Tire size calculator

Enter a tire size, get the diameter, width and revs per mile. Compare two sizes for the speedometer error.

	275/65R20	315/70R17	Diff
Diameter	34.1″	34.4″	+0.8%
Width	10.8″	12.4″	+14.5%
Sidewall	7.0″	8.7″
Circumference	107.0″	108.0″
Revs / mile	592	587

Recommended Tire Care & Emergency Gear

Protect your tire investment, prevent uneven tread wear, and stay prepared for highway roadside emergencies.

Driver Equipment

Roadside Power Roadside Must

NOCO Boost Plus GB40 1000A 12V Lithium Jump Starter

Ultra-compact 1000-amp lithium jump starter rated for gasoline engines up to 6 liters and diesels up to 3 liters. Delivers up to 20 jump starts per charge with spark-proof technology and reverse polarity safety protection.

Revives dead vehicle batteries instantly without needing a second jump car

Check Price on Amazon

Emergency Air Pro Compressor

Heavy-Duty 12V Metal Cylinder Portable Tire Inflator

High-output direct battery clamp air compressor delivering up to 120 PSI. Inflates standard passenger tires and oversized SUV/truck tires (up to 33 inches) in minutes with thermal overload cutoff protection.

Eliminates roadside strandings and avoids coin-operated gas station air pumps

Check Price on Amazon

Wheel Service Torque Spec

LEXIVON 1/2-Inch Drive Click Torque Wrench (10-150 ft-lb)

Pre-calibrated reversible click-stop torque wrench with electro-black finish and dual-range highlighted scale. Ensures wheel lug nuts are tightened to exact manufacturer torque specifications.

Prevents warped brake rotors, broken wheel studs, and loose lug nuts

Check Price on Amazon

Pressure Calibration Driver Essential

AstroAI Heavy-Duty Digital Tire Inflator Gauge (250 PSI)

Commercial pistol-grip inflator gauge with backlit LCD, heavy-duty brass air chuck, and quick-disconnect bleeder valve. Delivers precise 0.1 PSI resolution for exact cold-pressure balancing.

Maximizes tire contact patch, fuel efficiency, and tread longevity

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Puncture Repair Flat Defense

Boulder Tools Heavy-Duty All-Metal Tire Plug Repair Kit

Heavy solid steel T-handle spiral reamer and split-eye insertion needle with self-vulcanizing rubber repair strings. Plugs nail punctures and screw holes in steel-belted radial tires without demounting wheels.

Permanently seals tread punctures on the shoulder in under 10 minutes

Check Price on Amazon

Safety Inspection Wear Inspector

Digital Tire Tread Depth Gauge with Backlit LCD Screen

High-precision hardened probe gauge measuring remaining rubber depth in 32nds of an inch and millimeters. Features one-touch zero tare to detect early suspension camber wear.

Alerts you before tread wears past the critical 4/32" wet hydroplane threshold

Check Price on Amazon

Tire Maintenance & Wheel Service Matrix

Standard vehicle maintenance intervals to maximize tread lifespan, fuel economy, and highway braking stability.

Service Task	Recommended Equipment	Service Interval	Vehicle Safety & Handling Impact
Cold Pressure Calibration	Digital Inflator Gauge (0.1 PSI resolution)	Monthly or before highway road trips	Maintains uniform footprint, avoids edge-shoulder wear, and saves fuel.
Lug Nut Torque Verification	1/2-Inch Drive Click Torque Wrench	Every tire rotation or 50 miles after wheel installation	Eliminates wheel wobble, sheared wheel studs, and warped brake discs.
Tread Depth Inspection	Digital Tread Depth Gauge	Every 3,000 miles across all 4 tires	Catches toe or camber alignment issues before tires fail wet braking tests.
Roadside Puncture Sealing	T-Handle Vulcanizing Plug Kit	On-demand when encountering road debris	Seals nail punctures in the tread crown without waiting for roadside tow trucks.
Mobile Air Delivery	12V Battery-Clamp Air Compressor	Pre-trip safety and roadside emergency inflation	Re-inflates low tires after slow bead leaks or temperature drops.
Emergency Powertrain Start	1000A Lithium Jump Pack	Vehicle emergency glovebox standby	Provides standalone cranking amps during cold morning battery drains.

