The 205/55R16 to 225/45R17 swap is just 0.38% taller, and the speedometer reads 60.2 mph at a 60 mph reading. The 265/70R17 to 285/75R17 swap is 7.04% taller, and the same reading is an actual 64.2 mph. Diameter is the screening test, and fitment is a physical check.

What a tire comparison actually measures

Comparing two tire sizes is comparing two circles, and the comparison is decided by four numbers: overall diameter, sidewall height, circumference, and revolutions per mile. Overall diameter is the height of the tire standing on the ground, and it is the number that decides whether the swap is safe, because the car’s speedometer, odometer, gearing, and clearance were all designed around the original diameter. Sidewall height is the cushion of rubber between the rim and the road, and it decides ride comfort and rim protection. Circumference is how far the tire rolls in one turn, and revolutions per mile is how many turns it makes in a mile, which is the number the speedometer and odometer were calibrated against.

All four numbers come from one formula. The sidewall is the width times the aspect ratio divided by 100. The diameter is the rim diameter in inches times 25.4, plus two sidewalls, because there is one sidewall above the rim and one below it. The circumference is pi times the diameter. Revolutions per mile is 1,609,344 millimetres, which is one mile, divided by the circumference. The Tire Size Calculator works all four out for both sizes at once, and the difference that matters most is the diameter difference as a percentage: the new diameter minus the old, divided by the old, times 100. Most fitters treat plus or minus 3% as the safe window, and beyond it speedometer error, gearing changes, and clearance problems start to matter in ways that no diameter figure alone can excuse.

The 3% window is a convention, not a law of physics, and it should be treated as a screening test, not a permission. A swap inside the window is a candidate, not a confirmed fit. A swap outside it is a warning, not necessarily a disaster, but it needs a professional to confirm clearance, gearing, and load rating before it is fitted. The window exists because small diameter changes are absorbed by the car’s systems without noticeable effect, while large ones are not, and 3% is where the trade has drawn the line through experience. This guide uses the window the same way the hub page does: as the first check, followed by the fitment checks below, which no percentage can replace.

Plus-sizing: bigger rim, same diameter

Plus-sizing is the art of fitting a larger wheel while keeping the overall diameter the same, by fitting a tire with a shorter sidewall. Going from a 16-inch rim to a 17-inch rim adds 25.4 mm of wheel, and the sidewall must give back 12.7 mm on each side to keep the total unchanged, which means a lower aspect ratio on the same or a slightly wider tire. The classic example on the hub page does exactly this: 205/55R16 to 225/45R17. The rim grows by an inch, the sidewall shrinks from 112.75 mm to 101.25 mm, the width grows by 20 mm, and the overall diameter moves from 631.9 mm to 634.3 mm, a difference of just 0.38%. The car keeps its speedometer calibration, its gearing, and its clearance, and gains a larger wheel with a shorter, stiffer sidewall.

The trade in plus-sizing is ride against response. A shorter sidewall flexes less, so steering feels sharper and the car turns in more crisply, but the tire absorbs less of the road, so bumps reach the cabin more directly and the rim is less protected from potholes. A taller sidewall does the opposite: it absorbs bumps and protects the rim, at the cost of a softer, less precise feel. Neither is better in the abstract. The right choice depends on the roads the car drives, the driver’s priorities, and the maker’s approved sizes, and it should never be made on appearance alone, because a wheel that looks right and a tire that fits are two different things, and only the second one keeps the car safe.

Plus-one and plus-two are the usual steps: plus-one adds an inch of rim, plus-two adds two, and each step needs a lower aspect ratio to compensate. A 225/45R17 to 225/40R18 swap is a plus-one step that moves the diameter from 634.3 mm to 637.2 mm, a difference of 0.46%, which is inside the window. A plus-two step from the same starting size, to a 19-inch rim, needs an even lower aspect ratio, and the sidewall becomes short enough that pothole damage becomes a real risk on poor roads. The calculator’s equivalent-sizes tool lists sizes for the same wheel inside the window, and it is the right place to start a plus-sizing search, because it shows which combinations keep the diameter honest before any other question is asked.

