ElpiAutoTire & wheel · calculator
Tire Size Calculator
Type a tire size and see what the code actually means — overall diameter, section width, sidewall height, rolling circumference and revs per mile — drawn to scale, with every alternative size that fits your wheel inside the 3% rule.
Tire size
Accepts 225/45R17, 35X12.50R17, 31x10.5-15.
Drawn to scale
inches225/45R17 in real numbers
Every dimension the size code hides, spelled out in inches.
| Overall diameter | 24.97″ |
|---|---|
| Section width | 8.86″ |
| Sidewall height | 3.99″ |
| Circumference | 78.45″ |
| Revolutions per mile | 807.6 |
| Revolutions per km | 501.8 |
| Approved rim width estimate | 6–8.5″ |
Overall diameter is the controlling number — clearance, gearing, speedometer accuracy and odometer reading all follow from it. Revolutions per mile/km is what your ABS, speedometer and odometer actually count.
Alternative tire sizes for a 17″ wheel
Sizes within ±3% of your 24.97″ overall diameter — a straight swap for gearing and the speedometer. Grouped by wheel size so you can also see what fits if you change wheels.
14″ wheels
15″ wheels
16″ wheels
17″ (your wheel)
18″ wheels
19″ wheels
20″ wheels
21″ wheels
22″ wheels
Reference
How to read a tire size
A metric size like 225/45R17 packs three numbers and a letter. Only the wheel diameter is already in inches; the other two need converting before they mean anything on the car. Flotation sizes like 35X12.50R17 skip the maths — they state height, width and rim directly, all in inches.
225 — section width
Tread width in millimetres, measured sidewall to sidewall. Divide by 25.4 for inches: 225 mm ≈ 8.86 in.
45 — aspect ratio
Sidewall height as a percentage of the width: 45% of 225 mm = 101.3 mm ≈ 3.99 in. Lower number = shorter sidewall = firmer ride, sharper steering, more pothole risk.
R17 — construction & rim
R is radial construction (almost everything on the road). 17 is the wheel diameter in inches the tire is built to mount on.
The math
Where the numbers come from
Two sidewalls stack on top of the rim, so overall diameter is:
diameter(in) = rim + 2 × ( widthmm × aspect / 100 ) / 25.4For 225/45R17: 17 + 2 × (225 × 45 / 100) / 25.4 = 24.97 in.
revs / mile = 63360 / circumference(in)
A larger diameter covers more ground per revolution, so it makes fewer revolutions per mile — which is why a bigger tire makes the speedometer read low. To see that effect for a specific swap, use thetire size comparison.
Common questions
What do the numbers on a tire mean?
Take 225/45R17. 225 is the tread (section) width in millimetres. 45 is the aspect ratio — sidewall height as a percentage of that width, so 45% of 225 mm ≈ 101 mm (about 3.99 in). R is radial construction. 17 is the wheel diameter in inches. Overall tire diameter works out to about 24.97 in. A flotation size like 35X12.50R17 states height, width and rim directly, all in inches.
How do I convert a tire size to inches?
Overall diameter in inches = wheel diameter + 2 × (section width mm × aspect ratio ÷ 100) ÷ 25.4. For 225/45R17 that is 17 + 2 × (225 × 45 ÷ 100) ÷ 25.4 ≈ 24.97 in. Section width in inches is just the millimetre figure ÷ 25.4. Switch this calculator to inch mode and every dimension is shown converted.
What other tire sizes fit my wheel?
Any size whose overall diameter is within about 3% of the original keeps the speedometer, ABS and traction control close to their factory calibration. The calculator lists those alternatives for your wheel, split into a recommended band (within 1.5%) and a possible band (1.5–3%, check clearance).
What is "plus sizing"?
Going up a wheel inch while dropping the aspect ratio to keep overall diameter almost the same, for example 225/45R17 to 245/40R18. You gain a larger wheel and a shorter, stiffer sidewall without upsetting gearing or the speedometer. A "plus zero" keeps the same wheel and only changes width.
How many inches bigger of a tire can I fit?
As a rule of thumb, staying within ±3% of the original overall diameter keeps the dash honest — roughly ±0.75 in on a 25-inch tire, or a bit under an inch of total diameter. Going further usually needs a speedometer recalibration and a clearance check at full lock and full suspension compression.
Metric vs flotation — which is "bigger"?
They are just two notations. 285/70R17 and 33X11.50R17 are within a few tenths of an inch of each other. Off-road sizes are usually quoted in flotation (inches); road-car sizes in metric. Enter either one and the calculator normalises both.
Methodology & sources
Dimensions use the standard geometric tire model: overall diameter from rim plus two sidewalls, circumference as π·diameter, revolutions per mile/km as 63,360/39,370 ÷ circumference (inches). Flotation sizes are read directly as their stated dimensions. Approved rim-width range is estimated from section width (roughly section − 2.8″ to section − 0.35″) and is a guide only.
- Tire and Rim Association / ETRTO size designation standards for the metric size format; see the overview in Wikipedia's Plus sizing article.
- The ±3% overall-diameter figure is a widely repeated tire-industry guideline, not a regulation — see Performance Plus Tire's explanation(and their tighter 1% recommendation for ABS/traction-control accuracy). Alternatives here are split at ±1.5% (recommended) and ±3% (possible).
- Actual moulded tire dimensions vary by brand by roughly 1–3%; confirm load rating, wheel width and clearance on your own vehicle before fitting.
Last reviewed September 2026.