305/35R20, 315/35R20 and 325/35R20 differ in nominal width and nominal diameter. The aspect ratio stays at 35 percent, so increasing width also increases the calculated sidewall height. These calculations explain the size codes; they do not approve a tire for a wheel or vehicle.
An important comparison is easy to misread: 275/40R20 and 315/35R20 have nearly identical nominal diameters, not a difference of about an inch. The arithmetic below makes that distinction visible.
The nominal-size calculation
For these size codes:
- Nominal sidewall height in millimeters = width × aspect ratio ÷ 100.
- Nominal overall diameter in millimeters = wheel diameter in inches × 25.4 + twice the sidewall height.
- Divide millimeters by 25.4 to express the result in inches.
For 315/35R20, that is 315 × 0.35 = 110.25 mm of nominal sidewall, then 508 + 220.5 = 728.5 mm overall. Read the other code elements in our tire-size guide.
| Size | Nominal width | Nominal sidewall | Nominal diameter | Diameter in inches |
|---|---|---|---|---|
| 275/40R20 | 275 mm | 110.00 mm | 728.0 mm | 28.661 |
| 305/35R20 | 305 mm | 106.75 mm | 721.5 mm | 28.406 |
| 315/35R20 | 315 mm | 110.25 mm | 728.5 mm | 28.681 |
| 325/35R20 | 325 mm | 113.75 mm | 735.5 mm | 28.957 |
Original calculations from the nominal codes, rounded as shown. These are not manufacturer measurements, approved rim ranges, loaded rolling radii or measured rolling circumferences.
How much does the calculated diameter change?
Relative to 305/35R20, 315/35R20 is 7 mm taller in nominal overall diameter, about 0.97 percent. The nominal radius difference is 3.5 mm. Moving from 305/35R20 to 325/35R20 adds 14 mm of diameter, about 1.94 percent, and 7 mm of radius.
Compare 275/40R20 with 315/35R20 separately: the difference is just 0.5 mm of diameter, about 0.069 percent. This is why width alone cannot tell you how much taller a tire will be. The aspect ratio changes the result.
These percentages do not establish an allowable change. A small calculated difference can coexist with an unsuitable wheel width, insufficient clearance, wrong service description or incompatible axle combination.
What this means for a speedometer comparison
In a simplified model, the ratio of new to old rolling circumference changes distance traveled per revolution. If nominal diameter alone were used as a proxy, 728.5 ÷ 728.0 gives a factor of about 1.000687. At a reference 60 mph, the corresponding theoretical value is about 60.04 mph—not 62–63 mph.
That example isolates the arithmetic. It is not a prediction of the car’s displayed or actual speed. Real tire dimensions, rolling behavior, wear and vehicle calibration need their own evidence. Do not use a nominal-size calculation to certify speedometer accuracy.
Width, rim approval and clearance are separate checks
A 315 code does not identify the measured tread width, and it does not establish an approved wheel-width range. Obtain the technical specification for the exact tire model and size. Record its measuring rim separately from the manufacturer’s permitted rim range.
Our width and contact-patch guide explains the different measurements. The vehicle also needs appropriate wheel offset, load capacity, brake clearance and space through steering and suspension movement. A wider outer fender does not prove inner clearance.
For AWD, use the vehicle manufacturer’s matching requirements. Nominal percentage calculations cannot replace them. For different front and rear sizes, retain the approved combination instead of selecting each axle by appearance.
Put the cost beside the complete specification
When comparing VCORSA candidates, use the current size catalog and obtain the exact SKU’s released technical information. Confirm the required load and speed capability and permitted wheel application before ordering.
Request a complete installed quote through the buying checklist. This article does not attach a fixed fuel-economy penalty, grip gain or tire-weight difference to any width. Those outcomes require an appropriate comparison, not an inference from the first three digits on the sidewall.
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