Tire Size CalculatorTire Size Comparison Calculator & Diameter Converter
Type any tire size — metric like 265/70R17 or flotation like 33×12.50R17 — to get its diameter, width, sidewall height and revolutions per mile. Add a second size to compare them side by side, see your speedometer error, and find equivalent sizes that fit.
Flotation sizes (e.g. 35×12.50R17) work in the text box. Dropdowns are for metric sizes.
Tire 2 is 1.1 in taller than Tire 1.
Side-by-side tire size comparison
| Measurement | Tire 1 | Tire 2 | Difference |
|---|
Speedometer error with Tire 2
| Speedometer shows | You’re actually doing | Difference |
|---|
Alternative sizes within ±3% of Tire 1
Tap a size to load it as Tire 2.
What is a tire size calculator?
A tire size calculator converts a sidewall size such as 265/70R17 into real dimensions — overall diameter, section width, sidewall height, circumference and revolutions per mile. Overall diameter = (width × aspect ratio × 2 ÷ 25.4) + rim diameter, so a 265/70R17 tire is about 31.6 inches tall. Comparing two sizes shows the percentage change, which equals your speedometer error.
How to use the tire size comparison calculator
Find your size
Read it off the tire sidewall or the placard inside the driver’s door jamb — for example P265/70R17 113T.
Enter Tire 1
Type the size or pick width, aspect ratio and rim. This is your current or factory (OEM) tire.
Enter Tire 2
Add the size you’re considering — bigger, smaller, wider or a plus-size on a new wheel.
Read the verdict
Check the diameter difference, speedometer error and clearance change, then tap an alternative size if it’s over 3%.
How to read a tire size on the sidewall
Every passenger and light-truck tire carries the same coded size. Once you know what each part means, the tire size chart stops looking like a secret language.
| Code | What it means | In this example |
|---|---|---|
| P / LT | Tire type: P = passenger, LT = light truck. No letter usually means a Euro-metric tire. | Passenger tire |
| 265 | Section width in millimetres, sidewall to sidewall. | 265 mm = 10.4 in wide |
| 70 | Aspect ratio: sidewall height as a percentage of width. | 70% of 265 mm = 185.5 mm (7.3 in) |
| R | Construction — R is radial, used on almost every modern tire. | Radial |
| 17 | Wheel (rim) diameter in inches. | Fits a 17-inch wheel |
| 113 | Load index — the maximum weight each tire can carry. | 113 = 2,535 lb per tire |
| T | Speed rating — the maximum sustained speed. | T = 118 mph |
Tire diameter formula and how this calculator works
Metric tire sizes mix millimetres and inches, so the tire height calculator converts everything to inches first:
Overall diameter (in) = (2 × sidewall height) + rim diameter
Circumference (in) = diameter × π
Revolutions per mile = 63,360 ÷ circumference
Difference (%) = (new diameter − old diameter) ÷ old diameter × 100
Actual speed = speedometer reading × (new diameter ÷ old diameter)
Flotation sizes are already in inches: a 33×12.50R17 tire is 33 inches tall, 12.5 inches wide and fits a 17-inch wheel. Its sidewall is (33 − 17) ÷ 2 = 8 inches.
Example: 265/70R17 vs 285/70R17
Tire 1 — 265/70R17
Sidewall = 265 × 0.70 ÷ 25.4 = 7.30 in
Diameter = 2 × 7.30 + 17 = 31.61 in
Revs per mile = 63,360 ÷ (31.61 × π) = 638
Tire 2 — 285/70R17
Sidewall = 285 × 0.70 ÷ 25.4 = 7.85 in
Diameter = 2 × 7.85 + 17 = 32.71 in
Revs per mile = 63,360 ÷ (32.71 × π) = 617
The 285 is 1.10 in (3.49%) taller. At an indicated 60 mph you’d really be doing about 62.1 mph, the truck sits about 0.55 in higher at the axle, and the odometer under-records roughly 349 miles every 10,000. This is the default comparison loaded in the calculator above, so you can check every number.
