Will the new tires actually fit?
Put in the size you have and the size you are thinking about. You get the diameter change, how far off your speedometer will read, and whether it clears the 3% rule.
Tire size calculator
205/55R16
225/45R17
225/45R17 on a vehicle fitted with 205/55R16: This fits
Diameter moves 0.4%, comfortably inside the 3% rule. Your speedometer, ABS, traction control and gearing all stay in the range they were calibrated for. Your speedometer will read low, so you are going faster than it says.
| Measurement | 205/55R16 | 225/45R17 | Change |
|---|---|---|---|
| Overall diameter | 24.88″ | 24.97″ | +0.09″ (+0.4%) |
| Sidewall height | 4.44″ (112.8 mm) | 3.99″ (101.3 mm) | −11.5 mm |
| Tread width | 205 mm | 225 mm | +20 mm |
| Circumference | 78.16″ | 78.45″ | +0.30″ |
| Revs per mile | 811 | 808 | −3 |
| Speedometer error | baseline | +0.4% | reads low |
When your speedometer shows the indicated speed with 225/45R17 fitted, your actual speed is:
25 mph shown
25.1 mph actual
35 mph shown
35.1 mph actual
45 mph shown
45.2 mph actual
55 mph shown
55.2 mph actual
65 mph shown
65.2 mph actual
75 mph shown
75.3 mph actual
The 3% & 5% diameter rules
Tire professionals judge a swap by how much the overall diameter changes. That single percentage drives everything else: speedometer accuracy, clearance, gearing and electronic safety systems.
Green zone
The industry-standard safe range. Diameter stays close enough that your speedometer, odometer, ABS and stability control keep working as calibrated. Most plus-size upgrades live here.
Yellow zone
Workable, but do your homework. Expect measurable speedometer error and check wheel-well clearance at full lock and full compression, plus scrub radius and gearing effects.
Red zone
Avoid. Large diameter changes throw off speedometer and transmission calibration, strain driveline components, and risk rubbing. Recalibration is usually required to drive safely.
How to read a tire size
Find the code on your tire’s sidewall — for example 225/45R17. Each part tells the calculator what it needs.
225/45R17
225
Tread width
The tire’s nominal section width, in millimeters, measured sidewall to sidewall.
45
Aspect ratio
Sidewall height as a percentage of width. Here: 45% of 225 mm ≈ 101 mm of sidewall.
R17
Construction & wheel
R means radial construction; 17 is the wheel diameter in inches the tire mounts on.
Popular size-vs-size comparisons
Pre-computed comparison pages for the swaps drivers check most.
Not sure which size you have?
Browse a library of common tire sizes by wheel diameter — with overall diameter, circumference and revs per mile for every size, plus one-click comparisons.
Open the tire size libraryTire size questions, answered
- How much bigger can my tires be?
- Stay within 3% of your original overall diameter and everything calibrated to the factory size keeps working: speedometer, odometer, ABS, stability control. Between 3% and 5% usually works, but check clearance and accept some speedometer error. Past 5% you are looking at recalibration and a real chance of rubbing.
- Does changing tire size affect the speedometer?
- Yes, and the error tracks the diameter change exactly. A tire 3% larger turns 3% fewer times per mile, so an indicated 60 mph is really about 61.8 mph. Bigger tires make the speedometer read low, smaller tires make it read high.
- How do I read a tire size like 225/45R17?
- 225 is the tread width in millimeters. 45 is the aspect ratio, meaning the sidewall is 45% of that width, so about 101 mm. R is radial construction. 17 is the wheel diameter in inches.
- Can I fit wider tires on the same wheels?
- Usually yes, as long as you stay inside the wheel’s rated width range. The catch is that width and height move together: go wider at the same aspect ratio and the tire also gets taller. Check the diameter change here first, then confirm inner-fender and strut clearance.
How these numbers are calculated
Every figure on this site is worked out from the tire size code itself. No lookup table, no estimation. Sidewall height is tread width × aspect ratio ÷ 100. Overall diameter is the wheel diameter plus two sidewalls. Circumference is π × diameter, and revolutions per mile is 63,360 ÷ circumference. Speedometer error is simply the ratio of the two overall diameters, because a speedometer counts wheel rotations and assumes the factory rolling circumference.
These are nominal dimensions from the size code. Real molded dimensions vary by a few millimeters between manufacturers and models, and a mounted tire’s loaded radius is smaller than its free radius. Treat the results as accurate to within roughly 1% and confirm fitment against your vehicle’s tire and loading information placard.
Sources
- 49 CFR § 571.110 — Tire selection and rims, the federal standard behind the tire and loading information placard on your driver’s door jamb.
- NHTSA TireWise — Tire safety ratings and awareness, for tire maintenance, tread depth and load-rating guidance.
Last updated September 2026.