Tire Size Converter: Metric Decoder, Speedometer Error, and Inch-to-Metric
The only tire size tool that combines metric-to-inch decoding with a side-by-side stock vs new comparison showing speedometer error, actual speed at every highway speed, odometer drift per 1,000 miles, and ride height change — all in one free calculator.
🛞 Select Conversion Mode
📋 Quick Parse — Type Any Tire Code
Type any metric tire code and click Parse to auto-fill the fields below.
Calculator will also show a full speed table from 30 to 80 mph.
📐 Enter Inch Tire Size (Diameter x Width R Rim)
in
in
in
Common examples: 33×12.50R15 (Jeep off-road), 35×12.50R17, 37×13.50R17 (truck build). Enter as three separate numbers.
📊 Tire Size Results
🛞
Enter a tire code in the Metric Decoder to see all dimensions, use Stock vs New to check speedometer error and actual speed, or enter an inch size to find the metric equivalent.
Tire Dimension Comparison
Reading a Tire Sidewall: What Every Number on a US Tire Code Actually Means
Every tire sold in the United States carries a standardized code on its sidewall that contains six pieces of information packed into a compact string like P265/70R17. Most drivers know their tire size exists, see it printed on the door jamb sticker, and trust that the tire shop puts the right one on at replacement time. What very few people know is that these numbers directly determine how their speedometer reads, how their vehicle sits, how their odometer tallies miles, and whether their ABS and traction control systems are calibrated for the tire they are actually running on.
Breaking down P265/70R17: The leading P means Passenger vehicle tire (LT means Light Truck, which has a higher load rating and typically heavier construction). The 265 is the section width in millimeters — the widest point of the tire when mounted on the specified rim and inflated to the recommended pressure. The 70 is the aspect ratio, which means the sidewall height is 70 percent of 265 mm, or 185.5 mm (7.30 inches). The R means Radial construction, which is standard for virtually every consumer tire sold today. The 17 is the rim diameter in inches — and notably, this is the only measurement in the entire code given in inches rather than millimeters, a legacy of US automotive history that still confuses people today.
The overall tire diameter is not printed anywhere on the tire. You calculate it: two sidewall heights (top and bottom) plus the rim diameter. For P265/70R17, that is (7.30 x 2) + 17 = 31.61 inches. This number drives everything — it determines how fast the vehicle moves per wheel revolution, which is what the speedometer reads, and it determines how high the vehicle sits off the ground.
The Aspect Ratio: Why the Middle Number Is the Most Misunderstood
Many drivers read the 70 in 265/70R17 and assume it refers to some measure of the tire’s overall size or volume. It does not. It is a ratio — specifically, the sidewall height as a percentage of the section width. A lower aspect ratio means a shorter, stiffer sidewall that tends to handle better and look sportier (low-profile performance tires run 35 or 40). A higher aspect ratio means a taller, more cushioned sidewall that absorbs road impacts better and provides more off-road articulation (truck and SUV tires often run 70 or 75). For off-road enthusiasts, a higher aspect ratio is not just a style choice — it provides a taller sidewall that can be aired down for better grip on loose terrain without risking rim contact with rocks.
How the Three Conversion Modes Work for Different Situations
Mode 1: Metric Decoder — All Dimensions from One Tire Code
Enter any standard metric tire code — directly typed as 265/70R17, with the LT or P prefix included if you have it — and click Parse to auto-fill the fields. The decoder calculates section width in both millimeters and inches, sidewall height in both units, overall diameter, circumference, and revolutions per mile. The revolutions per mile figure matters for custom speedometer recalibration, GPS accuracy verification, and understanding how much more (or less) ground you cover per engine revolution after changing tire sizes.
Mode 2: Stock vs New Comparison — Speedometer Error and Actual Speed Table
Enter your current stock tire code and your new or proposed tire code. The comparison shows every dimension side by side, then calculates the speedometer error as a percentage. More usefully, it provides a complete speed table showing what you are actually doing at every 5 mph from 30 to 80 mph on your indicated speedometer. It also calculates ride height change (half the diameter difference) and odometer drift — how many extra or fewer actual miles you travel per every 1,000 miles shown on your odometer. On a vehicle with 200,000 lifetime miles at a 3% tire upsize, the odometer has shown 6,000 fewer miles than were actually driven.
