Tire Size Comparison Calculator

What the three numbers in 225/45R17 mean

A tire size is a three part code, and each part answers a different question. Read it left to right and it stops looking like noise.

• 225 is the section width in millimeters, measured across the widest part of the inflated tire, sidewall to sidewall. It is not the tread width, which is usually 20 to 30 mm narrower.
• 45 is the aspect ratio, a percentage of the section width. The sidewall is 225 x 0.45, or 101.25 mm tall. A lower number means a shorter sidewall and a stiffer ride, not a narrower tire.
• R17 is the wheel diameter in inches. The R stands for radial construction, which every passenger car tire sold in the US uses, and 17 is the rim size.

Two more numbers often follow the size, such as 94V. The first is the load index, where 94 means 1,477 lb per tire. The letter is the speed rating, where V means sustained 149 mph. Those two carry real safety weight, and dropping below the factory rating is a bad trade for a $40 difference in price.

When people say a tire is a 225, they mean the width. When they say a 45 series, they mean the aspect ratio. Mixing those two up is the most common reason a shopper orders a size that does not fit.

How to compare two sizes in four steps

Every comparison on this page comes from the same four lines of arithmetic. You can do them on a phone calculator in under a minute.

1. Sidewall height equals section width times aspect ratio, divided by 100. For a 225/45 that is 225 x 45 / 100 = 101.25 mm.
2. Rim height equals the wheel size times 25.4, because there are 25.4 mm in an inch. A 17 inch rim is 431.8 mm.
3. Diameter equals the rim plus two sidewalls, one above the wheel and one below. For 225/45R17 that is 431.8 + 202.5 = 634.3 mm.
4. Percent change equals the new diameter minus the old one, divided by the old one, times 100.

Run 225/45R17 against 235/45R17 and the numbers land here: sidewalls of 101.25 mm and 105.75 mm, diameters of 634.3 mm and 643.3 mm, or 24.97 and 25.33 inches. The difference is 9.0 mm, which is 0.35 inch, which is 1.42%. One size wider in the same aspect ratio and rim, and the tire grows by roughly the thickness of a pencil.

Because both tires share a 45 aspect ratio, the wider one is also the taller one. Width and height are tied together by that percentage, which is why a 235/40R17 would be shorter than the stock 225/45R17 instead of taller. Change one number and the others move with it.

Diameter, circumference and revs per mile

Diameter is the number people quote, but circumference is the number the car's computers use. Multiply diameter by pi, 3.14159, and you get the distance one revolution covers. A 225/45R17 rolls 78.45 inches per turn, and a 235/45R17 rolls 79.57 inches.

Divide 63,360, the number of inches in a mile, by that circumference and you get revolutions per mile. The stock tire turns 808 times in a mile. The wider one turns 796 times, twelve turns fewer. That gap of 1.5% is small per mile and adds up over a year of driving.

Circumference also drives the speedometer, the odometer, the trip computer's fuel economy figure and the shift points in an automatic. All of them assume the factory rolling radius. Nothing in the car knows you changed it, so every one of those readings drifts by the same percentage the diameter changed.

The last column is the one to watch. A taller tire covers more ground per revolution, so the car is moving faster than the dash says. The error is proportional, not fixed: 1.42% is 0.85 mph at 60 and 1.07 mph at 75.

Speedometer and odometer error in real numbers

The correction is one multiplication. Real speed equals indicated speed times new diameter divided by old diameter. With a 1.42% taller tire, an indicated 60 mph is 60.85 mph, and an indicated 75 mph is 76.07 mph.

The odometer drifts the same way, in the opposite direction from what most drivers expect. The car counts revolutions and multiplies by the factory circumference, so a taller tire means each counted mile is longer than a real mile. After 10,000 odometer miles you have covered 10,142 real miles. At 12,000 miles a year, the gap is 170 miles annually.

That matters in three places. A lease with a 12,000 mile annual cap is quietly under-counting your mileage by 170 miles a year, which works in your favor. A warranty that runs to 60,000 miles ends 852 real miles early. And a fuel economy display that reads 30 mpg is reporting 29.6 mpg, because it divides real gallons by short miles.

Under-reading also has a safety edge. If the dash reads 60 and you are doing 60.85, the margin over the limit is smaller than you think. At a 3% change, 60 becomes 61.8, and at 80 it becomes 82.4. A driver who sets the cruise at 5 over is now 7 over without touching the pedal.

Some shops can reprogram the computer for a new tire size, and some cannot. Ask before you buy, and get the answer in writing. A dealer flash is usually $100 to $200, which changes the math on a cheap set of tires.

Why a change over 3% is a problem

Three percent is the line most tire shops and plus-size charts use, and it is not arbitrary. It is roughly where the systems that read wheel speed start to disagree with the road.

