Tyre Size Converter
Tyre Size Converter
205/55 R16 into section width, sidewall height, overall diameter, rolling circumference and revolutions per kilometre — and then what a size change does to the speedometer, against the one-sided tolerance UNECE R39 actually sets. Imperial and pre-1972 codes handled; load index and speed rating refused.
Tyre sizes
205/55 R16, compared with 225/45 R17, at a rolling factor of 97.1%
Three units in one string
- width
- the first number, in millimetres. Nominal section width of the inflated unloaded tyre, not the tread
- aspect
- the second number, as a percentage OF THE WIDTH. The same 55 is a different sidewall on a 205 and a 225
- rim
- the last number, in inches, always — even on a fully metric code
- k
- the rolling-circumference factor. About 0.971 for a loaded passenger tyre; 1.000 if you want the geometric figure that most online tables print
- over-read
- the margin the manufacturer built into the speedometer. UNECE R39 permits up to 10% + 4 km/h high and forbids reading low, so the margin exists — and a bigger tyre spends it
Worked example
205/55 R16, compared with 225/45 R17, at a rolling factor of 97.1%
Sidewall = 205 × 55 ÷ 100 = 112.75 mm. The 55 is a percentage of the 205, which is the part people miss
Rim = 16 × 25.4 = 406.4 mm — inches, on a metric code
Overall diameter = 406.4 + 2 × 112.75 = 631.9 mm, the headline
Geometric circumference = π × 631.9 = 1,985.2 mm, giving 503.7 rev/km. At 97.1% the rolling circumference is 1,927.6 mm and 518.8 rev/km
225/45 R17 is 17 × 25.4 + 2 × 101.25 = 634.3 mm — only 2.4 mm bigger, or 0.380%. A textbook plus-one
So at an indicated 100 km/h the true speed becomes 100.38 km/h with no factory over-read — the needle now reads low by 0.38 km/h, which is the one thing UNECE R39 does not allow. Put your car's real over-read in and the picture changes
Reading each code, and which unit each part is in
| Code | Example | Width | Aspect ratio | Rim | Overall diameter |
|---|---|---|---|---|---|
| Metric, aspect stated | 205/55 R16 | 205 mm | 55% of 205 = 112.75 mm | 16 in = 406.4 mm | 406.4 + 2 × 112.75 = 631.9 mm |
| Metric, aspect omitted | 165 R13 | 165 mm | assumed 82% = 135.3 mm | 13 in = 330.2 mm | 600.8 mm |
| Imperial light truck | 31×10.50 R15 | 10.50 in = 266.7 mm | worked backwards: (31 − 15) ÷ 2 ÷ 10.50 = 76.2% | 15 in = 381 mm | stated: 31 in = 787.4 mm |
| Numeric, before 1964 | 6.50-15 | 6.50 in = 165.1 mm | 90% by convention = 148.6 mm | 15 in = 381 mm | 678.2 mm |
| Numeric, 1965 to early 1970s | 6.50-15 | 6.50 in = 165.1 mm | 80% by convention = 132.1 mm | 15 in = 381 mm | 645.2 mm |
| Alphanumeric | C78-15 | refused — the letter is a load and size class, not a width | 78% stated | 15 in | not computable without a letter-to-width table |
Rolling circumference is not π × D, and “published rev/km” is two different numbers
| Size | Overall diameter (mm) | Geometric circumference (mm) | Published Abrollumfang (mm) | Ratio (%) | rev/km from geometry | rev/km from the published figure |
|---|---|---|---|---|---|---|
| 195/65 R15 | 634.5 | 1,993.3 | 1,937 | 97.17 | 501.7 | 516.3 |
| 205/55 R16 | 631.9 | 1,985.2 | 1,928 | 97.12 | 503.7 | 518.7 |
The rule that actually binds, and why it is one-sided
| Where | Requirement | Source |
|---|---|---|
| UNECE Regulation No 39, paragraph 5.4 | 0 ≤ (V1 − V2) ≤ 0.1 V2 + 4 km/h, and “The speed indicated shall not be less than the true speed of the vehicle.” | UN R39 as published in the Official Journal of the EU |
| European Union | UN R39 applies through EU type approval | UN R39 |
| United Kingdom | Speedometers meeting Directive 75/443 (as amended by 97/39) or UN R39 are permitted; under the Motor Vehicles (Approval) Regulations 2001, for 25-70 mph the indicated speed may not exceed 110% of actual plus 6.25 mph and “the speedometer must never show an indicated speed less than the actual speed” | as summarised by Wikipedia, Speedometer |
| Australia | Vehicles built from 2007 “must conform to UNECE Regulation 39” | as summarised by Wikipedia, Speedometer |
| India | Not asserted here. The Central Motor Vehicles Rules 1989 carry a Rule 117 headed “Speedometer”, but its text could not be fetched, so no Indian tolerance is stated on this page | CMVR 1989 table of contents only |
The 3% rule, and what it is worth
| Claim | Status |