Mode 3: Inch to Metric — 33×12.50R15 to Metric Code
The off-road tire market in the US still widely uses inch sizing: a 33×12.50R15 is a 33-inch overall diameter, 12.50-inch wide tire on a 15-inch rim. Enter these three values separately and the calculator produces the standard metric equivalent with the nearest common aspect ratio. This matters when cross-shopping tires or checking whether an inch-sized tire fits within a lift kit’s clearance specifications that are defined in metric terms.
Three Real US Tire Upgrade Scenarios with Complete Size Analysis
Scenario 1: F-150 Owner Going from Stock 265/70R17 to 285/75R17
The 285/75R17 is one of the most popular tire upgrades for Ford F-150, Ram 1500, and Chevy Silverado owners. It fits under the stock fenders with minimal or no lift, gives a noticeably more aggressive look, and improves off-road grip. But what does it actually do to the truck’s calibration?
Measurement
Stock 265/70R17
New 285/75R17
Change
Overall diameter
31.61″
32.83″
+1.22″ taller
Section width
10.43″
11.22″
+0.79″ wider
Sidewall height
7.30″
8.42″
+1.12″ per side
Revolutions/mile
642
618
24 fewer per mile
Speedometer error
—
+3.86%
reads SLOW
At 60 mph indicated
60.0 mph actual
62.3 mph actual
2.3 mph faster than shown
Odometer drift / 1000 mi
exact
+38.6 mi
38 extra miles driven per 1K shown
At 3.86 percent speedometer error, this upgrade crosses the 3 percent threshold where most experts recommend ECU recalibration. The truck’s transmission shift points, ABS thresholds, and stability control activation speeds are all calibrated for the 31.61-inch stock tire. The recalibration process (done via a tuner like Forscan on Ford vehicles or the dealer’s scan tool) corrects all of these systems and should cost $100 to $150 at a reputable shop if not done with a DIY tuner.
Scenario 2: Honda Civic Stock 205/55R16 vs Performance Upgrade 225/45R17
Plus-sizing is the practice of moving to a larger rim while reducing the aspect ratio to maintain nearly the same overall tire diameter. This gives the vehicle a sportier look and typically improves handling by reducing sidewall flex, but it requires careful calculation to ensure the diameter stays within 1 to 2 percent of stock.
Measurement
Stock 205/55R16
New 225/45R17
Change
Overall diameter
24.88″
25.00″
+0.12″ (near-perfect)
Section width
8.07″
8.86″
+0.79″ wider
Sidewall height
4.44″
3.98″
-0.46″ shorter
Speedometer error
—
+0.48%
negligible
At 70 mph indicated
70.0 mph actual
70.3 mph actual
less than 0.4 mph difference
This is a textbook plus-sizing execution. The 225/45R17 is 0.79 inches wider and 0.46 inches shorter in sidewall, giving the Civic a much wider footprint for improved lateral grip, while the 0.12-inch diameter difference produces only 0.48 percent speedometer error — essentially imperceptible. The shorter sidewall reduces ride comfort slightly on rough roads but meaningfully reduces body roll in corners. The 17-inch rim pairs with lower-profile tires that are visually proportional to a sporty compact.
Scenario 3: Jeep Wrangler 255/75R17 Stock to 37×13.50R17 Off-Road Build
Serious off-road Jeep builds often use inch-sized tires because the off-road tire market has standardized on 33, 35, and 37-inch diameter designations. Converting 37×13.50R17 to metric requires the Inch to Metric mode.
Measurement
Stock 255/75R17
37×13.50R17
Metric Equiv.
