Tire Size Diameter Calculator | Plus-Size Chart, Speedometer Error, Wheel Poke (Flush Fitment) & How to Read the Date Code
Enter your stock tire size and the new tire size (e.g. 205/55R16 → 225/45R17) to calculate overall diameter, sidewall height, circumference and revolutions per km, along with the diameter difference (mm and %) and the change in ride height. It also shows the actual speed when the speedometer reads 40, 60 and 100 km/h, and whether it falls within the Japanese vehicle inspection (shaken) speedometer test (actual speed 30.9–42.5 km/h for vehicles made from 2007). Get a table of plus-size and minus-size options within ±3% of the stock diameter, grouped by rim diameter; a wheel fitment calculator that uses rim width (J) and inset to show how far the outer and inner edges of the wheel move and whether it pokes past the fender; and a reverse calculation of the inset that keeps the same position. It also decodes sidewall markings (load index, speed rating, XL) and reads the manufacturing week/year and age from the four digits in the oval.
Compare the stock size with the new size
Enter the marking exactly as written on the sidewall or the label inside the door, such as “205/55R16 91V” (the load index and speed rating are optional).
Diameter difference
+2.4mm
+0.38% (631.9 → 634.3mm)
Ride height change
+1.2mm
The body rises or drops by half the diameter difference
Actual speed at indicated 40 km/h
40.2km/h
Within inspection speedometer range (guideline)
Tire width
+20mm
About half of this spreads to each side, outward and inward
| Item | Stock 205/55R16 | New 225/45R17 | Difference |
|---|---|---|---|
| Overall diameter | 631.9 mm | 634.3 mm | +2.4 |
| Sidewall height | 112.8 mm | 101.3 mm | −11.5 |
| Rim diameter | 406.4 mm | 431.8 mm | +25.4 |
| Circumference (distance per revolution) | 1,985 mm | 1,993 mm | +8 |
| Revolutions per km | 503.7 revs | 501.8 revs | −1.9 |
Speedometer error and inspection guideline
The speedometer calculates speed from tire revolutions, so with a larger diameter the actual speed is faster than indicated, and with a smaller one it is slower.
If you don’t know, leave it at 40 (assumes an accurate speedometer). Most cars’ speedometers read slightly high, so entering a value measured with a GPS speedometer app makes it more accurate.
Result with stock tires: within range
| Speedometer reading | Actual speed (stock) | Actual speed (new) |
|---|---|---|
| 40 km/h | 40.0 km/h | 40.2 km/h |
| 60 km/h | 60.0 km/h | 60.2 km/h |
| 80 km/h | 80.0 km/h | 80.3 km/h |
| 100 km/h | 100.0 km/h | 100.4 km/h |
| 120 km/h | 120.0 km/h | 120.5 km/h |
Plus-size and minus-size options
Sizes within ±3% of the stock 205/55R16 diameter (631.9mm), listed by rim diameter from closest. Tap one to enter it as the “New” size.
14-inch(−2 inch)
15-inch(−1 inch)
16-inch(same as stock)
17-inch(+1 inch)
18-inch(+2 inch)
19-inch(+3 inch)
20-inch(+4 inch)
These are calculated options. Check the tire maker’s catalog for availability, whether the size fits your rim width (approved rim width), and whether the load index is at least stock.
Wheel poke (flush fitment) calculator
Calculates how far the outer and inner edges of the wheel move based on rim width (J) and inset. Inset is the same number formerly called “offset” in Japan.
Current rim width
New rim width
Rim outer edge
+14.7mm
moves out
Rim inner edge
+10.7mm
closer to the suspension
Outer edge incl. tire
+24.7mm
Adds half of the +20mm tire width difference
Fender clearance after change
-9.7mm
Pokes out by 9.7mm (calculated)
To keep the outer edge where it is now with 7.5J (without changing the flush position), use an inset of about 63mm. Because the rim is wider, the inner edge moves +25.4mm.
For passenger cars in Japan, the tire alone (excluding wheels and nuts) may protrude less than 10 mm (June 2017 safety standard revision; within 30° forward and 50° rearward of directly above the axle). Wheels, spacers and nuts must not protrude.
Read the sidewall markings (load index, speed rating, manufacturing week)
Enter the text on the sidewall and the four digits in the oval (manufacturing week/year) to see what they mean.
