Tire Size Comparison Calculator

How to compare tire sizes

The tire size comparison calculator takes two tire sizes in standard metric format and computes every measurement that matters when swapping tires: overall diameter, sidewall height, circumference, speedometer error, and ground clearance change. Enter your original size and the size you are considering, and the results tell you exactly what changes.

The metric tire size notation looks like this: 245/65R17. Reading left to right, 245 is the section width in millimetres, 65 is the aspect ratio (sidewall height as a percentage of section width), R means radial construction, and 17 is the rim diameter in inches. From those three numbers you can calculate everything else.

For a 245/65R17: sidewall height = 245 × 0.65 = 159.25 mm. Overall diameter = 17 × 25.4 + 2 × 159.25 = 431.8 + 318.5 = 750.3 mm (29.54 inches).

Reading the tire sidewall code

Every tire has its full specification moulded into the sidewall. The main size code is just the beginning. Here is the full string you might see:

P245/65R17 105T

• P: P-metric, meaning passenger vehicle specification. LT would mean Light Truck, which has stiffer construction and higher load ratings.
• 245: Section width in millimetres when mounted on the correct width rim and inflated to spec.
• 65: Aspect ratio. 65 percent of 245mm = 159.25mm sidewall height.
• R: Radial construction. Nearly all modern tires are radial.
• 17: Rim diameter in inches. This number never changes with metric to imperial conversions; it is always expressed in inches.
• 105: Load index. 105 corresponds to 925 kg (2039 lb) per tire. The load index table goes from 71 (335 kg) to 126 (1700 kg).
• T: Speed rating. T means the tire is rated for continuous speeds up to 190 km/h (118 mph). Common ratings: S=180, T=190, H=210, V=240, W=270, Y=300 km/h.

When comparing tire sizes for a swap, you need the size string to match the load and speed requirements of your vehicle. The door jamb sticker specifies the minimum load index and speed rating for each axle position.

Why people change tire sizes

There are four main reasons drivers use different tire sizes than the factory specification.

Lifts and levelling kits: Off-road enthusiasts fitting a 2 to 4 inch suspension lift can run taller tires for more ground clearance. A 4Runner or Tacoma might go from stock 265/70R17 (30.6 inches) to 285/70R17 (32.7 inches) or even 305/65R17 (32.6 inches). The taller tire provides better off-road capability and fills the wheel arch better with the lift.

Plus sizing for performance: Performance-focused drivers may go to a larger rim with a lower-profile tire to maintain similar overall diameter. A car with 225/55R17 might get upgraded to 245/45R18. The tires have nearly identical diameters but the wider 18-inch wheel improves lateral grip and braking performance.

Aesthetic fitment: The stance and custom wheel scene prioritizes the visual relationship between tires and fenders. Drivers may size up to fill fender arches or choose specific offset and width combinations for a flush or slight poke look. The size change is about appearance more than performance.

Winter tire strategy: Many drivers run a separate set of narrower tires on the same rim diameter for winter. A 235/60R17 summer setup might get replaced with a 215/65R17 winter set. The narrower winter tire maintains near-stock diameter while cutting better through snow. This is common in northern Europe and Canada.

The math behind diameter and circumference

The speedometer and odometer in your vehicle count wheel rotations and multiply by the tire’s circumference to get distance. When you change tire size, the circumference changes, and the vehicle electronics do not automatically know about it.

For a 245/65R17 with 750.3mm diameter: circumference = π × 750.3 = 2357.5mm per revolution.

For a 265/70R17 with 800.1mm diameter: circumference = π × 800.1 = 2513.6mm per revolution.

The new tire completes one rotation and travels 2513.6mm. The vehicle’s computer expected only 2357.5mm for that same rotation. The vehicle thinks it has traveled less distance than it actually has, so the speedometer reads low. The correction factor is:

In this example: (800.1 - 750.3) / 750.3 × 100 = +6.64%. This means at an indicated 60 mph, you are actually traveling 63.98 mph. The speedometer reads about 4 mph slow, which is the tire reading fast (you’re going faster than indicated).

The speedometer error percentage is also exactly the odometer error percentage. If your odometer has accumulated 100,000 miles on tires 8 percent larger than stock, the actual distance driven was about 108,000 miles. This matters for maintenance intervals and resale value.

Speedometer calibration after a tire change

Modern vehicles with electronic speedometers can often be recalibrated after a tire size change. Options include:

ECU tune or programmer: Many diesel trucks and performance vehicles have ECU reprogramming support from companies like HP Tuners, EFI Live, or DiabloSport. A tire size correction is a simple parameter change.

Plug-in speedometer correctors: Devices like SpeedoHealer or Dakota Digital modules intercept the VSS (Vehicle Speed Sensor) signal and apply a correction factor before it reaches the instrument cluster and ECU. These plug in-line with the speed sensor harness.

