🛞 Tire Size Calculator

How to Use the Tire Size Calculator

Enter your stock tire width, aspect ratio, and rim diameter, then the new size you are considering. Press Compare for both overall diameters, sidewall heights, section widths, circumference difference, and the speedometer error at 60 mph. The verdict line tells you whether the change stays inside the safe 3% rule.

Find stock size on the driver door jamb sticker or the tire sidewall itself. If you are plus-sizing (bigger rim, lower profile), enter the new combo exactly as sold. The calculator handles any pairing, including metric-to-flotation conversions you type in manually.

The Formulas

The aspect ratio is a percentage, not a measurement: a 70-series sidewall is 70% as tall as the tire is wide. Two sidewalls plus the rim make the diameter. Because the tire rolls on its circumference, a 3% diameter change is a 3% distance-per-revolution change, which is exactly the speedometer and odometer error.

Worked Example

Stock 265/70R17: sidewall = 265 × 0.70 ÷ 25.4 = 7.30 in; diameter = 17 + 14.61 = 31.61 in. New 285/70R17: sidewall = 7.85 in; diameter = 32.71 in. Difference: +1.10 in, +3.5%. At 60 mph indicated, true speed = 62.1 mph. That exceeds the 3% guideline, so expect to recalibrate the speedometer and check clearance.

The 3% Rule Explained

Vehicle systems are calibrated to the stock rolling diameter: speedometer, odometer, ABS pulse counts, traction control, transmission shift points, and even the computed MPG. Within ±3%, everything stays inside its tolerance band. Beyond it, errors compound: your odometer under-reads mileage (affecting warranty and resale records), shift points go lazy, and stability control intervenes at wrong thresholds.

The rule also protects gearing. Taller tires raise the effective gear ratio, making the engine lug; on trucks this costs towing performance and can overheat transmissions working outside their designed range. Re-gearing the axles restores performance but costs real money, so factor it into the upgrade budget.

Speedometer and Legal Exposure

A speedometer reading low is a speeding ticket waiting to happen. With +3.5% tires, 70 mph indicated is 72.5 actual, enough to matter in strict zones. The reverse (smaller tires) reads high, which is safe legally but racks up odometer miles faster than reality. Either way, verify with a GPS app after any change and recalibrate when beyond 3%.

Insurance and inspection angles exist too. Some policies require notifying them of size changes, and a few states inspect for tires protruding past fenders or rubbing. Keep the stock set if you lease; most lease agreements require returning the car on the original size.

Clearance Checks Before Buying

Diameter is only half the fitment question. Turn the steering to full lock and check the front tires against control arms, frame, and fender liners; wider tires rub where taller ones clear. Check suspension compression too: a tire that clears parked can shred a liner over a bump. Aftermarket wheels change offset, moving the tire inboard or outboard.

Load rating must meet or exceed stock. A bigger tire with a lower load index is a downgrade, not an upgrade, especially for towing. Speed rating matters less for trucks but should not drop below the vehicle original specification. Match or beat both ratings, not just the size.

Effects on MPG and Performance

Bigger tires cost fuel three ways: more rolling resistance, more rotational mass to accelerate, and taller effective gearing. Real-world losses of 1-3 MPG per size jump are typical on trucks. Aggressive all-terrain tread adds its own penalty over highway tread, sometimes another 1-2 MPG. Budget the fuel cost over the tire life before falling in love with the look.

Acceleration softens for the same reasons, most noticeably from a stop and when towing. Braking distances grow slightly since the brake system was sized for the stock diameter and mass. None of this forbids upsizing; it just prices it honestly so the decision includes the running costs, not only the purchase price.

Plus-Sizing Guide

Plus-sizing keeps overall diameter constant while increasing rim size: 265/70R17 becomes 265/60R19, for example. Done right, the speedometer never knows. The tradeoff is less sidewall, which sharpens handling but transmits more impact harshness and raises wheel damage risk on potholes. It is a looks-and-handling move, not a comfort one.

When plus-sizing, verify the new wheel clears the brake calipers; bigger brakes are exactly why many cars cannot downsize but can upsize. Keep load rating equal or better, and get an alignment afterward since scrub radius changes with offset. Done carefully, plus-one sizing transforms a car stance without breaking its engineering.

A Second Worked Example: The Proper Plus-Size

Stock 265/65R17 going to 20-inch rims. Sidewall stock: 265 times 0.65 divided by 25.4 is 6.78 in; diameter is 17 plus 13.56, or 30.56 in. Try 265/50R20: sidewall is 265 times 0.50 divided by 25.4, or 5.22 in; diameter is 20 plus 10.43, or 30.43 in. Difference: minus 0.13 in, minus 0.4 percent. Inside the 3% rule with room to spare. The speedometer barely notices, the gearing is untouched, and the truck gets the big-wheel look without the big-wheel problems.

This is what plus-sizing done right looks like: rim grows, sidewall shrinks, diameter stays. The wrong version grows everything and wonders why the transmission hunts and the speedometer lies. The calculator's verdict line exists to tell the two apart before money changes hands.

Load Index and Speed Rating: Do Not Downgrade

Diameter is not the only number on the sidewall. The load index, like 115, is the max weight each tire carries; the speed rating, like T or H, is the max sustained speed. When changing sizes, never go below the stock load index, especially on trucks and SUVs that tow. A tire that fits the rim but carries less weight is a blowout waiting for a hot highway day.

Speed ratings can move up freely but should not move down below what the vehicle needs. And check the manufacturing date code: tires age out around 6 to 10 years regardless of tread, so a "new" old-stock tire at a bargain price may already be halfway through its safe life. Diameter gets the attention; load index gets the safety.

Common Tire-Size Mistakes

First: comparing width alone. A 285 is not automatically taller than a 265; the aspect ratio decides the sidewall, and a 285/60 can be shorter than a 265/70. Compare diameters, not widths. Second: forgetting the spare. A full-size spare of the old diameter plus three new tires of a different diameter is a drivetrain problem on all-wheel-drive vehicles, which demand matched rolling diameters.

Third: ignoring load index, covered above but the most dangerous mistake on the list. Fourth: no clearance check at full lock and full compression. A tire that clears in the driveway rubs on the trail or with a full load. Fifth: skipping the recalibration. Beyond 3%, fix the speedometer; your odometer, warranty mileage, and speeding-ticket exposure all depend on it.