| Speedometer shows | Actual speed on 285/70R17 | Difference |
|---|---|---|
| 30 mph | 31.0 mph | +1.0 mph |
| 45 mph | 46.6 mph | +1.6 mph |
| 55 mph | 56.9 mph | +1.9 mph |
| 65 mph | 67.3 mph | +2.3 mph |
| 75 mph | 77.6 mph | +2.6 mph |
Tire size chart: popular metric sizes in inches
Common car, SUV and truck sizes converted to inches with the same formula the calculator uses. Revs per mile are geometric (unloaded) values.
| Tire size | Diameter | Width | Sidewall | Revs / mile |
|---|---|---|---|---|
| 195/65R15 | 25.0 in | 7.7 in | 5.0 in | 807 |
| 205/55R16 | 24.9 in | 8.1 in | 4.4 in | 811 |
| 215/55R17 | 26.3 in | 8.5 in | 4.7 in | 767 |
| 225/65R17 | 28.5 in | 8.9 in | 5.8 in | 707 |
| 235/65R17 | 29.0 in | 9.3 in | 6.0 in | 695 |
| 235/75R15 | 28.9 in | 9.3 in | 6.9 in | 698 |
| 245/75R16 | 30.5 in | 9.6 in | 7.2 in | 662 |
| 255/70R18 | 32.1 in | 10.0 in | 7.0 in | 629 |
| 265/70R17 | 31.6 in | 10.4 in | 7.3 in | 638 |
| 275/60R20 | 33.0 in | 10.8 in | 6.5 in | 611 |
| 275/70R18 | 33.2 in | 10.8 in | 7.6 in | 608 |
| 285/70R17 | 32.7 in | 11.2 in | 7.9 in | 617 |
| 285/75R16 | 32.8 in | 11.2 in | 8.4 in | 614 |
| 295/70R18 | 34.3 in | 11.6 in | 8.1 in | 589 |
| 305/70R17 | 33.8 in | 12.0 in | 8.4 in | 596 |
| 315/70R17 | 34.4 in | 12.4 in | 8.7 in | 587 |
What size is a 33-inch or 35-inch tire in metric?
There’s no single metric equal to a “33” — it depends on your wheel size. These are the closest common metric tire sizes by calculated diameter:
33-inch tire metric equivalents
- 16-inch wheel: 285/75R16 (32.8 in)
- 17-inch wheel: 285/70R17 (32.7 in) or 305/70R17 (33.8 in)
- 18-inch wheel: 275/70R18 (33.2 in) or 295/65R18 (33.1 in)
- 20-inch wheel: 275/60R20 (33.0 in)
35-inch tire metric equivalents
- 16-inch wheel: 315/75R16 (34.6 in)
- 17-inch wheel: 315/70R17 (34.4 in)
- 18-inch wheel: 315/70R18 (35.4 in)
- 20-inch wheel: 325/60R20 (35.4 in)
Type any of these into Tire 2 above to see exactly how it compares with your current size.
The 3% rule, plus sizing and what changes when you go bigger
Tire shops and fitment guides commonly recommend keeping a replacement within about 3% of the original overall diameter. Inside that window, speedometer error stays small and anti-lock brakes, stability control and transmission shift points usually behave as designed. Beyond it, plan on recalibrating and checking clearance.
Plus sizing (keeping the diameter)
“Plus one” means a 1-inch-bigger wheel with a lower-profile tire so the overall height stays almost the same. Example: 225/65R17 (28.5 in) → 235/55R18 (28.2 in). You get a sharper look and crisper steering, but a firmer ride and less sidewall to protect the wheel from potholes.
When you go taller
- Speedometer and odometer read low.
- Acceleration and towing feel weaker — the effective gear ratio gets taller.
- Fuel economy usually drops from added weight and rolling resistance.
- Braking distances can grow with heavier wheels and tires.
- Clearance: check fenders, control arms and full-lock steering.
Common tire size mistakes
- Comparing only the width. A 285 isn’t automatically taller than a 265 — aspect ratio and rim size decide height.
- Treating 33×12.50R17 and 285/70R17 as identical. They’re close, but the 285 is about 0.3 in shorter and 1.3 in narrower.
- Ignoring wheel width. Every tire has an approved rim-width range; a wide tire on a narrow wheel balloons and wears unevenly.
