| Spec | Old | New | Diff |
|---|---|---|---|
| Diameter | 621.5 mm | 627 mm | 5.5 mm |
| Revs/km | 512 | 508 | -4 |
This tire size is within the safe range. Your handling will remain stable.
When your speedometer shows 100 km/h, you are actually traveling at 100.88 km/h.
Welcome to the workshop! I’m often asked by drivers making minor modifications or simply replacing their original equipment tires (OEM): “Is moving up a size worth it?” It’s a great question, because while these two sizes might look similar on paper, the change in driving dynamics is significant. We’re comparing two popular 15-inch setups, but we are essentially trading soft comfort for sharp handling. Let’s peel back the rubber and see exactly what happens when you go wider.
Before we can compare, we need a baseline understanding. Think of these numbers like the specifications for a perfectly engineered shoe. Let's break down Tire A (185/65 R15) and Tire B (205/60 R15).
This is the most misunderstood number. It’s not the height in inches, but the sidewall height expressed as a percentage of the width.
Both tires use the same 15-inch radial rim, so you won’t need new wheels for this upgrade.
When you change tire size, you change the overall outer diameter of the tire assembly. Since your car's computer calculates speed based on how many rotations the wheel makes, a size change means your dashboard speedometer may become inaccurate.
When we crunch the numbers, the overall diameter of the 205/60 R15 (Tire B) is approximately 627 mm, while the 185/65 R15 (Tire A) is about 621.5 mm. Tire B is roughly 5.5 mm taller.
This translates to a difference of about 0.88%. What does that mean for your driving?
While this difference is minor and well within most vehicle safety tolerances and legal limits (which often allow up to 3%), it means you are traveling slightly faster than the speedometer indicates. For a Senior Engineer, I can confirm this is a negligible factor for safety, but it’s important context for understanding the full physical changes.
This is where the difference between the two sizes becomes most pronounced. It all comes down to the width and the stiffness of the tire’s structure.
Even though the actual physical sidewall height is similar, the 185/65 R15 feels noticeably softer. The narrower tread width means the internal structure and the sidewall have to absorb road irregularities across a smaller footprint. This results in:
The 205/60 R15 is designed for performance. The 20 mm increase in width significantly increases the tire’s contact patch, which is the amount of rubber touching the road at any given time. More contact equals more grip.
Physics dictates that wider tires require more effort to turn, and that effort comes directly from your fuel tank.
The 205/60 R15 has a larger contact patch, which means it generates greater rolling resistance. Rolling resistance is the energy lost when a tire rolls over a surface. Think of it like pushing a wider object through molasses—it requires more sustained energy.
Switching from Tire A to Tire B will almost certainly result in a small but measurable decrease in fuel economy, likely in the range of 1% to 3%, depending on the vehicle and driving style. If fuel efficiency is your absolute priority, Tire A is the clear winner.
Generally, the 185/65 R15 is a standard OEM size for many compact and subcompact vehicles, making it extremely high volume. The 205/60 R15 is often considered a performance upgrade or a specific size for mid-range sedans. High-volume, standard sizes (like Tire A) are almost always cheaper to manufacture and purchase.
As a tire engineer, I can’t tell you which tire is objectively "better"—only which tire is better for your specific needs. This decision hinges entirely on what you prioritize in your daily commute.
The jump from 185 to 205 might seem small, but it fundamentally shifts the vehicle’s character—from soft cruiser to confident handler. Drive safe!