Compare two tire sizes
Type the size printed on the sidewall of your current tire and the one you are thinking of. Everything is worked out in your browser from the size code alone.
Working it out…
Speedometer error
The speedometer counts wheel turns and assumes the current tire. With the new tire fitted:
| Speedometer shows | You are really doing | Difference |
|---|
Gearing: engine RPM with each tire, and the axle ratio that restores it
A taller tire covers more road per turn, so at the same road speed the axle and the engine turn slower, as if the axle ratio had dropped. Type your axle (ring-and-pinion) ratio: it is on the axle tag, the build sheet or in the handbook, written like 3.73 or 3.73:1.
The top-gear ratio defaults to 1.00, a direct-drive top gear. Overdrive top gears are below 1 (often 0.65–0.85); type yours from the transmission's ratio table to see cruising RPM, or leave 1.00 to compare the tires alone.
RPM = speed × axle ratio × top-gear ratio × wheel revs per mile ÷ 60 (speed in mph) restoring axle = axle × new diameter ÷ current diameter effective axle = axle × current diameter ÷ new diameter
Worked through for 265/70R17 (802.8 mm, 638.1 revs per mile) to 35x12.50R17 (889 mm, 576.2 revs per mile) on a 3.73 axle and a 1.00 top gear at 60 mph: 60 × 3.73 × 1.00 × 638.1 ÷ 60 = 2,380 rpm before, and 60 × 3.73 × 576.2 ÷ 60 = 2,149 rpm after, 9.7 % lower. The 35 makes the 3.73 behave like 3.73 × 802.8 ÷ 889 = 3.37; the ratio that brings back 2,380 rpm is 3.73 × 889 ÷ 802.8 = 4.13. The nearest common ratios are 4.10 (2,363 rpm, 0.7 % under the original) and 4.30 (2,478 rpm, 4.1 % over).
Common ring-and-pinion ratios it compares against: 3.21, 3.42, 3.55, 3.73, 3.91, 4.10, 4.30, 4.56, 4.88, 5.13, 5.38. Not every axle is offered in every one of these, and some axles have others. RPM here uses the unloaded tire; a loaded tire turns 2–3 % more often, so the real engine speed is that much higher, and an automatic with an unlocked torque converter slips a little more. It does not say whether a regear is needed, safe for your axle or what it costs.
Drawn to scale
What the code says
Load index and speed rating tables (ISO 4000-1 / ETRTO)
Load index: the maximum load one tire may carry at the reference inflation pressure. Speed symbol: the highest speed at which that load may be carried. Both tables are those of ISO 4000-1 and the ETRTO Standards Manual, which the US Tire and Rim Association uses for the same symbols. On LT tires a pair such as 121/118 gives the rating fitted single / fitted as a dual pair.
Sizes that fit
Type your tire size and pick the rim you want to run: every size within ±3 % of its overall diameter on that rim, closest first, with what the speedometer shows against your true speed.
The rule, the same one the size pages use: every size in a grid of section widths within ±20 mm of yours in 10 mm steps (for an inch-coded flotation size, around the metric width ending in 5 nearest its width in mm) and aspect ratios 25–80 in steps of 5, whose overall diameter is within ±3 % of yours. ±3 % is a rule of thumb many tire fitters use, not a law. The list is geometry, not a fitment promise: not every size is made by every maker, and a close diameter says nothing about clearance, wheel width or what the car maker approves. Any replacement needs a load index and speed rating at least the car maker's.
Read the whole sidewall
Type what is moulded on the tire, in any order: the size, the DOT number with its 4-digit date code, the treadwear, traction and temperature grades, snow and load marks. Each mark is explained with the rule it comes from, and the date code gives the week the tire was made and its age today. It does not say whether a tire is too old or safe to use, and it does not look up which factory a plant code belongs to. It all happens in this page; nothing you type is sent anywhere.
What this cannot tell you
- Whether the tire is too old or safe to use. The age is arithmetic from the date code. When to replace a tire is the tire maker's and car maker's call, and the inspection rules where you live; tread depth, cracks, bulges and how the tire was stored matter as much as the date.
- Which factory made it. Plant codes are shown as typed; NHTSA assigns them, and this page carries no plant list.
- Mileage. Treadwear is a rate relative to a reference tire, graded by the maker, and only comparable within one maker's range.
- Anything it did not recognise. Unread groups are listed as not read, never guessed.
Sources: 49 CFR 574.5 (DOT Tire Identification Number and date code), 49 CFR 575.104 (UTQG treadwear, traction and temperature grades), 49 CFR 571.139 (FMVSS 139 sidewall markings and the snow-tire symbol).