Mechanic Tip

Cold Inflation Calibration Rule: Tire pressure drops roughly 1 PSI for every 10 degrees Fahrenheit drop in ambient temperature. Furthermore, driving even 1 mile generates internal friction that raises tire pressure by 3 to 5 PSI above true ambient resting level. Never set your tire pressure to the "Max PSI" molded on the tire sidewall, which indicates the maximum mechanical burst rating of the casing. Always calibrate tire pressure when the vehicle has rested for at least 3 hours, matching the cold PSI specified on the yellow-and-white driver-side door jamb placard.

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This tire size calculator turns a stamped size like 275/65R20 into the numbers you actually need: overall diameter, section width, sidewall height, circumference and revolutions per mile. Compare a second size and it draws both outlines and shows the speedometer error. No ads above the fold, no "shop this tire" funnel. The arithmetic is the product.

How the tire size calculator reads a size

A metric size is three numbers. Width in millimetres, aspect ratio as a percent of that width, then the rim diameter in inches after the R. Sidewall height is width times aspect ratio. Overall diameter is the rim plus two sidewalls. Circumference is diameter times π. Revolutions per mile is 63,360 inches divided by that circumference. The how to read tire size page walks the rest of the sidewall (load index, speed rating, LT vs P).

A flotation size such as 33x12.50R15 is already in inches: overall diameter, section width, rim. Same geometry, one less conversion. Paste either stamp into the size field and the calculator splits it.

Worked example, the size this page opens on.
Input	Value	Result
Width	275 mm	10.8 in section width
Aspect ratio	65	7.0 in sidewall
Rim	20 in
Overall diameter		34.1 in
Circumference		107.0 in
Revs / mile		592

Tire size comparison and speedometer error

Open "Compare a second size" for a tire size comparison on the same drawing. The orange outline is the new size. Diameter difference is the speedometer error: a taller tire makes the speedometer under-read actual speed by that same percent. At 60 indicated, a 5% taller tire is doing 63 mph. The comparison calculator starts with that toggle already open. The speedometer calculator is the same pair with the error in prose under the tool.

A common swap on a half-ton is 275/65R20 (34.1 in) to something in the 35-inch family. 315/70R17 is 34.4 in, almost a wash. 35x12.50R17 is a true 35. The 33 vs 35 tires page is that pair in one place.

Tire size chart and diameter chart

The tire size chart lists every size in this catalog with diameter, width and sidewall. The tire diameter chart sorts the same list by height, which is the view you want when you are trying to stay inside a lift or a fender. Each row opens a converter page for that size, for example 275/65R20 in inches.

Those per-size pages exist because people search the stamp plus "in inches." The calculator is the same tool; the page is the numbers already filled in, plus the nearest neighbours so you can see what a +1% or +3% swap looks like.

How to read tire size, and what the numbers mean

Tire size meaning is the short version of the stamp. How to read tire size is the longer walk around the sidewall. Tire aspect ratio is the middle number on its own: why 65 and 55 on the same width are not the same tire, and why dropping aspect ratio on a larger rim (plus sizing) can keep diameter close.

The tire size converter is this calculator pointed at unit conversion: millimetres to inches, metric to flotation, and the usual "is 275/65R20 a 33?" question (it is 34.1, so no). Tire size conversion is the same job. If you already know the two stamps you want to compare, skip the explainer and use the toggle above.

Diameter is not a fitment guarantee

These numbers tell you whether two sizes are in the same family. They do not tell you whether a size clears your fenders, matches your wheel width, or is load-rated for the vehicle. Check the door-jamb sticker and the well before you buy. We do not have a per-vehicle fitment dataset and will not invent one.

Plus sizing, rims, load index

Plus sizing is a larger rim with a shorter sidewall, aimed at keeping overall diameter close to stock. What rim fits my tire is the width window a given section usually sits on, not a bolt-pattern lookup. Tire load index and the speed rating chart are the two tokens after the size that this calculator ignores on purpose: they are not geometry.

Taller, wider rubber changes more than the speedometer. Bigger tires and gas mileage is the rolling-resistance and gearing half. Mixing tire sizes is the axle and AWD warning. Neither page replaces a shop, but both beat a forum guess.

