Car tires come in a wide range of sizes and designs to suit different vehicles and driving needs. One specification that impacts performance is a tire’s cross-sectional thickness, or aspect ratio. Understanding tire sidewall size helps select optimal tires and keep them in top shape.
This article explains how tire thickness works, what the measurements mean, and how to gauge appropriate tire sizes for your car. Tips are also included on inspecting and maintaining healthy tire sidewalls for safety, handling and longevity.
Table of Contents
The Aspect Ratio Explained
The tire’s aspect ratio refers to the height of the sidewall, expressed as a percentage of the tread width. As an example, a 225/50R17 tire has a sidewall height 50% as wide as the tread.
Lower aspect ratios like 35 or 40 have shorter, stiffer sidewalls. Taller ratios around 60 or 70 create a larger tire cushion and flex. Performance tires typically use lower profiles while off-road and winter tires favor taller sidewalls.
The width, aspect ratio and construction impact a tire’s overall diameter. This must match OEM specifications to avoid clearance issues and maintain proper speedometer readings.
Measuring Sidewall Height
Tire sizes are often listed with slash marks like: 215/55R18. The numbers before the slash indicate tread width in millimeters. The aspect ratio follows the slash as a percentage.
So a 215 mm tread with a 55% aspect ratio gives a sidewall height of 215 x 0.55 = 118 mm. Overall diameter depends on rim size but is around 715 mm for this common size.
Measuring the visible sidewall yourself reveals current thickness, which may have increased with wear. Compare to new tire specs when replacing to get the proper aspect ratio.
Why Aspect Ratio Matters
The sidewall height affects tire characteristics:
- Ride Comfort – Taller sidewalls absorb bumps better while shorter ones transfer more road feel.
- Handling – Short flexible sidewalls cause more body roll while stiff, low profiles corner flatter.
- Tread Life – Thinner sidewalls typically flex more, generating heat that wears tread faster.
- Puncture Resistance – More rubber thickness resists sidewall punctures better.
Consider your priorities like comfort, responsiveness and longevity when selecting among aspect ratio options for replacement tires.
Typical Aspect Ratios by Vehicle
Here are some example aspect ratios for various vehicles to demonstrate the large range:
- Family sedans – 55% to 70%
- Performance cars – 35% to 45%
- Pickups and SUVs – 60% to 75%
- Crossovers – 50% to 65%
- Luxury cars – 45% to 55%
- Winter/all-season tires – 60% to 75%
Luxury vehicles favor lower profile for responsiveness while trucks use taller sides for load capacity and compliance.
Identifying When Tires Don’t Match
Installing mismatched tire sizes, including aspect ratios, causes handling issues. Visually inspect around the full tire to identify improper sizing:
- Overall tire diameter difference
- Lower profile appears stretched on rim
- Taller profile seems pinched with excessive rim gap
- Sidewall size seems thicker or thinner
- Speed rating, load index or construction differs
Consult door jamb stickers and owner’s manual for OEM spec when uncertain. Mixing tires throws off gearing, clearance, anti-locks and traction systems.
Potential Risks of Too Low Aspect Ratio
While lower sidewalls improve responsiveness, going excessively low has disadvantages:
- Harsher ride and more road noise
- Greater risk of wheel and tire damage from potholes and curbs
- Reduced load carrying capacity
- Increased chance of sidewall punctures
- Shorter tread life from excessive flex heat
- Poor performance in rain, snow and off-road use
Use the largest aspect ratio within fitment specs to optimize cushioning and wear. Consider increased diameter from plus-sizing tires.
Maintaining Healthy Tire Sidewalls
The condition of tire sidewalls directly impacts safety and drivability. Use these care tips to maximize service life:
- Inspect for cracks, scuffs or bulges – Indicate impending failure needing replacement
- Ensure proper inflation – Prevents excessive flexing heat and damage
- Rotate tires – Evens out wear patterns across tread and sidewalls
- Protect sidewalls – Avoid curbing wheels or abrasive scrubbing when cleaning
- Replace old tires – Rubber stiffens and cracks over time regardless of tread depth
Catching sidewall damage early keeps tires lasting longer while remaining safe.
Understanding tire sidewall heights represented by aspect ratios allows proper spec’ing when replacing tires. Select ratios appropriate for the vehicle type and intended use. Overall diameter is important for clearance and anti-lock systems.
Inspect sidewalls regularly and address any damage promptly. With proactive maintenance focused on your tires’ thickness and condition, they will deliver many miles of reliable performance.