How do I convert metric tire sizes to inches?

A metric tire like 285/70R17 means: 285mm width, 70% aspect ratio, 17" rim. The sidewall height is 285 × 0.70 = 199.5mm (7.85"). Total diameter = 17 + (2 × 7.85) = 32.7 inches. This calculator does all the math instantly.

What's the difference between metric and standard tire sizing?

Metric sizes (e.g., 285/70R17) list width in mm, aspect ratio as a percentage, and rim size in inches. Standard sizes (e.g., 35x12.50R17) list overall diameter and width in inches. Both describe the same tire, just different notation systems.

How much bigger is a 35 tire vs a 33?

A 35" tire has 2" more overall diameter than a 33", which translates to 1" more ground clearance. It also increases circumference by about 6%, which affects speedometer accuracy, gear ratio effectiveness, and fuel economy.

Will bigger tires affect my speedometer?

Yes. Larger tires cover more ground per revolution, so your speedometer will read lower than your actual speed. For example, 35" tires on a vehicle calibrated for 31" tires will create about a 13% speedometer error.

What tire size fits my truck without a lift?

Most half-ton trucks fit 33" tires on stock suspension with minor trimming. Wranglers and Broncos can often fit 33s stock. Going to 35" almost always requires at least a 2.5" lift and potential fender modifications.

What is 285/70R17 in inches?

About 32.7 in tall and 11.2 in wide on a 17 in wheel, with a 7.9 in sidewall.

Is a 285/70R17 a 33-inch tire?

Close. On paper it is 32.7 in, so people round it up to a 33. A true 33x12.50R17 is about 0.3 in taller and 1.3 in wider.

What does the aspect ratio mean on a tire?

It is the sidewall height as a percent of the tread width. On a 285/70R17 the sidewall is 70 percent of 285 mm, or 199.5 mm (7.85 in). A higher number means a taller sidewall.

How much speedometer error do bigger tires cause?

The same percent as the diameter change. Going from 265/70R17 (31.6 in) to 285/70R17 (32.7 in) is 3.5 percent bigger, so at an indicated 60 mph you are really doing about 62.1 mph, and at 70 about 72.4.

Interactive Tool

Tire Size and Diameter Calculator

This tire diameter calculator turns any metric size (like 285/70R17) or flotation size (like 35x12.50R17) into inches, then compares two sizes for speedometer error.

Quick Answer: A 285/70R17 tire is 32.7″ tall and 11.2″ wide. To convert metric to inches: multiply width (mm) by aspect ratio, divide by 25.4, double it, and add rim diameter. Use this calculator for instant conversions and side-by-side comparisons.

HOW IT WORKS

Enter Your Tire Size

Type any metric (285/70R17) or standard (35x12.50R17) tire size

Convert or Compare

Instantly see diameter, width, sidewall, and revs per mile

Share & Save

Copy a shareable link or save comparisons for later

Compare Two Tire Sizes

Enter any tire sizes (metric or standard) to see the differences

Current / Stock Tire

New / Upgrade Tire

Diameter Change

+5.4"

(+17.1%)

Width Change

+2.1"

Speedometer Error

+17.1%

Reading slow

At 70 MPH Indicated

81.9 MPH

Actual speed

265/70R17

Diameter31.6"

Width10.4"

Sidewall7.3"

Circumference99.3"

Rev/Mile638

37x12.50R17

Diameter37.0"

Width12.5"

Sidewall10.0"

Circumference116.2"

Rev/Mile545

Speedometer Calibration Recommended

With 17.1% speedometer error, consider calibrating your speedometer. Options include a tuner or inline calibrator.

Re-Gearing May Be Needed

A 17% increase in tire diameter will noticeably affect acceleration and fuel economy. Use our Gear Ratio Calculator to find the ideal gear ratio.

Save Your Results

Get results + personalized gear picks emailed to you.

Next step

Check your gearing

Bigger tires change your effective gear ratio. See if you need to re-gear.

Recalibrate your speedo

Your speedometer reads 17.1% off with the new tire. Fix it.

Understanding Tire Sizes

Metric Sizing (e.g., 285/70R17)

285 = Tire width in millimeters | 70 = Aspect ratio (sidewall height as % of width) | R = Radial construction | 17 = Rim diameter in inches

Standard Sizing (e.g., 35x12.50R17)

35 = Overall tire diameter in inches | 12.50 = Tire width in inches | R = Radial construction | 17 = Rim diameter in inches

Speedometer Error Calculator

Uses the two sizes in Compare Sizes: 265/70R17 (current) and 37x12.50R17 (new).

Diameter difference: +5.4 in (+17.1%)
At an indicated 60 mph you are actually going 70.2 mph
At an indicated 70 mph you are actually going 81.9 mph

Alternate Sizes Within 3%

Metric sizes on the same wheel as 265/70R17 whose overall diameter is within 3 percent, closest first.

