the new axle configuration.
- Consult the vehicle manufacturer: Work with the OEM or an authorized aftermarket integrator to ensure compatibility and warranty considerations.
- Check with the department of transportation: Obtain the necessary permits and understand inspection, signage, and routing requirements.
- Plan the weight distribution: Model how loads will be distributed across axles, and verify that the gross vehicle weight remains within permitted limits for all routes.
- Test and verify safety systems: Validate braking, steering, lights, and telemetry under the new configuration in controlled conditions before roadway operation.
- Secure updated insurance and compliance: Notify insurers and ensure all regulatory paperwork, inspections, and compliance measures are in place.
Proceeding carefully with these steps helps ensure that any axle change is safe, legal, and economically justifiable rather than a high-risk modification.
Summary
Moving from an 18-wheel to a 20-wheel configuration is not a straightforward upgrade. It involves substantial engineering changes, regulatory approvals, and cost, and in many cases is only feasible for highly specialized heavy-haul operations with clear permits and purpose-built equipment. For most fleets, maximizing payload and efficiency is better achieved through staying within the existing configuration, pursuing compliant heavy-haul solutions, or working with OEMs to design a purpose-built setup rather than retrofitting a standard 18-wheeler.
Is there a big difference between 18 and 20-inch wheels?
Smaller diameters, thicker tires, and more sidewall sizes are used in 18-inch wheels. The 20-inch wheels are quite larger in diameter, with thinner tires and sidewalls.
Do 20 wheels affect gas mileage?
Yes, 20-inch rims can negatively affect gas mileage primarily due to increased weight and potential changes in aerodynamics. Heavier rims require more power to accelerate and brake, which leads to higher fuel consumption. Additionally, the larger size and different designs of 20-inch wheels can create more wind resistance, further decreasing fuel efficiency.
Key reasons for the impact on gas mileage
- Weight: 20-inch rims and the tires that fit them are often heavier than smaller options. This increased rotating mass requires more energy, and therefore more fuel, to get the vehicle moving and to maintain speed.
- Aerodynamics: Larger wheels, especially off-road or more open-spoke designs, can catch more wind. This increased air resistance, or drag, means the engine has to work harder to overcome it, reducing your gas mileage.
- Rolling Resistance: The larger surface area of wider, 20-inch tires can increase friction with the road. This higher rolling resistance contributes to a lower MPG because more energy is needed to keep the tires moving.
- Potential for taller tires: If the 20-inch rims are paired with tires that are taller than the original equipment, it can also affect gear ratios, which can negatively impact fuel economy in stop-and-go city driving.
Factors that can influence the impact
- Tire and wheel design: The specific design matters. Lighter-weight 20-inch rims with an aerodynamic design will have less of a negative impact than heavy, less aerodynamic ones.
- Tire profile: The height of the tire's sidewall affects the ride and handling. Tires with lower, or "shorter," sidewalls that come with 20-inch rims can provide better performance but may result in a bumpier ride and still lead to a fuel economy decrease.
Can I upgrade my wheel size?
Upsizing typically involves changing both the tire and wheel size and may require recalibrating the speedometer or updating your vehicle's electronic systems. Keep in mind that larger tires can also create additional strain on components like brakes and suspension if they're not designed for the extra weight.
What are the disadvantages of 20-inch wheels?
20-inch wheels have several key disadvantages including increased replacement costs, susceptibility to damage from road imperfections, and possibly worse ride quality because of lower sidewall cushioning. This makes them less perfect for areas with plenty of road problems or hilly terrain.