Every dimension
| Measure | Current | New | Difference |
|---|
Revolutions per mile and per km are the geometric figures for the unloaded tire. Under a car's weight the tire squashes and turns roughly 2–3 % more often; a maker's datasheet figure is measured and wins over this one.
Size pages
Nineteen sizes worked out in full (passenger-car, pickup and SUV, and inch-coded flotation sizes), each with every size within ±3 % of its diameter on the same rim and one inch either side:
- 195/65R15 635 mm
- 205/55R16 632 mm
- 225/45R17 634 mm
- 225/40R18 637 mm
- 225/35R19 640 mm
- 215/65R15 661 mm
- 215/60R16 664 mm
- 215/55R17 668 mm
- 235/45R18 669 mm
- 235/40R19 671 mm
- 265/75R16 804 mm
- 265/70R17 803 mm
- 265/65R18 802 mm
- 265/60R19 801 mm
- 295/50R20 803 mm
- 225/70R16 721 mm
- 225/65R17 724 mm
- 245/55R18 727 mm
- 265/45R19 721 mm
- 275/40R20 728 mm
- 195/70R15 654 mm
- 205/60R16 652 mm
- 225/50R17 657 mm
- 245/40R18 653 mm
- 245/35R19 654 mm
- 31x10.50R15 787 mm
- 33x12.50R15 838 mm
- 33x12.50R17 838 mm
- 35x12.50R17 889 mm
All 29 size pages, grouped by ladder, with the rules that picked them
The formulas
A metric size like 225/45R17 is three numbers: the section width in millimetres (225), the aspect ratio — sidewall height as a percentage of width (45) — and the rim diameter in inches (17). The R means radial construction, not radius; ZR is a radial rated above 240 km/h.
sidewall = width × aspect ÷ 100 diameter = rim × 25.4 + 2 × sidewall circumference = π × diameter revs per mile = 1,609,344 ÷ circumference (mm) revs per km = 1,000,000 ÷ circumference (mm)
Worked through for 225/45R17: sidewall = 225 × 0.45 = 101.25 mm; diameter = 17 × 25.4 + 2 × 101.25 = 431.8 + 202.5 = 634.3 mm (24.97 in); circumference = π × 634.3 = 1,992.7 mm; 1,609,344 ÷ 1,992.7 = 807.6 revolutions per mile.
A flotation size like 33x12.50R15 is already in inches: 33 in overall diameter, 12.50 in section width, 15 in rim. The sidewall is what is left over: (33 − 15) ÷ 2 = 9.0 in (228.6 mm).
true speed = speedometer reading × new diameter ÷ current diameter
Going from 225/45R17 (634.3 mm) to 225/50R17 (656.8 mm) makes each wheel turn cover 3.55 % more road. When the speedometer shows 100 km/h you are doing 100 × 656.8 ÷ 634.3 = 103.5 km/h; when it shows 60 mph, 62.1 mph. The odometer under-counts by the same 3.4 %.
What this cannot do
- It does not say whether a size is right for your car. Clearance to struts, arches and brake calipers, wheel offset and width, steering lock and suspension travel all decide that, and none of them is in a tire size code. Check the car maker's approved sizes (door-jamb placard, handbook or the vehicle's registration papers), then the physical clearance.
- It does not check load capacity for your vehicle. The load index is decoded, but whether it is enough depends on your axle loads and inflation pressure. Never fit a lower load index or speed rating than the car maker specifies.
- Real tires differ from the nominal size. Makers build to tolerance bands (a few percent on diameter), rim width changes the section width by roughly 5 mm per half-inch of rim width, and tread wears: a new tire with 8 mm of tread is about 13 mm larger in diameter than the same tire worn to the 1.6 mm legal limit.
- Your speedometer was not exact to begin with. In the EU and UK (UN ECE Regulation 39) a speedometer may never read below true speed and may read high by up to 10 % + 4 km/h, so most cars already over-read by a few percent. A slightly taller tire eats into that margin; a smaller one adds to it. Many cars can have the tire circumference re-coded at a dealer.
- It does not say whether a regear is needed, safe or worth what it costs. The Gearing fold gives engine RPM and the axle ratio that restores it, which is arithmetic. Whether your engine and transmission cope with the lower RPM, whether a given ratio is made for your axle and carrier, what the ring and pinion, bearings and labour cost, and what a changed ratio does to the speedometer calibration, towing rating or warranty are questions for the vehicle maker and an axle specialist.
- It does not cover ABS, stability control or 4WD rules. Mixing sizes between axles on four-wheel-drive cars can strain the transfer case; that is a question for the car's handbook.