- Mixing sizes on AWD vehicles. Even small diameter differences between axles can strain the transfer case or center differential.
- Forgetting recalibration. Past about 3%, recalibrate the speedometer so you aren’t speeding without knowing it.
Related vehicle calculators
Tire size calculator FAQs
Multiply the width in millimetres by the aspect ratio (as a decimal), divide by 25.4 to get sidewall height in inches, double it, and add the rim diameter. For 265/70R17: 265 × 0.70 ÷ 25.4 = 7.30 in; 2 × 7.30 + 17 = 31.6 inches.
265 is the tire’s section width in millimetres, 70 is the aspect ratio (the sidewall is 70% as tall as the tire is wide), R means radial construction and 17 is the wheel diameter in inches. It works out to about 31.6 inches tall and 10.4 inches wide.
A common guideline is to stay within about 3% of the original overall diameter without other modifications. Going larger is possible, but you’ll typically need to confirm fender and suspension clearance, recalibrate the speedometer and sometimes add a leveling kit or lift.
It depends on the wheel. Close metric equivalents are 285/75R16 (32.8 in) on 16-inch wheels, 285/70R17 (32.7 in) on 17s, 275/70R18 (33.2 in) on 18s and 275/60R20 (33.0 in) on 20-inch wheels.
Popular 35-inch metric equivalents include 315/75R16 (34.6 in), 315/70R17 (34.4 in), 315/70R18 (35.4 in) and 325/60R20 (35.4 in). Metric “35s” often measure a little under a true 35 inches.
Yes. The speedometer reads low by the same percentage the diameter increased. With tires 3.5% taller, an indicated 60 mph is really about 62.1 mph, and the odometer under-records about 350 miles every 10,000.
Often, yes — on the same aspect ratio and rim, a 285 is about 0.8 in wider and 1.1 in taller (for 70-series on a 17-inch wheel, roughly 3.5%). Check that your wheel width is within the tire’s approved range and that the tire doesn’t rub at full steering lock or under suspension compression.
Aspect ratio is the sidewall height as a percentage of the tire’s width. A 55-series tire has a sidewall 55% as tall as it is wide; lower numbers mean a shorter, sportier sidewall and higher numbers a taller, more cushioned one.
Divide 63,360 (the number of inches in a mile) by the tire’s circumference in inches, which is diameter × π. A 31.6-inch tire has a circumference of about 99.3 inches, giving roughly 638 revolutions per mile unloaded.
Usually. Larger, heavier tires add rolling resistance and rotating mass, and aggressive tread adds drag. Your dashboard MPG will also look better than reality because the odometer under-counts miles, so measure fuel use with corrected distance.
They’re close but not the same. A 285/70R17 calculates to about 32.7 in tall and 11.2 in wide, while a 33×12.50R17 is nominally 33 in tall and 12.5 in wide — noticeably wider, which matters for clearance.
Look at the tire and loading information placard on the driver’s door jamb, in your owner’s manual, or on the sidewall of the tires currently fitted. The placard also lists the recommended cold tire pressure.
About the developer
Manzoor Ahmad
Web Designer & Calculator Developer · 13+ years of web development and SEO experienceI’m a professional pharmacist turned tech geek. I designed and coded this tire size calculator myself. I’m not a tire technician, so instead of relying on rules of thumb I built it on the standard sidewall sizing system used across the industry. Every output was checked by hand against the chart and worked example on this page, which are generated from the same formula.
Spotted a size that doesn’t parse, or a number that looks off? Let me know through my author page and I’ll fix and re-date it.
Sources
- National Highway Traffic Safety Administration — Tires: safety, sidewall information and placards
- The Tire and Rim Association — Tire and rim standards (US)
- European Tyre and Rim Technical Organisation — ETRTO standards
- Continental Tire — How to read your tire sidewall
- BFGoodrich — Reading a tire sidewall
Disclaimer: Results are calculated from nominal tire dimensions. Actual measured size varies by brand, tread design, wheel width and inflation. Always confirm fitment, load index and speed rating with your tire retailer or the vehicle manufacturer before buying.
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