• Anti-lock brakes and traction control. These compare wheel speeds to each other and to the vehicle's expected acceleration. A 3.5% taller tire changes the expected rate enough to shift when the system intervenes, and on some cars it logs a wheel speed code.
• Transmission shift points. The computer shifts on output shaft speed. A taller tire makes it shift later at the same real speed, which feels like a slipping transmission and raises fluid temperature.
• Clearance. A 225/50R17 is 22.5 mm taller than stock, which is 11 mm closer to the strut and the fender liner at each end. Add a passenger and a hard turn and that gap can close.
• Gearing and braking. Taller tires raise the effective final drive, so acceleration drops and the brakes work against a longer arm. A 3.5% change is small, but it stacks with worn pads and a loaded car.
• Load and inflation. The tire's load capacity is set at a stated inflation pressure, and a taller, heavier tire at the wrong pressure carries less than the sticker assumes. Federal safety rules cover the tire, not your fitment, so the matching is on you.

The 3% figure is a rule of thumb from the plus-size world, not a legal limit. A 2.98% change like 245/40R18 sits just inside it, and a 3.55% change like 225/50R17 sits outside. When a size lands between 3% and 5%, most shops will still mount it, and the honest answer is that you are trading dashboard accuracy and system calibration for looks or a softer ride.

Above 5%, the trade stops being cosmetic. Speedometer error passes 3 mph at 60, ABS behavior changes, and on a car with a tight wheel well the tire may rub at full lock.

Plus sizing without growing the diameter

Plus sizing means a bigger rim with a shorter sidewall, chosen so the overall diameter stays close to stock. The look changes, the gearing does not.

From a 225/45R17, a plus one is 225/40R18 and a plus two is 225/35R19. Both keep the width at 225 mm and shave the sidewall from 101.25 mm to 90 mm and then 78.75 mm, while the diameter moves less than 1%. That is the whole trick: every inch of rim comes out of the sidewall.

Two costs come with the look. A shorter sidewall protects the rim less, so a 35 series tire on a 19 inch wheel dents a wheel on a pothole that a 45 series would absorb. And the ride gets firmer, because the sidewall is the first spring in the system. Going from 101.25 mm to 78.75 mm of sidewall is a 22% reduction in that cushion.

Tire price also climbs with rim size. The same model in 225/35R19 typically costs $40 to $80 more per tire than in 225/45R17, so a set runs $160 to $320 more before installation.

Width, rim width and clearance

Going from 225 to 235 adds 10 mm of section width, which is 5 mm on each side of the wheel centerline. That sounds harmless until you remember the tire has to clear a strut on the inside and a fender lip on the outside.

Rim width matters as much as tire width. A 225 mm tire is designed for rims 7.0 to 8.5 inches wide, and a 235 mm tire for 7.5 to 9.0 inches. Mount a 235 on a 7 inch rim and the sidewall bulges, which dulls steering response and wears the shoulders. Mount a 225 on a 9 inch rim and the sidewall stretches, which exposes the rim to curbs.

Clearance checks worth doing before you order:

• Turn the wheel to full lock in both directions and look for a finger's width between the tire and the inner fender liner.
• Look at the gap between the tire shoulder and the strut, which is where a wider tire usually touches first.
• Check the rear arch with the car loaded, since a full trunk drops the body 15 to 25 mm.
• Confirm the wheel's offset. A 235 on a wheel with the stock offset sits 5 mm further out, and a lower offset pushes it out again.

A wider tire also changes how the car feels in a straight line. It tramlines on grooved pavement more, and it hydroplanes at a lower speed in standing water because the contact patch is wider with the same tread pattern. For a daily driver in a wet climate, a 10 mm jump is not free.

What to check before you buy a different size

Work through this list in order. It catches almost every mistake people make when they swap sizes.

1. Match or exceed the load index. A 94 load index carries 1,477 lb. A 91 carries 1,356 lb. Four tires at 91 still exceed most sedans, but a loaded SUV or a tow vehicle needs the original figure.
2. Match or exceed the speed rating. Going from V to H drops the sustained limit from 149 mph to 130 mph. That is fine on a sedan and not fine on a car that sees track days.
3. Keep the diameter within 3%. Past that, the speedometer, ABS and gearing all drift enough to notice.
4. Replace in pairs or all four. On an all-wheel-drive car, tire circumference needs to stay within 2/32 inch across all four, or the center differential works constantly and overheats.
5. Relearn the TPMS. New sensors need a relearn tool or a shop visit, and a mismatched sensor set throws a light that never goes out.
6. Check the spare. If the spare is a different diameter from the road tires, a long drive on it stresses the differential.

The federal rules that govern the tire itself are worth knowing. FMVSS 139 sets the endurance, high speed and bead unseating tests every new passenger tire must pass, and the text of the standard is public. The NHTSA tire safety pages cover load, inflation and the age of a tire, and the agency's tire buyer FAQ explains the sidewall markings in plain language.

Sizes, diameters and percentages here are arithmetic, not a fitment guarantee. Confirm clearance, load index and rim width with the tire maker or a shop before you buy, and check the door jamb sticker for the pressure the car was designed around.