A tire size like 245/45R18 has three numbers. Most people know the first is width and the last is wheel diameter. The middle number — 45 — is the aspect ratio. It is the single number that most determines whether your tire rides comfortably or handles precisely. It is also the number that causes speedometer errors when changed incorrectly.
What the aspect ratio number means
The aspect ratio is the sidewall height expressed as a percentage of the tire width. In 245/45R18: the width is 245mm; the sidewall height is 45 percent of 245mm, which is approximately 110mm.
A higher aspect ratio (60, 65, 70) means a taller sidewall. Taller sidewalls absorb road impacts better — more rubber between the wheel and the road means a cushioned ride. Standard family cars, trucks, and SUVs use 55 to 70 series tires.
A lower aspect ratio (35, 40, 45) means a shorter sidewall. Low-profile tires are stiffer — they transmit road feel directly to the driver. Sports cars and performance vehicles use 35 to 45 series tires for better handling response at the cost of ride comfort.
Why changing aspect ratio affects your speedometer
Your speedometer reads speed based on how many times the wheels rotate per mile — which depends on the tire's rolling circumference. That circumference depends on the overall tire diameter.
If you change to a larger aspect ratio (taller sidewall) without changing wheel diameter, the overall tire diameter increases. The speedometer under-reads — the car is going faster than it shows.
If you change to a smaller aspect ratio (shorter sidewall), the diameter decreases. The speedometer over-reads.
The correct approach when changing wheel size: use an online tire size calculator to find an aspect ratio that maintains the same or nearly the same overall diameter as the factory spec. Keeping the overall diameter within 3 percent of factory is the general guideline.
Frequently asked
Does a lower aspect ratio mean better performance?
In dry handling terms, yes — lower aspect ratio tires have less sidewall flex, which means more precise steering response. But they also ride rougher and are more susceptible to damage from potholes. Performance comes with a comfort trade-off.
Can I use a different aspect ratio than what the car came with?
Yes, but you need to compensate by changing width or wheel diameter to keep the same overall diameter. Use a tire size calculator. Going one ratio up (e.g., 45 to 50) while staying on the same wheel usually increases diameter slightly — acceptable if within 3 percent.
Keep reading
Last updated 2026-06-27. General guidance only — confirm specifics with a local shop for your exact vehicle.