Tire size by vehicle

If you came here with a car and not a stamp, start at tire size by vehicle. Honda, Toyota, F-150, Wrangler, Ram, Model 3 and the compact crossovers have their own tables of documented OEM sizes. The door sticker still wins. The table tells you what to look for.

What this calculator is not

Not a shop. There is no "buy this tire" button and there will not be one. The job is the number.
Not a fitment database. Wheel-size.com already does that, and the data is not ours to republish.
Not brand-specific. 275/65R20 is the same diameter on a Michelin as on a no-name. Load range and tread depth change the real-world height by a few tenths; we do not pretend otherwise.
Not a lift guide. Diameter tells you the tire grew. It does not tell you whether the control arm or the fender will accept it.

Use the tool, read the drawing, then decide. The default 275/65R20 result is in the HTML, so the page stays readable with JavaScript off. To put the same calculator on another site, copy the iframe from the embed page.

Frequently asked questions

What does tire size mean (for example 275/65R20)?

275 is the width in millimetres, 65 is the sidewall as a percent of that width, R means radial, and 20 is the rim diameter in inches. Overall diameter is the rim plus two sidewalls: 34.1 inches on a 275/65R20.

How do I read a tire size?

Read it left to right as it is stamped on the sidewall. Width, aspect ratio, construction letter, rim diameter, then load index and speed rating. The calculator uses only the first four. The how-to-read page covers the rest.

What is the difference between 265 and 275 tires?

Ten millimetres of section width, about 0.4 inches. If the aspect ratio and rim stay the same, the sidewall grows too, so overall diameter grows by twice that extra sidewall. Compare the two stamps above to see the exact percent.

Will a different tire size affect my speedometer?

Yes, if overall diameter changes. The speedometer is calibrated to the stock diameter. A taller tire under-reads actual speed by the same percent the diameter grew. A shorter tire over-reads.

What is plus sizing?

A larger rim with a shorter sidewall, chosen so overall diameter stays close to stock. Plus one is usually one inch more rim and a lower aspect ratio. The plus-sizing page works a common example.

How much bigger can I go without a lift?

That depends on the vehicle, the wheel offset and the well, not on a universal inch figure. Stay inside about 3% diameter of stock as a first filter, then check the well. We do not publish per-truck clearance numbers we have not measured.

Do bigger tires affect gas mileage?

Usually yes, and not in your favour. More rotating mass, a taller gearing effect, and often a more aggressive tread. The mileage page is the mechanism, not a promise of a specific mpg loss.

What is the difference between tire diameter and tire size?

Tire size is the stamped code (275/65R20). Tire diameter is one result of that code: how tall the tire is. Two different stamps can share a diameter. The diameter chart groups sizes that way.

How do I compare two tire sizes?

Enter both stamps and read diameter, width and the percent difference. The orange outline is the new size. If you care about speedometer error, look at the 60 mph line under the table.

What do the load index and speed rating mean?

They are the two tokens after the size, for example 116T. Load index is the maximum load per tire. Speed rating is the maximum sustained speed the tire is built for. Neither changes the diameter this calculator reports.

Can I mix tire sizes on the same axle?

Do not. Same axle should be the same size, load range and tread. Mixing left and right, or front and rear on AWD, is how you wear a differential. The mixing page lists the rare exceptions.

What size rim fits my tire size?

Each section width has an approved rim-width window, not a single number. A 275 tire commonly sits on an 8 to 9.5 inch rim. The rim page is the window, not your bolt pattern or offset.

How do I know if a tire will fit my wheel well?

Diameter and width tell you whether it is in the same family as stock. Fit in the well also depends on offset, fender height and suspension. Measure the well or ask a shop that has done that truck. This tool will not guess.

Is 275/65R20 a 33-inch tire?

No. 275/65R20 is 34.1 inches overall. A 33x12.50R15 is a true 33. People say "33s" loosely for anything in that height band; the calculator reports the geometry, not the slang.

How do I convert a tire size to inches?

Divide the millimetre width by 25.4, multiply by the aspect ratio for the sidewall, then add the rim and twice the sidewall. The converter page does that for any stamp and lists the high-volume sizes already done.