ElpiAutoTire & wheel · comparison
Tire Size Comparison
Put two tire sizes side by side and see everything that changes — overall diameter, width, sidewall, rolling circumference, revs per mile, speedometer error and how the swap behaves on the road — with a to-scale drawing. Metric or inch.
Two sizes
Accepts 225/45R17, 35X12.50R17, 31x10.5-15. Example sizes — type your own.
Both sizes, drawn to scale
1 grid = 1 inNote: These sit on different wheel sizes — you'd be changing wheels too, not just tires.
225/45R17 vs 245/40R18, measurement by measurement
Every dimension in inches, with the difference and which size is bigger.
| Measurement | 225/45R17 | 245/40R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 24.97″ | 25.72″ | +0.74″ | +3.0% |
| Section width | 8.86″ | 9.65″ | +0.79″ | +8.9% |
| Sidewall height | 3.99″ | 3.86″ | −0.13″ | −3.2% |
| Circumference | 78.45″ | 80.79″ | +2.34″ | +3.0% |
| Revs per mile | 807.6 | 784.2 | −23.4 | −2.9% |
| Revs per km | 501.8 | 487.3 | −14.5 | −2.9% |
| Approved rim width estimate | 6–8.5″ | 7–9.5″ | — | — |
Overall diameter is the controlling number — clearance, gearing, speedometer accuracy and odometer reading all follow from it. Revs per mile/km is what your ABS, speedometer and odometer actually count.
What changes on the road
Direction and rough size of each effect going from 225/45R17 to 245/40R18. Geometry only — real tires vary by brand and compound.
Speedometer & odometer error
What changes on the dash when the overall diameter changes.
A speedometer doesn't measure distance. It counts how fast the wheels turn and multiplies by a fixed factory number for tire circumference. Change the tire and that factory number is now wrong.
Going to a bigger tire, each turn covers more ground, so at a true 61.8 mph the wheels spin at the rate the factory calls 60. The dial reads low — you are speeding without knowing it. A smaller tire does the opposite, and the odometer also over-counts miles you didn't drive.
Over 10,000 indicated miles, this change means the odometer under-counts by roughly 289 miles.
Common tire size comparisons
Popular size changes and how far each one moves the overall diameter. Anything inside ±3% is usually a clean swap.
| Comparison | Diameter A | Diameter B | Difference | Change |
|---|---|---|---|---|
| 195/65R15 vs 205/55R16 | 24.98″ | 24.88″ | −0.4% | OE upsize, 15" to 16" |
| 205/55R16 vs 225/45R17 | 24.88″ | 24.97″ | +0.4% | OE upsize, 16" to 17" |
| 225/45R17 vs 245/40R18 | 24.97″ | 25.72″ | +3.0% | Plus one, 17" to 18" |
| 215/60R16 vs 225/55R17 | 26.16″ | 26.74″ | +2.2% | Plus one, taller sidewall kept |
| 235/45R18 vs 255/35R19 | 26.33″ | 26.03″ | −1.1% | Plus one, sport |
| 265/70R17 vs 285/70R17 | 31.61″ | 32.71″ | +3.5% | Truck / SUV, wider only |
| 245/75R16 vs 265/75R16 | 30.47″ | 31.65″ | +3.9% | Off-road, one size up |
| 275/55R20 vs 285/50R20 | 31.91″ | 31.22″ | −2.2% | Full-size truck |
Diameters are calculated from the size code with the standard geometric model; real moulded tires vary by brand by 1–3%.
Tire size chart by wheel diameter
Overall diameter, in inches, for common sizes on each wheel. Use it to spot sizes with a matching height before you compare them in detail.
15″ wheels
16″ wheels
17″ wheels
18″ wheels
19″ wheels
20″ wheels
How to read it
Which measurement matters when comparing tire sizes
A tire size like 225/45R17 is not a measurement you can compare directly. The first number is width in millimetres, the second is a percentage, and only the last is already in inches. Two sizes can look completely different and roll within a millimetre of each other.
Overall diameter is the one figure that decides whether a swap works. It sets ride height, gearing, and how far the tire travels per revolution — which is what the speedometer and odometer count. Keep the two sizes within about 3% of each other and the dash stays honest; past that, the speedometer drifts and you should check clearance at full lock and full suspension compression.
Section width and sidewall height matter for fit and feel — a wider tire needs a wider wheel and more arch clearance, a shorter sidewall sharpens steering and stiffens the ride — but they do not change how the car is geared. Compare diameter first, then width, then sidewall.
Common questions
How do I compare two tire sizes?
Enter both sizes above. The tool converts each code into real dimensions — overall diameter, section width, sidewall height, circumference and revolutions per mile — and shows the difference line by line, a to-scale drawing, the speedometer error and how the change behaves on the road. Overall diameter is the figure to watch: keep the two within about 3% for a clean swap.
Which tire is bigger, 245 or 255?
Not necessarily the one with the higher first number. That number is only the width in millimetres. A 245/45R18 and a 255/40R18 are within a tenth of an inch in overall diameter — the 255 is wider but has a shorter sidewall. Always compare overall diameter, not the width number alone.
What is an acceptable difference between two tire sizes?
Most fitment guides treat anything within ±3% of the original overall diameter as an acceptable substitute — the speedometer, ABS and traction control stay close to factory calibration. Some retailers tighten that to ±1%. Beyond ±3%, plan on a speedometer recalibration and check clearance at full lock and full suspension compression.
Will a bigger tire mess up my speedometer?
Yes, in proportion to the diameter change. A tire 3% larger makes the speedometer read about 3% low — an indicated 60 mph is really about 62. A smaller tire does the opposite and also makes the odometer over-count miles. The comparison shows the exact figure and a dial.
Can I compare a metric size with a flotation size?
Yes. Enter a metric size (225/45R17) in one field and a flotation size (33X12.50R17) in the other — the tool normalises both to inches and compares them directly.
Does a wider tire change the rolling circumference?
Only through its effect on overall diameter. Circumference is π × diameter, so two sizes with the same overall diameter roll the same distance per turn even if one is much wider. The revs-per-mile row makes this easy to check.
Methodology
Each size code is converted with the standard geometric model: overall diameter = wheel diameter + two sidewalls, where sidewall = section width (mm) × aspect ratio ÷ 100. Circumference is π × diameter; revolutions per mile and km are 63,360 and 39,370 divided by circumference in inches. Speedometer error is the ratio of the two diameters applied to an indicated speed. Flotation sizes are read as their stated dimensions. To decode a single size and see the alternatives that fit its wheel, use the tire size calculator.
The ±3% overall-diameter guideline is a widely repeated tire-industry rule of thumb, not a regulation. Actual moulded tire dimensions vary by brand by roughly 1–3%; always confirm load rating, wheel width range and physical clearance on your own vehicle before fitting.