Tire Size Calculator
The Tire Size Calculator compares two tire sizes (in the standard width/aspect-ratio/rim format) to show the difference in overall diameter and how it affects speedometer accuracy. The two most common reasons to run this comparison are "plus sizing" — moving to a larger wheel with a lower-profile tire while keeping the overall diameter close to stock — and simply checking fitment before buying a non-standard replacement size. Because anti-lock brakes, traction control, and stability control are all calibrated to the factory tire's rotational speed, a large enough diameter mismatch can throw off those systems in addition to your speedometer and odometer.
How This Tool Works
Enter both tire sizes in width/aspect-ratio/rim format (found on your tire's sidewall, e.g. 225/60R16) to compare their overall diameters.
Formula & Method
Sidewall height (mm) = tire width × (aspect ratio ÷ 100). Overall diameter (in) = (sidewall height × 2 ÷ 25.4) + rim diameter.
Example Calculation
Example: comparing a stock 215/65R16 to an aftermarket 225/60R17 ("plus one" fitment). Original: sidewall height = 215 × (65 ÷ 100) = 139.75 mm; diameter = (139.75 × 2 ÷ 25.4) + 16 ≈ 27.00 in. New: sidewall height = 225 × (60 ÷ 100) = 135 mm; diameter = (135 × 2 ÷ 25.4) + 17 ≈ 27.63 in. The new tire is about 2.32% larger in diameter — within the commonly recommended 3% tolerance — and your speedometer would read 60 mph at a true speed of about 61.4 mph, since the larger tire covers more ground per wheel rotation than the (still stock-calibrated) speedometer accounts for.
About the Tire Size Calculator
A tire size such as 225/60R16 is a compact code for three measurements: the tread width in millimeters, the sidewall height as a percentage of that width, and the diameter of the wheel in inches. What most people want to know when comparing two sizes is simpler than any of those individual numbers: will the new tire be about as tall as the old one? This calculator converts both codes into an overall diameter in inches, compares them, and shows the percentage difference along with what your speedometer will really mean after the change.
Overall diameter matters because your vehicle's speedometer, odometer, and several electronic systems are calibrated to the rate at which the original tires turn. A taller tire covers more ground with each revolution, so the dash shows a lower speed than you are truly traveling. A shorter tire does the opposite. Anti-lock brakes, traction control, and stability systems also compare wheel speeds, and a big mismatch from the factory size can confuse them, which is why many tire professionals suggest staying within a few percent of the original diameter.
The tool is especially useful for plus sizing, where you fit a larger wheel and a lower-profile tire so that the overall diameter barely changes. It is equally handy for checking whether a cheaper or more available size is a reasonable substitute, or for estimating how much a taller off-road tire will throw off your speedometer. It answers the diameter question precisely, but remember that it does not check clearance, load rating, or wheel width, which also decide whether a tire is suitable.
When to Use It
Plus-sizing to larger wheels
An owner who wants 17-inch wheels instead of the stock 16s needs a tire with a shorter sidewall to keep the overall height nearly unchanged. Entering the stock size and a candidate size shows whether the new tire's diameter lands within a fraction of a percent of the original, preserving speedometer accuracy and gearing.
Fitting a lifted truck or off-road tire
A truck owner adding taller all-terrain tires can see exactly how far the diameter grows and how far the speedometer will read low. A figure of several percent indicates that the speedometer will need recalibration, and that fender or suspension clearance deserves a physical check before purchase.
Buying a replacement when the stock size is hard to find
When the exact factory size is unavailable or expensive, a nearby size may work. By comparing the diameter of the stock tire to alternatives, a buyer can pick a substitute that stays within a small percentage and avoids problems with the speedometer, gearing, and driver-assist systems.
Preparing winter tire and wheel packages
Many drivers buy a second set of wheels for winter, often one inch smaller with a taller sidewall. Comparing the winter size against the summer size shows whether the two sets have close to the same rolling diameter, so the car behaves consistently when switching between them.
How to Use It, Step by Step
- Find your current tire size on the sidewall of a tire, or on the sticker inside the driver's door jamb. It looks like 205/55R16, where 205 is width, 55 is aspect ratio, and 16 is rim diameter.
- Enter the first number as Original width, in millimeters. Valid values in this tool run from 100 to 400, which covers nearly all passenger car, SUV, and light truck tires.
- Enter the second number as Original aspect ratio. This is the sidewall height expressed as a percentage of the width, accepted from 20 to 100. A lower number means a shorter, sportier sidewall.
- Enter the number after the R as Original rim size, in inches. The tool accepts rim sizes from 10 to 30 inches.
- Repeat the three entries for the tire you are considering, using New width, New aspect ratio, and New rim size. Take these from the retailer's listing or the sidewall of the candidate tire.
