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Guide
Tire Dimensions & Speedometer Tool

Tire Size Comparison Calculator

Compare tire dimensions, overall diameter, sidewall height, and speedometer error percentage between two tire sizes. Keep your speedometer accurate and protect your transmission and stability control systems.

Tire Size Comparison & Speedometer Variance
Tire 1 (Stock / Current)225/45R17
Tire 2 (New / Proposed)245/40R18
SpecificationTire 1 (225/45R17)Tire 2 (245/40R18)Difference
Overall Diameter634.3 mm (24.97")653.2 mm (25.72")+18.9 mm (3.0%)
Section Width225 mm (8.86")245 mm (9.65")+20 mm
Sidewall Height101.3 mm (3.99")98.0 mm (3.86")-3.3 mm
Circumference1993 mm (78.5")2052 mm (80.8")+59 mm
Revolutions per Mile808 revs/mi784 revs/mi-23 revs/mi

Speedometer Calibration Analysis

When your speedometer reads the indicated value, your true vehicle road speed is:

Variance: +2.98%
Indicated30 mphActual30.9 mph
Indicated45 mphActual46.3 mph
Indicated60 mphActual61.8 mph
Indicated70 mphActual72.1 mph
Indicated80 mphActual82.4 mph
Tire diameter difference is 3.0%. This is well within the acceptable industry standard tolerance of ±3%.

How to Decode Metric Tire Sizing

Automotive tire sidewalls use standardized ISO metric sizing. For instance, in the size 245/40R18 97Y:

  • 245 (Section Width): The nominal cross-sectional width of the tire measured in millimeters from sidewall to sidewall when mounted on its designated rim width.
  • 40 (Aspect Ratio): The sidewall height expressed as a percentage of the section width. Here, sidewall height is 40% of 245 mm = 98 mm (3.86 inches).
  • R (Internal Construction): Radial tire internal ply cords running perpendicular to the tread direction.
  • 18 (Rim Diameter): The wheel rim diameter in inches that the tire beads seat onto.
  • 97Y (Service Description): Load index (97 = 1,609 lbs per tire) and speed rating (Y = up to 186 mph / 300 km/h).

The Science of Speedometer Calibration & Rolling Circumference

Modern vehicles do not measure road speed using radar or GPS. Instead, vehicle speed sensors (VSS) count the revolutions of the transmission output shaft or wheel speed tone rings. The engine computer assumes a fixed rolling tire circumference.

When you mount taller tires, every rotation covers greater linear distance along the road surface. As a result, your vehicle travels faster than indicated on the dashboard gauge cluster. Conversely, shorter tires cause the speedometer to read faster than true speed while artificially inflating odometer mileage.

Rules for Safe Plus-Sizing

Plus 1 Upgrade

Moving from 215/55R16 to 225/45R17. The wheel diameter grows 1 inch, while tire profile drops from 55 to 45 series. Total diameter change is under 0.6%, preserving steering response and speedometer precision.

Plus 2 Upgrade

Moving from 215/55R16 to 225/40R18. Wheel diameter grows 2 inches, while profile drops to 40 series. Provides sharper turn-in feel, reduced sidewall flex, and fills the wheel arch with zero speedo calibration errors.

Tire Size Frequently Asked Questions

How do you calculate overall tire diameter from tire size?

The formula is: Overall Diameter (mm) = 2 x (Section Width x Aspect Ratio / 100) + (Rim Diameter in x 25.4). For example, with a 225/45R17 tire: Sidewall = 225 x 0.45 = 101.25 mm. Rim = 17 x 25.4 = 431.8 mm. Total Diameter = (2 x 101.25) + 431.8 = 634.3 mm (24.97 inches).

What is the acceptable percentage difference when changing tire sizes?

Automotive engineers strongly recommend keeping tire diameter difference within ±3% of the factory original equipment (OEM) tire size. Staying within 3% ensures your speedometer, odometer, anti-lock braking system (ABS), and electronic stability control (ESC) operate within intended calibration margins.

Will larger tires make my speedometer read slower or faster?

Installing tires with a larger overall diameter means each tire revolution travels farther along the road. Consequently, your speedometer will read slower than your actual vehicle road speed. If your speedometer indicates 60 mph on 3% larger tires, your car is actually traveling at 61.8 mph.

What does "Plus Sizing" (Plus 1, Plus 2) mean?

Plus sizing is the practice of upgrading to a larger diameter wheel rim while simultaneously reducing tire aspect ratio (lower profile sidewall) to keep overall outer tire diameter nearly identical to factory specs. Plus 1 upgrades 1 inch in wheel diameter (e.g. 17" to 18"), while Plus 2 upgrades 2 inches (e.g. 17" to 19").

Can differing tire diameters damage All-Wheel Drive (AWD) systems?

Yes. In full-time AWD vehicles (such as Subaru Symmetrical AWD, Audi Quattro, or Porsche AWD), all four tires must have virtually identical rolling circumferences (typically within 2/32" of tread depth or <1% rolling variance). Significant diameter differences between front and rear axles will cause center differential overheating and premature failure.