Data sheet TS-02 · Calculator
Tire size comparison
This tire size comparison calculator compares up to four sizes to scale, showing nominal diameter, width, sidewall, revolutions per mile and speedometer changes. The drawing shows size differences, not vehicle clearance. Published maker measurements can differ from the formula.
Table 1 — 3 sizes side by side
285/70R17 is 32.71″ tall, +3.49% against 265/70R17. At 60 mph on the dial you drive 62.1 mph.
| Item | 1265/ | 2285/ | 3315/ |
|---|---|---|---|
| Overall diameter | 31.61″ | 32.71″ | 34.36″ |
| vs size 1 | — | +3.49% | +8.72% |
| Section width | 10.43″ | 11.22″ | 12.40″ |
| Sidewall | 7.30″ | 7.85″ | 8.68″ |
| Rim | 17″ | 17″ | 17″ |
| Circumference | 99.3″ | 102.8″ | 108.0″ |
| Revs per mile | 638 | 617 | 587 |
| Dial 60 mph = | — | 62.1 | 65.2 |
| Axle height vs size 1 | — | +0.55″ | +1.38″ |
Fig. 1 — To scale, on one ground line
§ 1
How do you read the comparison?
Start with overall diameter, then compare section width and sidewall height. The scale drawing shows how much taller or wider each nominal size is relative to the reference tire.
A larger diameter means fewer revolutions per mile and a higher true speed at the same indicated speed. Use the speedometer calculator to examine that change; the comparison uses unloaded nominal dimensions, not a maker's measured tire.
§ 2
What changes from 265/70R17?
Use 265/70R17 as the reference for both comparisons below. These dimensions and changes are nominal, calculated by the formula.
At 60 mph indicated, nominal true speed becomes 62.1 mph with 285/70R17 or 65.2 mph with 315/70R17. At 100 km/h indicated, the corresponding speeds are 103.5 km/h and 108.7 km/h. The formula gives odometer error magnitudes of 3.5% and 8.7%, respectively.
- 285/70R17: diameter rises from 31.6 to 32.7 in (803 to 831 mm), adding 1.10 in or 3.5%. Width rises from 10.4 to 11.2 in, adding 20 mm. Sidewall rises from 7.3 to 7.9 in, adding 14 mm. Revolutions per mile fall from 638 to 617. Nominal axle ground clearance increases 0.55 in (14 mm).
- 315/70R17: diameter rises from 31.6 to 34.4 in (803 to 873 mm), adding 2.76 in or 8.7%. Width rises from 10.4 to 12.4 in, adding 50 mm. Sidewall rises from 7.3 to 8.7 in, adding 35 mm. Revolutions per mile fall from 638 to 587. Nominal axle ground clearance increases 1.38 in (35 mm).
§ 3
How do maker measurements differ?
Nitto, Firestone, Kumho, Toyo and Mickey Thompson list 28 tires in 285/70R17 across their size tables. Their published diameters range from 32.76 to 33.15 in, against 32.71 in by the formula: a difference of +0.2% to +1.3%. The listings include 21 LT and 7 P-metric or passenger versions.
The same makers list 15 tires in 315/70R17, including 14 LT and 1 P-metric or passenger version. Their published diameters range from 34.4 to 34.61 in, against 34.36 in by the formula: +0.1% to +0.7%. These maker tables were checked on October 15, 2026.
§ 4
What does the ±3% window mean?
The ±3% overall-diameter window is a tire-shop rule of thumb, not a law or a vehicle maker specification. The neighbor tables use it for the same rim diameter or a rim 1 in larger or smaller.
The reference 265/70R17 sits inside its own comparison window. Both larger examples are outside it: 285/70R17 is +3.5%, and 315/70R17 is +8.7% by the formula. A diameter window alone does not establish suitability; fit depends on the vehicle, wheels and suspension.
Index
Truck and SUV sizes with their own sheet
38 sizes, by rim. Overall diameter by the formula.
15″ rims
Q&A
Common questions
Why does a tire comparison calculator differ from maker specs?
The calculator uses unloaded nominal size math. Nitto, Toyo, Kumho, Mickey Thompson and Firestone publish tire-specific dimensions that can differ from those formula results.
Do bigger tires change the speedometer?
Yes. By the formula, changing from 265/70R17 to 285/70R17 makes 60 mph indicated equal 62.1 mph true; 315/70R17 makes it 65.2 mph true.
Does added tire diameter equal added ground clearance?
No. By the formula, the 1.10 in diameter increase to 285/70R17 adds 0.55 in (14 mm) of ground clearance at the axle.
Is staying within 3% a legal requirement?
The ±3% guideline is a tire-shop rule of thumb, not a law. It is also not a vehicle maker specification.
Notes
Method and sources
Each number and rule comes from a source with the date it was checked.
- Sidewall = width × aspect ratio ÷ 100
- Diameter = rim × 25.4 + 2 × sidewall (mm)
- Revs per mile = 63,360 ÷ (π × diameter, in)
- True speed = dial × new diameter ÷ old diameter
- Clearance change = half the diameter change
Numbers are nominal: the tire unloaded, on its measuring rim. Revs per mile published by makers run higher than the formula. “Within 3%” is a rule of thumb tire shops use, not a law. Tire pressure: the placard on your door jamb.
Related
Other calculators and tables
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- Metric ↔ inchesMetric to inches, and 33x12.50R20 to metric.
- Speedometer errorTrue speed after a size change.
- Gear ratio and RPMEngine RPM and the ratio to regear to.
- Wheel offset and pokeBackspacing, poke and inner clearance.
- DOT date codeHow old a tire is, from the DOT number.
- Load index chartLoad index in pounds and kilograms.
- Bolt patternMeasure it, convert inches to mm.
- Rim size and tire fitWhat 17x8 ET35 means, which sizes go on it.
- What fits my carStart from the placard on the door.
- Tire size chartCommon sizes in inches, by rim.
- Read a sidewallEvery mark on the sidewall.