- Press Compare Tire Sizes and look at Original tire diameter and New tire diameter, both shown in inches, to see the absolute heights of the two tires.
- Check the highlighted Diameter difference. A small percentage close to zero means a near-identical rolling diameter; the tool's own note flags differences above about 3 percent as significant for speedometer accuracy and gearing.
- Read True speed when speedometer reads 60 mph to see the real-world effect on speed and odometer accuracy, then confirm width, load rating, and clearance with a tire professional.
Understanding Your Results
Original tire diameter and New tire diameter are computed from the sidewall height, doubled to cover top and bottom, converted from millimeters to inches, and added to the rim diameter. These are nominal calculated sizes. Real tires differ slightly by brand and by the width of the wheel they are mounted on, and a tire measured with a tape may be a few tenths of an inch off the figure shown.
Diameter difference is the headline number. A positive percentage means the new tire is taller; negative means shorter. Within roughly plus or minus 1 percent is effectively the same for practical purposes. Between about 1 and 3 percent is usually acceptable but worth noting. Beyond 3 percent, the speedometer error, gearing change, and potential interference with electronic systems become real concerns, and you should talk to a tire shop or the vehicle manufacturer.
The last line converts that percentage into a speed. If it says your true speed is 64.2 mph when the speedometer shows 60, the speedometer reads about 7 percent low, and your odometer will under-report distance by the same proportion. That matters for speeding tickets, service intervals, and fuel-economy tracking. A shorter tire has the reverse effect: the speedometer reads high, which is safer for tickets but still inaccurate.
More Worked Examples
Plus-one upgrade that barely changes height
Scenario: Original 205/55R16 compared with a 225/45R17 on a larger wheel.
Enter 205, 55, 16 as the original and 225, 45, 17 as the new size. The tool shows an Original tire diameter of 24.88 in and a New tire diameter of 24.97 in. The Diameter difference is 0.38 percent, and the true speed at a 60 mph speedometer reading is 60.2 mph. That is an excellent match; the speedometer error is a couple of tenths of a mile per hour, well inside normal tolerance.
Taller tires on a light truck
Scenario: Original 265/70R17 compared with a 285/75R17 on the same rim size.
Enter 265, 70, 17 and 285, 75, 17. The original diameter is 31.61 in and the new diameter is 33.83 in, a Diameter difference of 7.04 percent. At a speedometer reading of 60 mph, the true speed is 64.2 mph. The new tire is about 2.2 inches taller, so the wheel center sits about 1.1 inches higher, and fender clearance, gearing, and speedometer recalibration all deserve attention.
Large-wheel upgrade with a low-profile tire
Scenario: Original 195/65R15 compared with a 215/40R18 on a big wheel.
Enter 195, 65, 15 and 215, 40, 18. The original diameter is 24.98 in and the new diameter is 24.77 in. The Diameter difference is negative 0.84 percent, and the true speed at a 60 mph reading is 59.5 mph, so the speedometer reads slightly high. The overall diameter is nearly unchanged, but a 40 series sidewall transmits more road shock, and a thin sidewall offers less protection against potholes.
Tips for Better Results
- Aim to keep the new diameter within about 3 percent of the original, and ideally within 1 percent, so that your speedometer, odometer, and driver-assist systems keep behaving as designed.
- When plus sizing, pick the new aspect ratio so the overall diameter stays near stock. As the wheel diameter goes up by an inch, the sidewall must typically shrink by roughly the same total height, which is why aspect ratios drop as wheels get larger.
- Check the load index and speed rating on any replacement tire. This tool compares only diameter, and a tire with a lower load capacity than the original can be unsafe even if its diameter matches perfectly.
- Verify clearance physically for any significantly wider or taller tire. Turn the steering lock to lock and compress the suspension, because a tire that fits when parked can rub in a turn or over a bump.
- Match tire sizes across an axle, and be especially careful with all-wheel-drive vehicles. Many AWD systems are sensitive to differences in rolling diameter between tires, and mismatches can cause drivetrain wear.
- If you do change diameter, consider having the speedometer recalibrated, or use a GPS speed reading to check the true speed. Many modern vehicles can be recalibrated by a shop or dealer.
Common Mistakes to Avoid
- Ignoring wheel width. A tire needs to be mounted on a wheel within an approved width range, and this tool does not check that. Look up the manufacturer's approved rim widths for any new tire.
- Reading the aspect ratio as a height in millimeters. It is a percentage of the width, so a 55 aspect ratio on a 205 tire means a sidewall of 55 percent of 205 mm, not 55 mm.
- Using only the width to judge size. A wider tire is not necessarily a taller one; the aspect ratio and rim size determine diameter, so always compare the overall diameter rather than the first number alone.
- Assuming a small percentage is always harmless. Even a 3 percent difference matters on vehicles with tight tolerances, and some manufacturers specify tighter limits, so check your owner's manual.