- • Section width 225mm, aspect ratio 45% (sidewall height 101.3mm), rim diameter 18 in
- • R: Radial construction.
- • Load index 95: each tire can support up to 690kg
- • Speed rating W: up to 270 km/h (at the specified pressure and load)
- • XL (Extra Load) / RF (Reinforced): A tire with higher load capacity at higher pressure. It needs higher pressure than a standard tire to achieve the same load index.
Made in week 25 of 2023 (week of 6/19)
About 3.3 years since manufacture
Less than 5 years since manufacture. Check tread depth (wear indicators) and pressure as you use it.The first two digits are the week and the last two are the last two digits of the year. A three-digit code means it was made before 2000.
Load index chart (60–120)
Speed rating chart
ABOUT
About Tire Size Diameter Calculator | Plus-Size Chart, Speedometer Error, Wheel Poke (Flush Fitment) & How to Read the Date Code
Tire Size Diameter Calculator works out a tire’s overall diameter, circumference and ride-height change from a marking such as “205/55R16 91V” found on the sidewall or the label inside the door. Diameter is “rim diameter (in) × 25.4 + section width × aspect ratio × 2”, so 205/55R16 gives 406.4 + 112.75 × 2 = 631.9 mm. Comparing the stock size with a new size shows the diameter difference (mm and %), how much the ride height rises or drops, and the actual speed when the speedometer reads 40 or 100 km/h, along with a guideline for whether it stays within the Japanese vehicle inspection (shaken) speedometer test (for vehicles made from January 2007, actual speed 30.9–42.5 km/h at an indicated 40 km/h). From a table of plus-size and minus-size options within ±3% of the stock diameter, grouped by rim diameter, one tap fills in the new size. For wheels, it uses rim width (J), inset and spacer thickness to show how many mm the outer and inner edges of the rim move and whether the wheel pokes past the fender (flush fitment calculation), and back-calculates the inset that keeps the current position. It also explains the load index (LI), speed rating and XL, and reads the manufacturing week/year and age from the four digits in the oval.
How to use Tire Size Diameter Calculator | Plus-Size Chart, Speedometer Error, Wheel Poke (Flush Fitment) & How to Read the Date Code
- 1Enter your stock (current) tire size, or pick an example such as kei car, compact or minivan
- 2Enter the new (candidate) tire size (you can also tap one in the plus-size options table)
- 3Check the diameter difference, ride-height change, the actual speed at an indicated 40 km/h, and whether it is within the inspection guideline
- 4If you know your stock speedometer error measured with GPS or similar, enter it to make the actual-speed table more accurate
- 5If you are changing wheels, enter the current and new rim width (J), inset, spacer and fender clearance to check for poke
- 6Enter the sidewall marking and the four digits in the oval to check the load index, speed rating and manufacturing week
- 7Use “Copy comparison” to keep a note, or “Copy URL with these settings” to send it to a shop or a friend
Great for
- Find tire sizes with a similar diameter when plus-sizing from 16 to 17 or 18 inches
- Find usable sizes when going one inch smaller (minus sizing) for winter tires
- See how far the speedometer reading drifts from the actual speed after changing size
- Check, as a guideline, whether a larger diameter still passes the inspection speedometer test
- Calculate whether new wheels with a given rim width and inset will poke past the fender (or sit flush)
- Find out how far your current wheels will stick out with a spacer of a given thickness
- Look up the manufacturing year of used or hand-me-down tires from the four-digit code
- Check what “91V” or “95W XL” on a tire means (load index and speed rating)
Good to know
- Diameter is a nominal value calculated from the tire marking. Even for the same size it can vary by a few mm depending on maker, model, pressure and wear.
- A diameter difference within ±1% is practically the same, and choosing within ±3% is the common rule of thumb. Even within 3%, check on the actual car or at a shop that the tire does not hit the fender or suspension.
- The speedometer calculation assumes the speedometer derives speed from tire revolutions. Speedometers often read slightly high even with stock tires, so if you know it, enter “actual speed at an indicated 40 km/h on stock tires”.
