Tire Size Comparison Calculator

About Tire Size Comparison Calculator

How Tire Sizes Work

Passenger tire sizes follow a standard format defined by the European Tyre and Rim Technical Organisation (ETRTO) and the Tire and Rim Association (TRA). A typical size like 225/45R17 breaks down into three key measurements:

Part	Example	Meaning
225	225 mm	Section width - the distance across the tire from sidewall to sidewall
45	45%	Aspect ratio - sidewall height as a percentage of the width (225 x 0.45 = 101.3 mm)
R17	Radial, 17"	Radial construction, fits a 17-inch wheel

From these three numbers you can calculate every other dimension. The sidewall height is the width multiplied by the aspect ratio percentage. The overall diameter is the rim diameter (converted to millimeters by multiplying by 25.4) plus two sidewall heights (one on each side). The circumference is pi times the overall diameter, and revolutions per mile is 1,609,344 divided by the circumference in millimeters.

Understanding these calculations matters because the aspect ratio is a percentage, not a fixed height. A 45-series tire on a 225 mm width gives a 101.3 mm sidewall. The same 45 on a 255 mm width gives a 114.8 mm sidewall. This is why you cannot just swap widths without checking what happens to the overall diameter.

Some tire codes include a prefix letter before the width. P indicates P-metric (common in North America for passenger vehicles), LT means light truck, and ST means special trailer. European-market tires usually omit the prefix. The calculator handles all of these formats.

If you want to decode a single tire size in more detail - including load index and speed rating - the tyre size decoder breaks down every character in the code.

How Tire Size Affects Your Speedometer

Your vehicle's speedometer is calibrated for the factory tire size. It counts how many times the wheels rotate and multiplies by the tire circumference to calculate speed and distance. When you fit a different size, the rotation count stays the same but the distance per rotation changes.

The formula is straightforward:

Actual speed = Displayed speed x (New circumference / Original circumference)

Here is a worked example. Say your car came with 205/55R16 tires and you are moving to 225/45R17:

Measurement	205/55R16	225/45R17
Sidewall height	112.8 mm	101.3 mm
Overall diameter	631.9 mm	634.3 mm
Circumference	1,985.2 mm	1,992.7 mm
Revolutions per mile	811	808

With these numbers, when your speedometer shows 60 mph your actual speed is 60 x (1,992.7 / 1,985.2) = 60.2 mph. That is a 0.4% difference, well within the acceptable range. The swap works because the slightly wider tire with a lower profile produces almost the same overall diameter.

If the circumference difference is larger, the effect adds up. A 3% increase in diameter means at a displayed 70 mph you are actually doing 72.1 mph. Over a long highway drive, that also means your odometer under-reads by 3%, which affects trip distance, fuel economy calculations, and service interval tracking.

Legally, speedometer accuracy requirements vary by region. Under EU regulation (UN ECE R39), a speedometer must never under-read - it can show a higher speed than actual, but never a lower one. The US has no federal regulation on speedometer accuracy for passenger cars, but most states reference SAE standards. In practice, factory speedometers typically over-read by 1-3% already, so a slightly larger tire may actually bring your displayed speed closer to your true speed. A significantly larger tire, though, can push the reading the other direction.

The speedometer error also applies to your odometer. If your tires are 2% larger in circumference, your odometer records 2% fewer miles than you actually drive. This means your fuel economy appears better than it really is, your service intervals come later than they should, and lease mileage limits are easier to stay under (though your tires and brakes wear at their actual rate regardless of what the odometer says).

Safe Tire Size Changes

The general rule across the industry is to stay within 3% of the original overall diameter. This threshold exists for several practical reasons:

• Speedometer accuracy - Most countries allow speedometers to over-read but not under-read. A larger tire makes you go faster than displayed, which can mean unintentional speeding.
• ABS and traction control - These systems compare wheel rotation speeds. If tire diameters differ significantly from what the ECU expects, the system may activate incorrectly or fail to activate when needed.
• Clearance - A larger tire may rub against fenders, suspension arms, or wheel wells, particularly during full steering lock or when the suspension compresses over bumps.
• Gearing - The effective gear ratio changes with tire diameter. A larger tire is like a taller gear - slightly less acceleration but lower engine RPM at highway speed.

Your vehicle's door jamb sticker or owner's manual typically lists two or three approved tire sizes. These have been tested by the manufacturer for clearance, load capacity, and system compatibility. When in doubt, stick to those approved sizes.