Overall diameter
32.07″
37.00″
+4.93″ taller
Section width
10.04″
13.50″
340/72R17 (approx)
Speedometer error
—
+15.4%
RECALIBRATE REQUIRED
At 60 mph indicated
60.0 mph actual
69.2 mph actual
9.2 mph faster than shown
Odometer drift / 1000 mi
exact
+154 extra miles
154 more miles driven per 1K shown
A 15.4 percent speedometer error on a 37-inch tire Jeep running stock gearing is not just a calibration inconvenience — it is a safety issue. At an indicated 60 mph, the driver is actually doing 69.2 mph, well above the legal limit. A full recalibration via Tazer Mini, Diablo Sport iDash, or dealer reflash is mandatory, not optional. Most serious Jeep builders also re-gear the axles from the stock 3.73 or 4.10 to 5.13 or 5.38 gears to restore lost low-end torque from the much heavier, larger tires — a separate but critical upgrade that this type of build requires.
Three Critical Tire Size Facts US Drivers Get Wrong
Tip 1: The Aspect Ratio Is NOT About Quality or Grade — It Is Purely a Height Ratio
A common misconception is that a higher aspect ratio (like 75 or 80) means a better-quality tire, while a lower ratio (like 35 or 40) means a sportier, high-performance tire. Aspect ratio has nothing to do with build quality, tread life ratings (the UTQG treadwear number), or handling performance directly. It simply describes sidewall height as a percentage of width. A 75-series truck tire and a 75-series economy tire have the same proportional sidewall; one may be dramatically better in quality, warranty, and traction. The aspect ratio tells you geometry, not grade.
What aspect ratio does meaningfully affect is ride quality and vulnerability to rim damage. A 55-series tire on a 16-inch rim has 4.53 inches of sidewall cushion between the road and the rim. A 35-series tire on an 18-inch rim has only 3.38 inches. On rough urban roads, low-profile tires expose rims to pothole damage far more readily — a real-world cost of performance-oriented tire choices that offset some of the handling gains.
Tip 2: Your Speedometer Has Always Been Slightly Off — Even from the Factory
Federal Motor Vehicle Safety Standard 101 allows US speedometers to read up to 10 percent fast but prohibits them from reading slow at all. Manufacturers typically calibrate speedometers to read 2 to 4 percent fast on new vehicles with brand-new tires at the specified inflation pressure, in order to ensure compliance even as tires wear and tire diameter decreases over their lifetime. This means that even on a stock vehicle with stock tires, your speedometer is probably showing 62 to 63 mph when you are actually doing 60 mph.
Aftermarket upsized tires that add another 3 to 5 percent of diameter can push the total indicated error to 5 to 8 percent, which means your GPS-verified actual speed is meaningfully faster than your speedometer shows. For reference, radar speed signs typically measure actual vehicle speed via Doppler effect — if you have ever noticed a radar sign reading 3 to 5 mph faster than your speedometer even on a stock vehicle, this factory speedometer bias is exactly why.
Tip 3: Tire Diameter Affects Far More Than Speed — It Changes Gear Ratio, Braking, and TPMS
When you change tire diameter, you effectively change your vehicle’s final drive ratio. Larger tires mean each wheel revolution covers more ground, so the engine works harder to accelerate from a stop and at low speeds. A truck geared 3.73:1 with 33-inch tires behaves noticeably differently than the same truck with 37-inch tires at the same gear ratio — it will lose significant low-end torque and towing capacity without an axle regear to compensate. Serious truck builders match their axle gear ratio to the tire size using established formulas to restore factory-equivalent performance.
TPMS (Tire Pressure Monitoring System) sensors are calibrated to the wheel position and rim size on US-market vehicles since 2008. Changing rim diameter requires reprogramming or replacing TPMS sensors. Changing only tire size (same rim) may not trigger a TPMS warning on its own but affects the rolling radius used in the vehicle’s wheel speed sensor calibration, which feeds the ABS, stability control, and all-wheel-drive torque distribution systems. Vehicles with modern torque-vectoring AWD systems (like some Jeep Grand Cherokees, Ford Edge STs, and BMW xDrive models) are particularly sensitive to wheel diameter mismatches between axles.