- The inspection speedometer standard used here is the one for vehicles made on or after January 1, 2007: “at an indicated 40 km/h, actual speed is 30.9–42.5 km/h (10(V1−6)/11 ≦ V2 ≦ 100/94×V1)”. Vehicles made earlier have a different standard. The real pass/fail is decided at inspection, so the result here is only a guideline.
- Plus-size options are calculated and not limited to sizes actually on sale. Check the tire maker’s catalog for availability, whether the size fits your rim width (approved rim width), and whether the load index is at least stock.
- Lowering the aspect ratio when plus-sizing may require XL (extra load) tires and higher pressure to achieve the same load index. Set pressure according to the tire maker’s inflation table.
- Wheel poke is compared using the distance from the hub mounting face to the rim edge (half the rim width ± inset). Any change in tire width is added assuming half spreads to each side. Actual bulge varies with rim width and tire model.
- For passenger cars, the June 2017 revision of Japan’s safety standards allows the tire (excluding wheels, nuts and spacers) to protrude less than 10 mm (within 30° forward and 50° rearward of directly above the axle).
- As a guideline recommended by the Japan Automobile Tyre Manufacturers Association (JATMA), tires in use for 5 years or more should be inspected every year, and tires 10 years past manufacture should be replaced even if tread remains.
- Your input is only saved in this browser and is never sent to a server.
Questions this tool answers
Use Tire Size Diameter Calculator | Plus-Size Chart, Speedometer Error, Wheel Poke (Flush Fitment) & How to Read the Date Code for searches like these:
- Calculate tire diameter from a tire size
- Find plus-size tire sizes based on diameter
- See speedometer error after changing tire size
- Check whether a different diameter still passes inspection
- Calculate wheel poke from inset and rim width
- Find the spacer or inset needed for a flush fitment
- Look up a tire’s manufacturing year from the four digits
- Understand load index and speed rating
FAQ
- How do I calculate tire diameter?
- Diameter = rim diameter (in) × 25.4 + section width × aspect ratio ÷ 100 × 2. For 205/55R16, the rim diameter is 16 × 25.4 = 406.4 mm and the sidewall height is 205 × 0.55 = 112.75 mm, so 406.4 + 112.75 × 2 = 631.9 mm. Circumference is diameter × π, about 1,985 mm.
- How much does the diameter change when plus-sizing from 205/55R16 to 225/45R17?
- The diameter of 225/45R17 is 431.8 + 101.25 × 2 = 634.3 mm, which is 2.4 mm (about 0.38%) larger than 205/55R16 (631.9 mm). Ride height rises by about 1.2 mm, and when the speedometer reads 40 km/h the actual speed is about 40.2 km/h, a difference small enough to call practically the same.
- Will a different tire diameter make my car fail inspection?
- There is no fixed limit on diameter itself, but the car must pass the speedometer test. For vehicles made from January 2007, it passes if the actual speed is 30.9–42.5 km/h when the speedometer reads 40 km/h. On a car with an accurate speedometer, a diameter about 6% larger exceeds 42.5 km/h, so keeping the difference within ±3% is the safe choice. Also check that nothing pokes past the fender and that the load index is at least stock.
- Are inset and offset different things?
- They are the same. It is the distance (mm) from the center of the wheel’s width to the hub mounting face; in Japan it used to be called “offset” and is now generally called “inset”. The smaller the number, the further out the wheel sits. If you widen the rim by 1J (25.4 mm) and don’t want the outer edge to move, increase the inset by about 12.7 mm.
- How far does a wheel stick out with a spacer?
- It is the same as reducing the inset by the spacer thickness, so both the outer and inner edges of the wheel move outward by that many mm. For example, a 7J inset 48 wheel with a 10 mm spacer sits in the same position as a 7J inset 38.
- Where can I find a tire’s manufacturing date?
- There is a four-digit number in an oval on the sidewall: the first two digits are the week and the last two are the last two digits of the year. For example, “2523” means week 25 of 2023 (the week of June 19). A three-digit code means it was made before 2000.
- What do “91V” and “95W XL” mean?
- The number is the load index (LI), the weight one tire can support (91 = 615 kg, 95 = 690 kg). The letter is the speed rating, the maximum speed for that tire (H = 210 km/h, V = 240 km/h, W = 270 km/h). XL is the extra load standard, a tire with higher load capacity at higher pressure.
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