285/65R17 (-0.1%)
335/55R17 (-0.3%)
245/75R17 (-0.4%)
305/60R17 (-0.6%)
235/80R17 (+0.6%)
315/60R17 (+0.9%)
225/80R17 (-1.4%)
225/85R17 (+1.4%)
255/75R17 (+1.4%)
295/65R17 (+1.6%)
275/65R17 (-1.7%)
325/55R17 (-1.7%)
255/70R17 (-1.7%)
275/70R17 (+1.7%)
295/60R17 (-2.1%)
235/75R17 (-2.3%)
325/60R17 (+2.4%)
245/80R17 (+2.6%)

How Tire Diameter Is Calculated

A metric size has three numbers. In 285/70R17, 285 is the tread width in millimeters, 70 is the aspect ratio (the sidewall height as a percent of the width) and 17 is the wheel diameter in inches. Here is the math:

Sidewall height: 285 x 0.70 = 199.5 mm, divided by 25.4 = 7.85 in
Two sidewalls (top and bottom): 7.85 x 2 = 15.7 in
Add the wheel: 15.7 + 17 = 32.7 in overall diameter

Flotation sizes do the math for you: a 35x12.50R17 is about 35 in tall and 12.5 in wide on a 17 in wheel. These are on-paper numbers. Real tires vary a little by brand and tread, so check the maker's spec sheet when clearance is tight.

Common 4x4 Tire Sizes in Inches

Size	Diameter (in)	Width (in)	Sidewall (in)	Revs per mile
245/75R16	30.5	9.6	7.2	662
265/70R16	30.6	10.4	7.3	659
265/75R16	31.6	10.4	7.8	637
285/75R16	32.8	11.2	8.4	614
315/75R16	34.6	12.4	9.3	583
245/70R17	30.5	9.6	6.8	661
265/65R17	30.6	10.4	6.8	660
265/70R17	31.6	10.4	7.3	638
275/70R17	32.2	10.8	7.6	627
285/70R17	32.7	11.2	7.9	617
295/70R17	33.3	11.6	8.1	606
315/70R17	34.4	12.4	8.7	587
35x12.50R17	35.0	12.5	9.0	576
37x12.50R17	37.0	12.5	10.0	545
265/65R18	31.6	10.4	6.8	639
265/70R18	32.6	10.4	7.3	619
275/70R18	33.2	10.8	7.6	608
285/65R18	32.6	11.2	7.3	619
285/70R18	33.7	11.2	7.9	598
295/70R18	34.3	11.6	8.1	589
35x12.50R18	35.0	12.5	8.5	576
275/60R20	33.0	10.8	6.5	611
275/65R20	34.1	10.8	7.0	592
285/65R20	34.6	11.2	7.3	583
295/60R20	33.9	11.6	7.0	594
35x12.50R20	35.0	12.5	7.5	576
37x12.50R20	37.0	12.5	8.5	545
37x13.50R22	37.0	13.5	7.5	545

On-paper figures from the standard formula. Flotation sizes are nominal, so a tire sold as a 37 often measures a bit under 37 in.

WHAT CHANGES WHEN YOU GO BIGGER

A tire size change isn't just about looks. Every inch of diameter you add triggers a chain reaction across speedometer accuracy, gearing, fuel economy, and clearance.

Speedometer Error

Your speedometer is calibrated to a specific number of wheel revolutions per mile. Larger tires cover more ground per revolution, so your speedo reads slower than you're actually going. Smaller tires do the opposite.

Stock → 33"

~3-4%

Minor

Stock → 35"

~10-13%

Noticeable

Stock → 37"

~16-19%

Must fix

At 70 mph indicated with 35" tires on a 31" stock vehicle, your actual speed is closer to 79 mph. That's ticket territory.

Fix options: performance tuner, inline calibrator, or dealer reflash.

Gearing Impact

Bigger tires effectively make your gearing taller, your engine works harder to turn those larger, heavier wheels. The result: sluggish acceleration, transmission gear-hunting, and higher RPMs on the highway.

Tire Increase	Effective Gear Change	How It Feels	Regear?
31" → 33"	3.73 → ~3.50 effective	Slightly lazy off the line	Optional
31" → 35"	3.73 → ~3.30 effective	Noticeably sluggish, hunts gears	Recommended
31" → 37"	3.73 → ~3.12 effective	Painful. Transmission overheats.	Required

Use our Gear Ratio Calculator to find the right regear ratio for your setup.

Rubbing & Clearance Risk

Every tire size has three clearance checkpoints: fender well (at full lock), upper control arm (at full compression), and frame/body mounts. Rubbing usually starts at full steering lock on bumps.

NO-LIFT ZONE

Up to 33"

Most trucks and Wranglers. May need minor fender trimming or a leveling kit. Check tire-to-lift guide.

MODERATE LIFT

34"–35"

2.5"–3.5" lift typically required. Fender flares or trimming common. Wheel offset matters, aggressive offset pushes tires outward.

SERIOUS BUILD

37"+

4"+ lift, fender mods, and likely a regear. Check wheel fitment before buying.

Ground Clearance Math

Every 2 inches of tire diameter adds roughly 1 inch of ground clearance, half above, half below the axle centerline. But don't confuse tire size with approach/departure angles, which depend on bumper design.

Ground Clearance Gain

(New Diameter − Stock Diameter) ÷ 2 = inches gained

31" → 33"

+1.0"

31" → 35"

+2.0"

31" → 37"

+3.0"

31" → 40"

+4.5"

Fuel Economy Impact

Bigger, heavier tires increase rolling resistance and rotational mass. Expect 1–3 MPG loss depending on tire weight, tread pattern, and driving conditions. Mud-terrains hit harder than all-terrains.

Run the numbers: MPG Impact Calculator →