16 Tire Size Conversion and Speedometer FAQs for US Drivers
In 265/70R17: 265 is the section width in millimeters (widest point of the tire). 70 is the aspect ratio, meaning the sidewall height equals 70% of 265mm, or 185.5mm (7.30 inches). R means Radial construction, used in virtually all modern consumer tires. 17 is the rim diameter in inches. The overall tire diameter, which is not directly stated in the code, is calculated as (2 x sidewall height in inches) + rim diameter: (2 x 7.30) + 17 = 31.61 inches. Use the Metric Decoder mode to get all dimensions from any code instantly.
Most automotive engineers and tire professionals consider up to 2 percent speedometer error to be within acceptable tolerance, since factory speedometers are already calibrated 2 to 4 percent fast. Error between 2 and 5 percent is noticeable on GPS-verified speed checks but typically does not trigger performance degradation in ABS or traction control. Error above 5 percent begins to meaningfully affect how ABS, stability control, and transmission shift points behave. Error above 10 percent (which occurs with very large tire upgrades like 33-inch to 37-inch on Jeep builds) requires mandatory recalibration of the vehicle’s ECU or PCM to restore correct operation of all speed-dependent systems.
On most F-150 model years from 2009 to present, a 285/75R17 (approximately 32.8 inches in diameter) fits under stock fenders without a lift kit, but may require minor trimming of the plastic fender liner at full steering lock and full suspension compression. Many F-150 owners report a clean fit on 2015 and newer SuperCrew models without any modification. Older body-on-frame F-150s (2004-2008) generally require at least a 1-inch leveling kit to clear the 285/75R17 without rubbing. The safest approach is to check F150Forum.com or a model-specific forum where owners document their exact fitment experience before purchasing. The size comparison in this calculator shows you the 1.22-inch diameter increase and 0.79-inch width increase so you can assess whether your fender clearance accommodates the change.
The P prefix (Passenger) and LT prefix (Light Truck) indicate different load-bearing specifications and construction standards. LT tires are built to higher load ratings, with heavier internal ply construction that resists damage from heavy loads and off-road impacts. They carry a load range designation (B, C, D, or E) indicating how much air pressure they can hold and how much weight they can support. P-metric tires are designed for passenger car loads and comfort. On trucks and SUVs rated for towing or hauling, LT-rated tires are recommended because they maintain their structural integrity under high loads where P-metric tires can flex excessively and overheat. The dimensional math is the same for both types — the conversion formulas in this calculator work identically for LT and P-metric tires of the same size code.
Enter 33 in the diameter field, 12.5 in the width field, and 15 in the rim field in the Inch to Metric mode of this calculator. The result is 325/60R15 as the metric equivalent (section width of 317mm rounds to 320mm or 325mm; aspect ratio calculates to approximately 57-60%). Off-road tires sold in inch sizes rarely have exact metric equivalents because the inch sizing convention does not follow the same standardized aspect ratio increments as metric sizing. The calculator provides the nearest standard metric code and shows you the exact verified diameter so you can confirm the conversion is within acceptable tolerance of your intended 33-inch outer diameter.
Yes, but the effect depends on whether diameter or width changes more, and in which direction. Larger diameter tires reduce rotational frequency (fewer revolutions per mile), which reduces internal energy losses in the tire at highway speeds — a theoretical fuel efficiency gain. However, larger and wider tires increase rolling resistance due to greater contact patch size and heavier tire weight, which typically more than offsets the diameter benefit. As a rough real-world guideline: upgrading from stock 265/70R17 to 285/75R17 on a full-size truck typically costs 0.5 to 1.5 MPG in highway fuel economy due to the added weight, rolling resistance, and aerodynamic frontal area. Mud-terrain tires in any size typically cost 1 to 3 MPG versus highway tires of the same size due to their aggressive tread pattern’s higher rolling resistance.
