Tire size comparison
When you change tire size, the number that matters most is overall diameter. BFGoodrich and Continental both say to keep it within about 3% of the original, and Continental says to match the original rolling circumference as closely as possible.[1 · Oct 29, 2026][2 · Oct 29, 2026]
Why diameter matters
A taller tire travels farther in one turn, so the speedometer and odometer read low. BFGoodrich says going beyond the guideline can affect the speedometer and odometer, anti-lock brakes and stability control, and on some vehicles transmission shift points.[1 · Oct 29, 2026] For cars and vans, Tire Rack also gives 3% as the limit and says that's about ¾ in.[3 · Oct 29, 2026]
Popular swaps, calculated
True speed is how fast you're actually going when the speedometer, set for the first size, says 60 mph.
| Swap | Overall diameter | Difference | True speed at an indicated 60 mph | Ride height change | vs the 3% guideline |
|---|---|---|---|---|---|
| 265/70R17 → 285/70R17 | 31.61 → 32.71 in | +3.5% | 62.1 mph | +0.55 in | Over 3% |
| 265/70R17 → 275/70R17 | 31.61 → 32.16 in | +1.7% | 61.0 mph | +0.28 in | Within 3% |
| 265/70R17 → 285/75R17 | 31.61 → 33.83 in | +7.0% | 64.2 mph | +1.11 in | Over 3% |
| 225/65R17 → 235/65R17 | 28.52 → 29.03 in | +1.8% | 61.1 mph | +0.26 in | Within 3% |
| 225/65R17 → 225/60R18 | 28.52 → 28.63 in | +0.4% | 60.2 mph | +0.06 in | Within 3% |
| 235/65R17 → 235/60R18 | 29.03 → 29.10 in | +0.3% | 60.2 mph | +0.04 in | Within 3% |
| 205/55R16 → 225/45R17 | 24.88 → 24.97 in | +0.4% | 60.2 mph | +0.05 in | Within 3% |
| 275/55R20 → 275/60R20 | 31.91 → 32.99 in | +3.4% | 62.0 mph | +0.54 in | Over 3% |
| 215/55R17 → 225/55R17 | 26.31 → 26.74 in | +1.6% | 61.0 mph | +0.22 in | Within 3% |
| 245/75R16 → 265/75R16 | 30.47 → 31.65 in | +3.9% | 62.3 mph | +0.59 in | Over 3% |
The math (our own arithmetic): sidewall height = width × aspect ratio ÷ 100; overall diameter = rim × 25.4 + 2 × sidewall (mm), ÷ 25.4 for inches; revs per mile = 63,360 ÷ (π × diameter in inches); true speed = indicated speed × new diameter ÷ original diameter. These are nominal figures from the size code. Tire Rack notes that real tires differ from the formula, so use the tire maker's published spec when clearance is tight.[4 · Oct 29, 2026]
How to compare two sizes yourself
- Work out each size's overall diameter: rim + 2 × (width × aspect ÷ 100 ÷ 25.4).
- Divide the new diameter by the old, subtract 1, and multiply by 100 for the percent difference.
- Multiply your indicated speed by the same ratio to get your true speed.
- Half the diameter difference is how much the vehicle rises or drops.
Or let the decoder do it: type both sizes and it shows all of this at once.
Diameter isn't the only check
- Load: the new tire's load capacity must meet or exceed the original.[1 · Oct 29, 2026][2 · Oct 29, 2026]
- Speed rating: equal to or higher than the original.[2 · Oct 29, 2026]
- Rim width: use only tire and rim width combinations approved by the industry standards.[2 · Oct 29, 2026]
- Clearance: make sure the tire clears the body and chassis under all service conditions.[2 · Oct 29, 2026]
- Pressure: BFGoodrich says the right pressure after a size change has to be worked out from load and inflation tables, and to confirm any change with a tire professional.[1 · Oct 29, 2026]
Pickups and SUVs: Tire Rack writes that they're usually engineered to handle up to a 15% oversize tire, while cars and vans should stay within 3%.[3 · Oct 29, 2026] Bigger swaps on trucks still change the speedometer reading, so check them in the decoder.
Affiliate links: if you buy through a link in this box we may earn a commission, at no extra cost to you. It does not change the sizes or sources on this page. How we handle affiliate links.
Sources: 1. BFGoodrich, Changing tire size. 2. Continental Tire, PSIB 06-03: Tire plus-sizing for passenger and light truck vehicles (PDF, 2006). 3. Tire Rack, How do I select the right tires?. 4. Tire Rack, How do I calculate tire dimensions?. Checked Oct 29, 2026. Size dimensions, revolutions per mile and speedometer figures are our own arithmetic from the size code. Trademarks belong to their owners; figures are restated in our own words and tables, not reproduced.