Dealership programming: Many dealerships can change the tire diameter setting in the ECU for common size changes. Ford, GM, and Ram trucks are particularly well-supported. Call ahead with your original and new tire sizes before booking.

GPS speed reference: While not a calibration method, a GPS speed display (phone app or dash unit) shows your true ground speed regardless of what the speedometer reads. This lets you know exactly how much error your tires introduce.

For off-road use or occasional recreational driving, a speedometer error under 5 percent is usually tolerable. For everyday commuting, especially in areas with strict speed limit enforcement, correcting for errors above 3 percent is worthwhile.

Ground clearance effects

Ground clearance increases by half the diameter difference. If the new tire is 50mm larger in diameter, ground clearance increases by 25mm (roughly one inch). This relationship is straightforward because the axle sits exactly at the wheel center, which is the tire radius above the ground.

However, ground clearance is not the only clearance that matters. Taller tires also reduce the clearance between the tire and the inner fender liner. A lift kit typically provides the extra arch clearance needed for taller tires. Without a lift, running a significantly taller tire risks tire contact with the inner fender at full suspension compression or full steering lock.

The lateral clearance is controlled by wheel offset and tire width, not diameter. A wider tire may clear the fender vertically but still rub the inner fender liner or control arms if the section width increases significantly.

Load rating considerations when changing tire sizes

The load index printed on the sidewall must meet or exceed the minimum requirement specified for your vehicle. This requirement is listed on the door jamb sticker and in the owner’s manual.

When you go to a physically larger tire, it does not automatically mean higher load capacity. A 285/70R17 with load index 116 carries 1250 kg per tire. A 245/65R17 with load index 107 carries 975 kg per tire. The 285 is rated higher. But you cannot assume that just because a tire is bigger, it carries more load without checking the specific load index number.

For trucks and SUVs used for towing or carrying payload, the load index difference between P-metric and LT specification tires matters significantly. An LT tire of the same size typically has a load index 6 to 8 points higher than the equivalent P-metric tire.

Rim width compatibility

Each tire size has an approved rim width range. The 245/65R17, for example, is typically approved for rims from 7 to 8.5 inches wide. Mounting on a rim outside this range causes the sidewall to bulge inward or outward, which changes the actual section width, aspect ratio behavior, and handling characteristics.

The rim width also affects speedometer accuracy slightly. A tire mounted on a narrower rim will have a slightly smaller contact patch diameter than the same tire on a wider rim. The difference is small, typically under 1 percent across the approved rim width range, but it is another reason the speedometer error calculation in this tool is an approximation rather than an exact figure.

If you are also changing rim width along with tire size, keep the new rim within the approved range for your new tire size. Tire manufacturer fitment guides (available from Michelin, Bridgestone, Continental, etc.) provide the approved rim range for each tire size.

AWD and 4WD special considerations

AWD vehicles monitor wheel speed on all four corners to detect slip and activate the rear or front drive. Consistent speedometer input from all four wheels is critical for proper operation.

When all four tires are the same size with consistent wear, wheel speeds match and the AWD system stays in background standby. When tires differ in diameter (because one is a different size, or because tires have different wear levels), the AWD controller sees a constant speed difference and interprets it as wheel slip. Depending on the system, this can result in:

• Constant activation of the AWD coupling, generating heat and wear
• Dashboard warning lights
• Reduced AWD system life (transfer case, clutch packs, viscous coupling)
• In tight-turning situations, binding and driveline wind-up

Most AWD manufacturers specify that all four tires must be within 1/4 inch (about 6mm) of circumference or within 5/32 inch of tread depth difference. This is why buying all four tires at once and rotating on schedule is especially important on AWD vehicles.

If you change tire sizes on an AWD vehicle, all four tires must change to the same new size at the same time.

Rolling resistance and fuel economy

Tire size affects fuel economy through weight, contact patch area, and rotational inertia. The relationships are not large in absolute terms, but they accumulate over tens of thousands of miles.

A heavier tire has more rotational inertia, requiring more energy to accelerate. A 10-kilogram increase in rotating mass is roughly equivalent to adding 25 to 30 kilograms of vehicle weight for the purposes of acceleration fuel consumption.

A wider contact patch generally increases rolling resistance. Rolling resistance is the deformation energy absorbed as the tire flexes through its contact area. A wider tire flattens over a larger area per revolution, deforming more rubber. However, a taller tire with more flexible sidewall may have lower rolling resistance than a wider one, depending on construction.

In practice, going from a 245mm tire to a 275mm tire of similar construction typically reduces fuel economy by 1 to 2 percent on a highway cycle. Over 15,000 miles per year at 30 MPG and $3.50 per gallon, that is about $35 to $70 per year in additional fuel cost. Meaningful over many years, but not the dominant factor in most tire purchase decisions.