245/45R20 vs 275/40R20
The verdict
Inside the ±3% overall-diameter band vehicle makers calibrate to. Speedometer, ABS and stability control all stay inside the error they were validated for.
SOURCE·Metric size geometry + UN ECE R39 speedometer tolerance·both tyres drawn to one scale from their size designations; the ±3% ring is the industry fitment convention, not a legal limit·REVIEWED ETRTO STANDARD
- Diameter change
- −0.5mm
- Ride height
- −0.3mm
- Speedo at 100
- 99.9km/h true
- Section width
- +30mm
What the speedometer does
−0.1 km/h At an indicated 100 km/h you would really be doing 99.9 km/h.
That is the tolerated direction: the needle now reads high, and R39 allows a speedometer to over-read by up to 10% + 4 km/h. At 100 km/h true, the legal ceiling on the dial is 114 km/h, so this change has room in it — but it also means you are driving slower than you think, which is its own problem in traffic.
Indicated against true, right across the range
| Speedo reads | You are actually doing | Error | In mph |
|---|---|---|---|
| 50 km/h | 50.0 km/h | −0.0 km/h | 31 shown → 31.0 true |
| 60 km/h | 60.0 km/h | −0.0 km/h | 37 shown → 37.3 true |
| 80 km/h | 79.9 km/h | −0.1 km/h | 50 shown → 49.7 true |
| 90 km/h | 89.9 km/h | −0.1 km/h | 56 shown → 55.9 true |
| 100 km/h | 99.9 km/h | −0.1 km/h | 62 shown → 62.1 true |
| 110 km/h | 109.9 km/h | −0.1 km/h | 68 shown → 68.3 true |
The shorter tyre covers less ground per revolution, so the needle reads high — you are going slower than the dial says. This is the direction the regulation tolerates.
SOURCE·Computed from rolling-diameter ratio·instrument assumes the original circumference·REVIEWED ETRTO STANDARD
Side by side
| 245/45R20 | 275/40R20 | Change | |
|---|---|---|---|
| Overall diameter | 728.5 mm | 728.0 mm | −0.5 mm (−0.07%) |
| Sidewall height | 110.3 mm | 110.0 mm | −0.3 mm |
| Section width | 245 mm | 275 mm | +30 mm |
| Rim diameter | 20″ | 20″ | same |
| Rolling circumference | 2289 mm | 2287 mm | −2 mm |
| Revolutions per km | 436.9 | 437.2 | +0.3 |
| Ground clearance at the hub | half the diameter change | −0.3 mm | |
| Odometer per 1,000 km shown | true distance travelled | 999 km | |
SOURCE·Metric size geometry + UN ECE R39 speedometer tolerance·every row computed from the two size designations·REVIEWED ETRTO STANDARD
How the figures are derived
- Read the size as the specification it is: section width in millimetres, aspect ratio as a percentage of that width, rim diameter in inches.
- Sidewall height = width × aspect ÷ 100. This is the number the size string is really about, and it is the one nobody quotes.
- Overall diameter = rim × 25.4 + two sidewalls — one at the top of the wheel and one at the bottom.
- Circumference = π × overall diameter, which gives revolutions per kilometre and per mile directly.
- Nothing above is measured, estimated, averaged or dated. It is the definition of the designation, so it does not go stale and it has no source to cite beyond the standard itself.
sidewall (mm) = width × aspect ÷ 100 overall diameter (mm) = rim × 25.4 + 2 × sidewall speedo true speed = indicated × (new diameter ÷ old diameter)
The full method, the ±3% rule, and what geometry cannot tell you →
What this cannot tell you
Rolling diameter is the part of a fitment that arithmetic owns completely. The rest of it is not in the size string and this page will never pretend otherwise:
- Rim width. Every tyre section width has a range of rim widths it is approved for. A 275 mm tyre on a rim meant for 245 mm will mount, and it will also sit wrong.
- Arch and suspension clearance. 0.3 mm of extra radius has to go somewhere at full lock and full compression.
- Load index and speed rating. Fitting below your vehicle's placard figures is the one change here that is unambiguously unsafe. The load table →
- The placard. Your car states its own sizes on a plate in the driver's door aperture. That plate outranks every website, including this one.
245/45R20 vs 275/40R20 — the questions that follow
Can I fit 275/40R20 instead of 245/45R20?
Geometrically the change is −0.07% on rolling diameter, which is within tolerance against the ±3% convention vehicle makers calibrate to. That is only half the question: rim width, arch clearance, load index and your car's own tyre placard decide the rest, and none of those are in a size string.
How much will my speedometer be out?
At an indicated 100 km/h you would really be doing 99.9 km/h — 0.1 km/h slower than the dial shows. The odometer drifts by the same 0.07%, so 1,000 km displayed is really 999 km.
Which is taller, 245/45R20 or 275/40R20?
245/45R20 is taller, by 0.5 mm on overall diameter. That is 0.3 mm of ride height, because only half the diameter change sits between the hub and the road.
Is the difference wider as well as taller?
Yes. 275/40R20 is 30 mm wider at its widest point — 1.18″. Section width is not tread width, but a wider section still needs the arch and the rim to accommodate it.
Other sizes people weigh against 245/45R20
Reading it the other way: fitting 245/45R20 to a car built for 275/40R20 is +0.07%. The geometry here is exact; whether a tyre fits YOUR car is not something geometry can answer. Arch clearance, suspension travel, offset, load rating and your vehicle's own placard all decide that, and none of them are in a size string. The ±3% rule quoted throughout is an industry convention, not a law. Confirm any change against the tyre placard on your door pillar and with a tyre fitter before you buy.