There are also ride quality trade-offs to consider. A lower-profile tire (smaller aspect ratio) has a shorter, stiffer sidewall that responds more precisely to steering inputs. The downside is less cushioning - you feel bumps and road imperfections more, and the tire is more vulnerable to damage from potholes and curb strikes. Higher-profile tires absorb road irregularities better and tend to be quieter, but they flex more during hard cornering, which can feel less precise. For daily driving on typical roads, the comfort difference between a 55-series and a 45-series tire is noticeable.

Width changes have their own trade-offs. A wider tire puts more rubber on the road, which generally improves dry grip and cornering stability. But wider tires are more prone to hydroplaning in wet conditions because there is more surface area for water to collect under. They also increase rolling resistance slightly, which can reduce fuel economy by 1-2%. In snow, narrower tires actually perform better because they concentrate the vehicle's weight onto a smaller contact patch, cutting through to the road surface rather than riding on top of the snow.

If you are evaluating the broader cost impact of different tire sizes on a vehicle purchase, the total cost of ownership calculator can help you factor in tire replacement costs alongside fuel, insurance, and depreciation.

Common Tire Size Upgrades

Plus-sizing - moving to a larger rim with a lower-profile tire - is the most popular type of tire size change. The goal is to increase the rim diameter by one or two inches while keeping the overall diameter as close to original as possible. Here are some common factory-to-upgrade paths:

Original Size	Plus-Size Option	Diameter Change	Speedo Error at 60 mph
195/65R15	205/55R16	-2.6 mm (-0.4%)	-0.2 mph
205/55R16	225/45R17	+2.4 mm (+0.4%)	+0.2 mph
215/55R17	225/50R17	-11.5 mm (-1.7%)	-1.0 mph
225/45R17	235/40R18	+10.9 mm (+1.7%)	+1.0 mph
215/60R16	225/55R17	+14.9 mm (+2.2%)	+1.3 mph
235/55R17	245/45R18	-12.6 mm (-1.8%)	-1.1 mph

The first two swaps in the table keep the diameter change under 0.5%, which is ideal. The other combinations push closer to 2%, which is still within the 3% guideline but produces a noticeable speedometer error of about 1 mph at highway speeds.

Another common reason for changing tire size is fitting winter tires on a dedicated set of smaller, cheaper steel wheels. Going from 225/45R17 summer tires down to 205/55R16 winter tires is a typical approach - the narrower tire cuts through snow more effectively, and the smaller rim is less vulnerable to pothole damage in poor conditions. The overall diameter difference between these two sizes is under 0.5%, so speedometer accuracy is not an issue.

For comparing running costs between vehicles with different tire requirements, the EV vs gas calculator factors in tire wear as part of total ownership costs. Electric vehicles typically use wider, low-rolling-resistance tires that wear faster due to the instant torque from electric motors.

When shopping for new tires, always match the load index and speed rating to the vehicle manufacturer's specification or higher. A wider tire does not automatically have a higher load capacity - the internal construction and inflation pressure matter more than the tread width. Check the numbers on the sidewall, not just the physical size of the tire.

One detail that catches people out is that tire width is not the same as tread width. The section width (the first number in the tire code) measures from sidewall bulge to sidewall bulge, which is wider than the actual rubber contact patch. A 225 mm tire typically has a tread width of about 195-200 mm. This means that going from 205 to 225 adds roughly 15 mm of tread contact, not 20 mm. The exact tread width also depends on the rim width - the same tire mounted on a wider rim spreads the tread out more than it would on a narrower one. Tire manufacturers publish a recommended rim width range for each size, and staying within that range ensures the tire sits correctly and wears evenly.

Finally, remember that any tire size change affects your spare. If your car has a full-size spare in the trunk, it needs to match the rest. A temporary spare (space-saver) is designed for the factory tire size, and using it alongside significantly different tires can cause handling problems. If you plus-size your wheels, check that your spare arrangement still makes sense.

Sources

• ETRTO - European Tyre and Rim Technical Organisation Standards Manual
• U.S. Tire Manufacturers Association - Tire Sizing and Specifications
• UNECE Regulation No. 39 - Speedometer Equipment Requirements
• NHTSA - Tire Safety and Labelling (FMVSS 139)
• GOV.UK - Tyre load index and speed rating
• Tire Rack - Plus Sizing Concept and Guidelines