Guides & answers
All 295 guides →How to tell whether a new tire size will work
Short answer: convert both sizes to overall height in inches and compare. Within about 3% is a close match; 3 to 5% needs a clearance and speedometer check; over 5% is a big change. SidewallMath gives you that verdict first, then the details.
The formula
Worked example: 265/70R17 to 285/70R17
| Step | Result |
|---|---|
| 265/70R17: 17 + 2 × (265 × 0.70 ÷ 25.4) | 31.61 in tall |
| 285/70R17: 17 + 2 × (285 × 0.70 ÷ 25.4) | 32.71 in tall |
| Difference | +1.10 in = +3.49% |
| 60 mph on the speedometer | 60 × 32.71 ÷ 31.61 = 62.09 mph |
| Verdict | Check clearance: ride height +0.55 in |
The 3% rule of thumb is quoted by Discount Tire’s tire size calculator and 1010tires. It is a guideline, not a guarantee of fit. Real tires vary by brand and model.
Why SidewallMath
- The answer first: a verdict, % change, true speed and ride height before any table.
- Any format: 265/70R17, LT285/75R16, 225 45 17, 33x12.50R15.
- The follow-ups: sizes that fit your wheel, regearing, and wheel offset.
- Share, embed and print. No ads.
Frequently asked questions
How do I compare two tire sizes?
Convert each to overall diameter in inches: rim + 2 × (section width in mm × aspect ratio ÷ 100) ÷ 25.4. Then compare. 265/70R17 is 31.6 in tall and 285/70R17 is 32.7 in, a difference of 1.10 in or 3.5%.
How much can I change tire size?
A common rule of thumb, used by Discount Tire's and 1010tires' calculators, is to stay within about 3% of the original overall diameter. It is a guideline, not a guarantee of fit: also check rim width, clearance, load index and speed rating.
Will bigger tires change my speedometer?
Yes. The speedometer counts wheel turns, so a taller tire covers more ground per turn and the speedometer reads low. Going from 265/70R17 to 285/70R17, 60 mph on the speedometer is really 62.1 mph.
What does 265/70R17 mean?
265 is the section width in millimetres, 70 is the sidewall height as a percentage of the width, R means radial construction and 17 is the wheel diameter in inches. That works out to a tire 31.6 in tall and 10.4 in wide.
What size tire is 33 inches tall?
Many metric sizes land near 33 inches. On a 17 in wheel, 255/80R17 is 33.1 in tall, and on 20 in wheels 305/55R20 is 33.2 in. These are calculated: confirm a tire is actually made in that size.
Do I need to regear for bigger tires?
Not always. Taller tires lower the effective gear ratio, so the engine turns slower at a given speed. The gear ratio calculator shows the ratio that restores stock and the nearest common ratio, plus engine RPM at your cruising speed.
Does SidewallMath store my sizes?
No. Everything stays in your browser. Your sizes appear in the page link only so you can share a result.
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