Quick Answer
- Plus sizing is a bigger wheel with a lower aspect ratio tire so the overall diameter stays near stock. For 225/45R17 (24.97 in) on 18 inch wheels the closest sizes at the same width or up to 20 mm wider are 225/40R18 (+0.5%), 245/35R18 (-0.9%), 235/40R18 (+1.7%).
- Continental says a rolling circumference deviation above 3% can affect speedometer accuracy, ABS and stability control.
- The new tire needs a load index and speed rating equal to or higher than the original, an approved rim width and clearance at full suspension travel.
Enter your current tire size and the wheel diameter you want. The sizes below keep the overall diameter closest to stock.
Plus-1 options for 225/45R17
Stock overall diameter 24.97 in. Closest to stock first.
| Size | Diameter | Change | Within 3% | Sidewall | Continental rim width |
|---|---|---|---|---|---|
| 255/35R18 | 25.03" | +0.2% | 3.51" | 8.5 to 10" | |
| 225/40R18 | 25.09" | +0.5% | 3.54" | 7.5 to 9" | |
| 215/40R18 | 24.77" | -0.8% | 3.39" | 7 to 8.5" | |
| 245/35R18 | 24.75" | -0.9% | 3.38" | 8 to 9.5" | |
| 205/45R18 | 25.26" | +1.2% | 3.63" | 6.5 to 7.5" | |
| 235/40R18 | 25.40" | +1.7% | 3.70" | 8 to 9.5" | |
| 235/35R18 | 24.48" | -2.0% | 3.24" | 8 to 9.5" | |
| 205/40R18 | 24.46" | -2.1% | 3.23" | 7 to 8" | |
| 215/45R18 | 25.62" | +2.6% | 3.81" | 7 to 8" | |
| 245/40R18 | 25.72" | +3.0% | 3.86" | 8 to 9.5" | |
| 225/35R18 | 24.20" | -3.1% | 3.10" | 7.5 to 9" | |
| 255/30R18 | 24.02" | -3.8% | 3.01" | 8.5 to 9.5" |
Continental says a rolling circumference deviation above 3% can affect speedometer accuracy, ABS and stability control, and that the new tire needs a load index and speed rating equal to or higher than the original. Continental rim widths are from its 2025 databook; other makers can list a different range, and a size marked Not in table is not in that databook.
Get a plain-English explanation of your results and what they mean for your vehicle.
Tire Plus Sizing Chart: Closest Size by Wheel Diameter
For each stock size, the closest size to its overall diameter at the same width or up to 20 mm wider, with the change from stock. Minus-1 is for winter downsizing.
| Stock size | Stock diameter | Plus-1 | Plus-2 | Minus-1 |
|---|---|---|---|---|
| 205/55R16 | 24.88" | 225/45R17 (+0.4%) | 215/40R18 (-0.4%) | 225/55R15 (-0.5%) |
| 215/55R17 | 26.31" | 235/45R18 (+0.1%) | 235/40R19 (+0.3%) | 235/55R16 (-0.5%) |
| 225/45R17 | 24.97" | 225/40R18 (+0.5%) | 245/30R19 (-0.7%) | 225/50R16 (-0.5%) |
| 225/50R17 | 25.86" | 225/45R18 (+0.4%) | 245/35R19 (-0.4%) | 225/55R16 (-0.4%) |
| 235/45R18 | 26.33" | 235/40R19 (+0.3%) | 235/35R20 (+0.6%) | 235/50R17 (-0.3%) |
| 235/55R18 | 28.18" | 235/50R19 (+0.3%) | 255/40R20 (-0.5%) | 235/60R17 (-0.3%) |
| 245/40R18 | 25.72" | 245/35R19 (+0.1%) | 245/30R20 (+0.3%) | 245/45R17 (-0.1%) |
| 255/45R19 | 28.04" | 255/40R20 (-0.0%) | 255/35R21 (-0.0%) | 255/50R18 (+0.0%) |
| 265/70R17 | 31.61" | 265/65R18 (-0.1%) | 265/60R19 (-0.3%) | 265/75R16 (+0.1%) |
| 275/55R20 | 31.91" | 275/50R21 (-0.3%) | 275/45R22 (-0.5%) | 275/60R19 (+0.3%) |
Sizes are calculated, not a list of what every maker sells, so confirm the size exists for your tire. The diameter and speedometer comparison of old and new sizes checks any pair.