|---|---|
| New tyres should be within 3% of the factory overall diameter | A trade rule of thumb. Jalopnik states it as “the 3% rule states that new tires should never be 3% larger or smaller in diameter than the factory-spec rubber” and calls it “An industry standard for tire replacements”, but names no standards body — and none could be found. It is printed here as what it is. |
| 3% keeps the speedometer legal | Not necessarily. Whether the indicated speed stays at or above true speed depends entirely on how much over-read the manufacturer built in. With a 5% factory over-read you can add 5% of diameter before the indication reaches true speed; with none, you cannot add any. The calculator works this out from your own figure. |
| 3% keeps ABS and stability control happy | Plausible and unverified. These systems compare wheel speeds with each other and with modelled vehicle speed, and a uniform change on all four corners is the mildest case. A mismatch between axles is a different and worse problem, and nothing on this page addresses it. |
| Load index and speed rating can be treated as preferences | No. See the refusals below. |
What this page refuses, and why
| Asked for | Why not |
|---|---|
| Load index → kilograms | The full index table (from 60-something up past 120) is a published table in the ETRTO Standards Manual and the equivalent TRA and JATMA documents, all copyrighted, and none was obtainable. A partial or remembered table is worse than none here, because fitting a tyre below the load index on your vehicle’s own placard is a safety matter and, in most jurisdictions, an insurance and roadworthiness one. Read the placard in the door shut, and match or exceed it. |
| Speed rating letter → km/h | Same reason. The letter series is a published table and it is not linear, and a tyre below the vehicle’s specified rating is again a safety and insurance question rather than a preference. Match or exceed the placard. |
| Alphanumeric sizes such as C78-15 | The letter is a load and size class, not a width, and no letter-to-width table could be sourced. Without it the section width is unknown and nothing can be computed. Measure the tyre, or find the manufacturer’s own chart. |
| Whether a size will fit your car | Not computable from a tyre code. Clearance depends on the arch, the suspension travel, the steering lock, the wheel’s own offset and width, and the vehicle’s approved fitment list. For the wheel side of that arithmetic see the wheel offset and backspacing converter, and then test fit. |
| Rolling resistance, wet grip or noise class | These are measured properties of a specific tyre model, printed on the EU tyre label. They are not functions of the size code. |
Millimetres, a percentage and inches in one string
The code mixes three units, which is the only reason it is hard. In 205/55 R16 the 205 is millimetres, the 55 is a percentage of that 205, and the 16 is inches. So the sidewall is 205 × 0.55 = 112.75 mm, the rim is 16 × 25.4 = 406.4 mm, and the overall diameter is the rim plus two sidewalls: 631.9 mm. Once you have that, everything else on this page is arithmetic. The consequence people forget is that the aspect ratio is relative: a 55 on a 225 is 123.75 mm of sidewall, not 112.75, so widening a tyre at the same aspect ratio makes it taller as well.
The speedometer question is a ratio, and the regulation behind it is one-sided. A speedometer counts wheel revolutions and multiplies by a constant that was set for the original tyre. Fit a tyre with a larger rolling circumference and the wheel turns fewer times per kilometre, so the needle reads low — true speed is indicated speed times the ratio of the circumferences. That direction is the one people get backwards. And it runs into UNECE Regulation No 39, whose paragraph 5.4 requires “0 ≤ (V1 − V2) ≤ 0.1 V2 + 4 km/h” and states that “The speed indicated shall not be less than the true speed of the vehicle.” High is permitted, low is not. Manufacturers therefore leave themselves a margin — which is why your needle usually sits a few per cent above a GPS — and a bigger tyre spends it. This page asks for your own measured over-read, because how much headroom you have is a fact about your car and not about tyres in general. At zero over-read, any increase at all puts you under.