- Forgetting about the spare tire. A full-size spare of a different diameter can upset the drivetrain on AWD vehicles if used for long, and the right choice depends on the vehicle's guidelines.
Assumptions and Limits
- Diameter is calculated from the nominal printed numbers using twice the sidewall height plus the rim diameter. Actual tire dimensions depend on the manufacturer, the tread depth, and the wheel width, so measured diameters can differ by a few tenths of an inch.
- The speed conversion assumes the speedometer is calibrated to the original tire and that the vehicle reads exactly right with it. Many speedometers are intentionally set slightly high, so the real-world error with a new size may differ.
- The tool compares only diameter. It does not evaluate load capacity, speed rating, clearance, ride quality, braking performance, or whether the combination is approved by your vehicle manufacturer.
- Tire wear changes diameter over time. A worn tire is smaller than a new one by a fraction of an inch, so a precise comparison should account for the tread depth. Consult a tire professional for any safety-related decision.
Key Terms
- Aspect ratio
- The height of the tire's sidewall expressed as a percentage of its width. A lower aspect ratio means a shorter sidewall and a sportier, stiffer feel.
- Section width
- The width of the tire from sidewall to sidewall in millimeters, shown as the first number in the tire code.
- Rim diameter
- The diameter of the wheel on which the tire is mounted, in inches. It is the number following the R in the tire size.
- Overall diameter
- The total height of the mounted tire from the ground to the top, calculated as rim diameter plus two sidewall heights.
- Plus sizing
- Installing a larger wheel with a lower-profile tire so that overall diameter stays close to the original. Plus one means a wheel one inch larger.
- Load index
- A number on the sidewall that represents the maximum weight a tire can carry. A replacement should meet or exceed the original load index.
Related guides
Frequently Asked Questions
What do the numbers in a tire size mean?+
In "225/60R16," 225 is the tire width in millimeters, 60 is the aspect ratio (sidewall height as a % of width), and 16 is the rim diameter in inches.
How much diameter difference is safe?+
Most tire professionals recommend staying within about 3% of the original overall diameter to avoid speedometer errors and clearance or handling issues.
Does a different tire size affect my speedometer?+
Yes — since speedometers calculate speed from wheel rotations, a larger tire diameter travels farther per rotation, making your speedometer read slower than your true speed.
Does a tire size change affect fuel economy, not just my speedometer?+
It can. Larger, heavier tires increase rotating mass and rolling resistance, which can modestly reduce real-world fuel economy. They also change your effective gearing, since the engine now turns fewer revolutions for the same distance traveled — which can affect both MPG and how the vehicle feels off the line.
What does 'plus sizing' mean?+
Plus sizing means fitting a larger-diameter wheel with a lower-profile (lower aspect-ratio) tire so the tire's outer diameter stays close to the original — 'plus one' is one inch larger wheels, 'plus two' is two inches larger, and so on. Use this calculator to confirm the resulting diameter stays within a few percent of stock before buying.
Will bigger wheels make my car faster or slower?+
Larger wheels do not by themselves increase speed. If the overall tire diameter stays the same, gearing is unchanged. Heavier wheels and tires add rotating mass, which can slightly dull acceleration and braking response. If the overall diameter grows, the effective gearing becomes taller, which can reduce acceleration slightly and may improve cruising revs, while the speedometer reads lower than your true speed.
What happens to my odometer if I change tire size?+
The odometer counts wheel revolutions and converts them to distance using the original tire's circumference. With a taller tire, each revolution carries you farther than the odometer assumes, so it under-reports distance. With the 7.04 percent change in the large-truck example, 10,000 odometer miles would actually be about 10,700 real miles, which affects warranty mileage and service schedules.
Can I mix tire sizes on the front and rear axles?+
Some vehicles, especially performance cars, come from the factory with different sizes front and rear, in which case the manufacturer's staggered setup is correct. For most other vehicles, matching sizes is expected, and all-wheel-drive models are particularly sensitive to differences in rolling diameter. Check your owner's manual or ask a tire professional before using a combination that differs from the factory specification.
How do I read the DOT date code on a tire?+
Look for the letters DOT followed by a string of characters, ending in four digits. Those four digits show the week and year of manufacture, so 2423 would mean the 24th week of 2023. Tires age even when unused, as rubber compounds degrade over time, so age is worth checking when buying and when deciding whether older tires should be replaced.
Does tire size affect fuel economy and ride comfort?+
It can influence both. Wider tires usually have more rolling resistance, and heavier tires and wheels take more energy to spin, which can modestly reduce fuel economy. Lower-profile tires have less sidewall to absorb bumps, giving a firmer ride and making wheels more vulnerable to pothole damage. A taller, narrower tire often rides more softly. The effect size varies by model, so rely on a tire test rather than a rule of thumb.
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