Three swaps, worked in full

The hub page ships with three preset swaps, and they are worth working through because two pass and one fails, and the failure is as instructive as the passes. All diameters below come from the same formula, and the speedometer figures assume a 60 mph reading, which is the speed most drivers check first.

The first swap is the plus-sizing example, and it is an easy pass: a 2.4 mm diameter difference, thinner than a coin, with a speedometer error of 0.2 mph at 60. The second swap is a width and rim change that goes the other way, slightly shorter, with a speedometer that over-reads by 0.2 mph at 60, which is equally harmless. The third swap is a large truck tire change, wider and taller on the same rim, and it fails by a wide margin: 56.5 mm taller, 7.04% larger, with a speedometer that under-reads by 4.2 mph at 60, an odometer that under-records mileage, gearing that is effectively taller, and clearance that must be checked at every point of the suspension travel. A swap like the third needs a professional assessment of the whole vehicle, and it should never be fitted on the strength of a diameter figure alone, because the diameter figure is the warning, not the permission.

Two more swaps show the edges of the window. Changing only the aspect ratio, 205/55R16 to 205/60R16, keeps the width and rim and raises the diameter from 631.9 mm to 652.4 mm, a difference of 3.24%, which is just outside the window, with a speedometer reading of 61.9 mph actual at 60 shown. It looks like a small change, one step of aspect ratio, and it is already too much for the convention. Changing only the width, 205/55R16 to 225/55R16, raises the sidewall from 112.75 mm to 123.75 mm and the diameter to 653.9 mm, a difference of 3.48%, which is also outside. Width changes diameter too, because the sidewall is a percentage of the width, and that is the single most overlooked fact in tire comparison: nothing about a tire size changes alone.

Speedometer, odometer, and gearing

A taller tire rolls farther per revolution, so the car travels farther than its instruments think, and the speedometer under-reads: the car is going faster than the dial says. A shorter tire does the opposite, and the speedometer over-reads. The error scales with the diameter change: actual speed equals the speedometer reading times the new diameter divided by the old diameter. At a 0.38% difference, a 60 mph reading is an actual 60.2 mph, which is negligible. At a 7.04% difference, the same reading is an actual 64.2 mph, which is enough to matter for speed limits, for following distances, and for any driver who trusts the dial. The speedometer table on the hub page shows the actual speed at a range of readings, so the error can be checked at the speeds the car actually drives, not just at 60.

The odometer makes the same error in the other direction: a taller tire under-records mileage, because fewer revolutions cover each mile, and a shorter tire over-records it. Over the life of the car, a persistent 3% error changes the recorded mileage by 3,000 miles in 100,000, which matters for service intervals, for resale, and for any warranty measured in miles. Gearing changes too: a taller tire is effectively a taller final gear, so acceleration softens and the engine turns more slowly at a given road speed, while a shorter tire is effectively a shorter gear, with sharper acceleration and higher revs. None of these effects is catastrophic inside the window, and all of them grow with the diameter difference, which is why the window exists and why it should be respected.

The fitment checklist: beyond the diameter

Diameter is the first check, and the checklist below is the rest. Work through it in order, and do not skip a step because the diameter passed, because a tire can match on diameter and fail on every other measure.

One: clearance. Turn the steering to full lock in both directions and check the tire against the wheel arch, the suspension, and the brake lines, at full suspension travel if the car can be checked that way. A tire that clears standing still can rub under load, on a bump, or at full lock, and rubbing damages the tire and the car. Two: rim width. Every tire width suits a range of rim widths, and a tire fitted to a rim outside its range will not seat or wear correctly. The rim width is stamped on the wheel, and a tire professional can confirm the range for the tire. Three: load index and speed rating. Neither may drop below the placard and the original tire, as the companion guide explains in full. Four: overall width and offset. A wider tire on the same rim may clear the suspension and still foul the arch, and a wheel with different offset moves the tire inboard or outboard, changing both clearances at once.