Rolling circumference is not π × D, and “published rev/km” is two different numbers. A loaded tyre flattens at the contact patch and advances less than its geometric circumference per turn; ISO 17269 is the standard that defines how to measure it on a loaded new tyre. A published German Abrollumfang table gives 1928 mm for 205/55 R16, against 1985 mm of geometry — 97.1%, and 97.2% for 195/65 R15. Meanwhile plenty of reference sites print the geometric figure and call it revolutions per kilometre: byways.org gives 504 rev/km for the same tyre, which is π × D exactly. Both are above, with the factor as an input, so you can tell which kind of number a source has given you. For a comparison between two sizes it cancels out; for an absolute rev/km figure it is a 3% difference.
The other codes, including one this page refuses. A metric size printed without an aspect ratio — 165 R13 — is taken as 82%, the documented convention. The imperial light-truck code works backwards: in 31×10.50 R15 the 31 is the overall diameter in inches and the 10.50 the width, so the aspect ratio is (31 − 15) ÷ 2 ÷ 10.50 = 76.2%. Older numeric sizes such as 6.50-15 carry an assumed ratio that depends on their date: 90% before 1964, 80% from 1965 into the early 1970s, which makes the same printed size two different tyres. Alphanumeric sizes like C78-15 are refused, because the letter is a load and size class rather than a width and no letter-to-width table could be sourced.
And two things this page will not tell you, deliberately. It will not convert a load index into kilograms or a speed rating letter into km/h. Those tables live in the ETRTO, TRA and JATMA standards manuals, all copyrighted and none obtainable here — and more importantly, fitting a tyre below the load index or speed rating on your vehicle’s own placard is a safety matter and, in most jurisdictions, an insurance and roadworthiness one rather than a preference. The placard in the driver’s door shut is the authority; match it or exceed it. It also will not tell you whether a size will clear your arches, which depends on the suspension, the steering lock and the wheel. For that half of the problem see the wheel offset and backspacing converter, and then test fit.
Frequently asked questions
What does 205/55 R16 mean?
205 is the nominal section width in millimetres. 55 is the aspect ratio — the sidewall height as a percentage of that width, so 112.75 mm here. R means radial construction. 16 is the rim diameter in inches. The code mixes millimetres, a percentage and inches in five characters, which is exactly why nobody can do it in their head. Overall diameter = 16 × 25.4 + 2 × 112.75 = 631.9 mm.
How much does a bigger tyre change my speedometer?
In proportion to the rolling circumference, which for a given construction is in proportion to the overall diameter. A tyre 2% larger makes the wheel turn 2% fewer times per kilometre, so the needle reads about 2% low — at an indicated 100 km/h you are doing about 102. The odometer under-counts by the same 2%, which matters for warranty, resale and service intervals. The direction is the thing to get right: bigger tyre, lower reading.
Is it legal to fit a bigger tyre?
The tyre is not what the regulation is about — the speedometer is. UNECE Regulation No 39, paragraph 5.4, requires “0 ≤ (V1 − V2) ≤ 0.1 V2 + 4 km/h” and states that “The speed indicated shall not be less than the true speed of the vehicle.” That is one-sided: high is allowed, low is not. Manufacturers build in a margin so the needle never goes under, which is why your speedometer usually reads a few per cent above a GPS. A bigger tyre spends that margin, and once it is gone the indication is below true speed. How much you have depends on your car, so put your own measured over-read into the calculator. Beyond that, whether a particular size is permitted on your vehicle is a question for the manufacturer’s fitment list and your local roadworthiness rules, not for this page.
Where does the “3% rule” come from?
From the trade, as far as anybody can tell. Jalopnik states it as “new tires should never be 3% larger or smaller in diameter than the factory-spec rubber” and calls it an industry standard, but names no standards body, and no standards document setting a 3% figure could be found. It is a sensible working limit — 3% of a 632 mm tyre is 19 mm of diameter and 9.5 mm of ride height — and this page prints it as a rule of thumb rather than a regulation. The thing that is written down is the speedometer tolerance above.
Why do published revolutions-per-kilometre figures disagree?
Because they are two different quantities. A loaded tyre flattens at the contact patch and advances less than π × D per turn, and ISO 17269 defines how to measure that loaded rolling circumference. A published German Abrollumfang table gives 1928 mm for 205/55 R16 against a geometric 1985 — 97.1%. But plenty of online tables simply print π × D: byways.org gives “504 per km” for the same tyre, which is the geometric figure exactly. Both numbers appear above, with the factor as an input, so you can see which one a source has handed you. For a speedometer comparison it barely matters, because the factor cancels in the ratio of two sizes.