What to Check Before You Plus Size
Continental's plus sizing guidance for passenger and light truck vehicles in the US, Canada and Mexico.
| Check | What Continental says |
|---|---|
| Load capacity | Use a tire with a load carrying capacity equal to or greater than the original equipment tire. Check the door placard or owner's manual for the OE size, maximum load and inflation pressure. Some applications need a higher pressure than the placard to keep the same load capacity, and the pressure must never exceed the maximum on the sidewall. |
| Inflation pressure | Keep pressure at or above the vehicle manufacturer's recommendation, and keep the same pressure relationship between the front and rear axles. |
| Speed rating | Use an equal or greater speed rating than the original. Winter tires with a lower speed rating may be acceptable, but that limits the vehicle's maximum speed. |
| Rolling circumference | Match the original as closely as possible. A deviation greater than 3 percent can affect speedometer accuracy, ABS, stability control and other safety systems. On AWD and 4WD vehicles replace all four tires with the same size unless the vehicle is designed for staggered fitments. |
| Tire and rim combination | Use only industry-approved tire size and rim width combinations from the data book. A rim that is too narrow or too wide affects handling, stability, wear and durability, and Continental approves no combination outside TRA, ETRTO or JATMA standards. |
| Body and chassis clearance | Check clearance at full suspension travel, at steering lock and under load, allow for tire growth and deflection in cornering, and check chain clearance if you use chains. |
Inflation change when an XL tire replaces an SL tire
| Original SL load index | XL replacement load index | Required inflation change |
|---|---|---|
| Same as OE | Same | +0.4 bar (6 psi) |
| SL index | +1 | +0.3 bar (4 psi) |
| SL index | +2 | +0.2 bar (3 psi) |
| SL index | +3 | +0.1 bar (2 psi) |
| SL index | +4 or more | No change |
Never exceed the maximum pressure on the sidewall. Check the load index, the rim width range for a tire, the wheel offset and the inner and outer clearance estimate before you buy.
How to Calculate a Plus Size Tire
Stock overall diameter
Rim + 2 x width x aspect / 100 / 25.4. For 225/45R17: 17 + 2 x 101.25 / 25.4 = 24.97 in.
Target sidewall on the new wheel
(Stock diameter - new rim) / 2. For an 18 inch wheel: (24.97 - 18) / 2 = 3.49 in = 88.5 mm.
Aspect ratio for a chosen width
Target sidewall (mm) / width x 100. At 225 mm: 39.4%, so round to the nearest size that exists (40 gives 225/40R18). At 245 mm: 36.1%.
Change from stock
Change % = (new diameter / stock diameter - 1) x 100. Rolling circumference changes by the same percentage.
Sidewall heights for any size are on the sidewall height calculator, and the shorter-sidewall trade-offs are in the guide to low profile tires.
How to Use the Plus Size Tire Calculator
- 1Type your current tire size from the sidewall, for example 225/45R17.
- 2Pick the new wheel diameter. The label next to it shows Plus-1, Plus-2, Plus-0 or Minus-1 for winter downsizing.
- 3Read the table: sizes are ranked by how close the overall diameter is to stock, with the change in percent and a check mark when it is within 3%.
- 4Compare each size with the Continental rim width column and your wheel width, then check load index, speed rating and clearance before buying.
Frequently Asked Questions
Source for the checks and the 3% statement: Continental Tire, Tire Plus-sizing for Passenger and Light Truck Vehicles (US, Canada and Mexico). Sizes are calculated from the nominal size code. Always check the sizes your vehicle manufacturer approves and do a physical fitment check.