Five: the spare and the tools. A larger tire may not fit the spare wheel well, and a space-saver spare has its own speed and distance limits that must be respected if it is ever used. Six: tire pressure monitoring. A new wheel and tire combination must work with the car’s pressure monitoring system, whether it reads pressures directly or infers them from wheel speeds, and a swap that confuses the system will warn constantly or, worse, stay silent when a tire loses pressure. Seven: insurance and legality. In some markets, fitting a size outside the maker’s approved list must be declared to the insurer, and in others it can affect roadworthiness. Check both before buying, because a tire that fits the car but voids the cover has not saved any money.

The equivalent-sizes tool on the hub page helps with the first part of this checklist and only the first part. It lists standard sizes for the same wheel inside the 3% window, up to 24 of them, ordered by closeness to the original diameter, and it shows the diameter range the window covers. Treat its list as candidates. The rest of the checklist needs the car, the wheel, and a professional, and no calculator can do it from typed numbers alone. That limit is stated on the hub page, and it is repeated here because it is the most important sentence in this guide: diameter is a screening test, and fitment is a physical check.

When not to swap at all

Do not swap when the only reason is appearance and the size is outside the maker’s approved list. Do not swap when the load index or speed rating would drop. Do not swap when the car is under a warranty or a lease that restricts tire sizes, without checking the terms first. Do not swap a single tire to a different size from the other three, because mismatched diameters across an axle confuse traction control and anti-lock braking, and mismatched sizes front to rear change the car’s balance in ways the driver cannot predict. And do not swap on the strength of a price, because the cheapest tire in a size is not a bargain if the size is wrong for the car. The right size, at a fair price, fitted by a professional who has checked the list above, is the only swap worth making, and the calculator on the hub page is the tool for finding the candidates, not for making the decision.

Frequently asked questions

What is the safe window for a tire size change?

Most fitters treat plus or minus 3% of overall diameter as the safe window. Inside it, speedometer, odometer, and gearing changes are small enough to be absorbed. Beyond it, the speedometer error, clearance, and gearing changes need professional assessment. The window is a screening test, not a permission: a swap inside it still needs the fitment checks in this guide.

How does a bigger tire affect my speedometer?

A taller tire rolls farther per revolution, so the car goes faster than the speedometer says: actual speed equals the reading times the new diameter divided by the old diameter. At a 0.38% difference, a 60 mph reading is an actual 60.2 mph. At a 7.04% difference, the same reading is an actual 64.2 mph. A shorter tire does the opposite, and the speedometer over-reads.

What is plus-sizing?

Plus-sizing is fitting a larger wheel while keeping the overall diameter the same, by fitting a tire with a shorter sidewall. Going from 205/55R16 to 225/45R17 adds an inch of rim and shortens the sidewall from 112.75 mm to 101.25 mm, and the diameter moves by only 0.38%. The steering feels sharper, but the ride is firmer and the rim is less protected from potholes.

Can I change just the rim size and keep the same tire width?

Only if the aspect ratio drops enough to keep the diameter inside the window. A 225/45R17 to 225/40R18 swap is a plus-one step that moves the diameter by 0.46%, which is inside the window. Changing the rim without changing the aspect ratio raises the diameter by the full rim increase, which is far outside the window and is not a swap at all.

Does changing tire width change the diameter?

Yes, because the sidewall height is a percentage of the width. Going from 205/55R16 to 225/55R16 raises the sidewall from 112.75 mm to 123.75 mm and the diameter by 3.48%, which is outside the window, even though the aspect ratio and rim never changed. Nothing about a tire size changes alone, and every change must be checked on the calculator.

What else must I check besides the diameter?

Clearance at full lock and full suspension travel, rim width suitability, load index and speed rating, overall width and wheel offset, spare wheel fit, tire pressure monitoring, and insurance and roadworthiness rules. The equivalent-sizes tool on the hub page checks diameter only. The rest of the checklist needs the car, the wheel, and a tire professional.