How do I read 31×10.50 R15?
It is the imperial light-truck convention and it works the other way round: 31 is the overall diameter in inches, 10.50 is the section width in inches, R is radial and 15 is the rim in inches. The aspect ratio is not stated, so it has to be recovered: (31 − 15) ÷ 2 is the sidewall in inches, 8, and 8 ÷ 10.50 is 76.2%. Set the code family above to the light-truck option and the page does that for you.
What if the size has no aspect ratio, like 165 R13?
It is taken as 82%. That is the documented convention for a metric size with the ratio omitted — “If the information is omitted, it is assumed to be 82%”. Older numeric sizes like 6.50-15 are different again: they carried a 90% ratio before 1964 and 80% from 1965 into the early 1970s, so the same printed size means two different tyres depending on its date. Both options are in the code-family list.
Why won’t this page convert a load index or a speed rating?
Because the tables are copyrighted, could not be obtained, and matter too much to guess at. The load index is a coded kilogram capacity and the speed rating a coded maximum speed, and fitting a tyre below the index or rating on your vehicle’s own placard is a safety matter and, in most places, an insurance and roadworthiness one — not a preference you trade off against price. The placard in the driver’s door shut is the authority. Match it or exceed it.
Related calculators
References
- UNECE Regulation No 39 (uniform provisions concerning the approval of vehicles with regard to the speedometer and odometer equipment), paragraph 5.4, read 27 September 2026 from its publication in the Official Journal of the European Union (OJ L, 2025/1902, eur-lex.europa.eu). The requirement is two things at once: “0 ≤ (V1 − V2) ≤ 0.1 V2 + 4 km/h”, and “The speed indicated shall not be less than the true speed of the vehicle.” The test is run on “one of the types normally fitted to the vehicle” at “normal running pressure”. That one-sided tolerance is the whole reason a bigger tyre is a legal question and not just an inconvenience.
- Wikipedia, Speedometer (read 27 September 2026), for how R39 is applied: the UK permits speedometers meeting Directive 75/443 as amended by 97/39 or UN R39, and under the Motor Vehicles (Approval) Regulations 2001 for 25-70 mph “the speedometer must never show an indicated speed less than the actual speed”; Australian vehicles built from 2007 “must conform to UNECE Regulation 39”. India’s Central Motor Vehicles Rules 1989 carry a Rule 117 headed “Speedometer”, but its text could not be fetched, so no Indian tolerance is stated on this page.
- Wikipedia, Tire code (read 27 September 2026), for the code itself: the section width is “in millimeters”, the aspect ratio is “the sidewall height as a percentage of the nominal section width”, the last number is “the diameter, in inches, of the rim”, and the overall diameter is “(2×T×A/100) + (W×25.4)” in millimetres. Where a metric size omits the aspect ratio, “it is assumed to be 82%”. Numeric sizes such as 6.50-15 used “a 90% aspect ratio before 1964, then an 80% aspect ratio from 1965 to early 1970s”. Alphanumeric sizes (C78-15) state the ratio but encode the width as a letter, which is a load class rather than a width — which is why this page refuses them.
- Schmidt Tuner-Felgen (Bad Segeberg), published Abrollumfang table (t3-infos.de/images/Abrollumfang.pdf, read 27 September 2026): 205/55 R16 1928 mm and 195/65 R15 1937 mm. Against the geometric π × D those are 97.1% and 97.2%, so the loaded rolling circumference of a passenger tyre runs about 3% short of the geometric one. ISO 17269, Passenger car tyres — Methods for measuring rolling circumference — Loaded new tyres, is the standard that defines that measurement; it is copyrighted, was not fetched, and is cited by number only.
- byways.org’s 205/55R16 reference page (read 27 September 2026), which publishes “78.2″ (1984.5 mm)” of circumference and “810 per mile” / “504 per km”. Those are π × D exactly, with no load deflection in them at all. Two published revolutions-per-kilometre figures for the same tyre, 2.9% apart, because they are two different quantities — which is why the rolling-circumference factor is an input here and not a constant.
- Jalopnik, What the 3% rule means for your tires (read 27 September 2026): “the 3% rule states that new tires should never be 3% larger or smaller in diameter than the factory-spec rubber”, described as “An industry standard for tire replacements”. No standards body is named in it, and none could be found, so this page prints the 3% figure as a trade rule of thumb and computes the R39 consequence beside it, because the R39 tolerance is the part that is actually written down.