Common stock tire sizes for popular US vehicles: Ford F-150 (base): 265/70R17 (31.6 inch). F-150 Tremor/Raptor: 315/70R17 (34.0 inch). Ram 1500: 275/65R18 (32.1 inch) or 275/60R20 (32.0 inch). Chevy Silverado 1500: 265/65R18 (32.5 inch). Jeep Wrangler JL: 255/75R17 (32.1 inch). Toyota Tacoma: 265/70R16 (30.6 inch). Toyota Tundra: 275/65R18 (32.1 inch). Honda Pilot: 245/50R20 (29.0 inch). Ford Explorer: 255/50R20 (29.1 inch). These are approximate — exact sizes vary by trim level and model year. Check your driver’s door jamb sticker for your specific vehicle’s required tire size.
Changing tire size from the manufacturer’s specification technically voids the towing and payload ratings published in your vehicle’s towing guide, because those ratings are calculated using the specific tire size, gear ratios, and braking distances of the stock configuration. In practice, a modest tire upsizing (within 3 percent of stock diameter, same load rating) has a minimal real-world effect on towing safety, but an aggressive upsize to 35 or 37-inch tires materially changes the effective gear ratio, reduces low-end torque availability, and alters braking distance — all of which matter when towing near the rated limit. Before major tire upgrades on a truck used for towing, consult your towing capacity calculator and verify that the new effective gear ratio still puts the engine in the appropriate RPM range for sustained highway towing.
Plus-sizing means increasing wheel diameter while simultaneously decreasing the tire’s aspect ratio to keep the overall tire diameter close to stock. Plus-one sizing (e.g., 16-inch to 17-inch wheels) reduces aspect ratio by approximately 5 to 8 points. Plus-two sizing (e.g., 16-inch to 18-inch) reduces aspect ratio by 10 to 15 points. The goal is to maintain the same overall tire diameter within plus or minus 1 inch (roughly 3 percent) of stock. Use the Stock vs New comparison in this calculator to enter your current size and proposed plus-sized tire to verify the diameter change stays within acceptable range. The general rule: overall diameter should stay within 3 percent of stock to avoid significant speedometer error, and within 1 percent for vehicles with advanced driver assistance systems (ADAS), adaptive cruise control, or torque-vectoring AWD.
Maximum fitment widths vary significantly by vehicle, lift height, and wheel offset. General guidelines for popular trucks: stock Ford F-150 (no lift): 285mm (11.22 inches) wide; with 2-inch leveling kit: 305-315mm. Stock Chevy Silverado (no lift): 285mm; with 2-inch leveling: 305mm. Stock Ram 1500 (no lift): 285mm. Jeep Wrangler JL (no lift): 285mm on 17-inch stock wheels; with 2-inch lift: 315mm; with 3.5-inch lift: 35-inch tire (approximately 325mm). These are community-verified fitments. Actual clearance also depends heavily on wheel backspacing and offset — a wider wheel with less backspace may allow wider tires than a narrower wheel with more backspace. Always verify fitment on model-specific forums before purchasing.
The recalibration method depends on your vehicle. Ford trucks (2011 and newer): Forscan software with an OBD2 adapter (free, DIY, under $25 in hardware) can update the tire size parameter in the ABS module and instrument cluster. Chevy/GMC trucks: EFI Live, HP Tuners, or an EZ Lynk device can update tire size in the PCM. Jeep Wrangler JL/JT: Tazer JL Mini plug-in device handles tire size update. Ram trucks: DiabloSport iDash or HP Tuners. For most modern trucks, the dealer can also reflash the speedometer calibration parameter via their scan tool, usually for $75 to $150. For older vehicles without advanced ECU controls, mechanical speedometer recalibration via a speedometer gear change in the transmission is the only option. After any tire size change, verify calibration using a GPS speedometer app on your phone, which shows actual speed based on satellite positioning, independent of your vehicle’s wheel speed sensors.
Load range letters correspond to the number of internal plies and maximum inflation pressure the tire is rated for: Load Range C is 6-ply rated, maximum 50 PSI. Load Range D is 8-ply rated, maximum 65 PSI. Load Range E is 10-ply rated, maximum 80 PSI. Higher load range means higher maximum load capacity and more robust construction that resists punctures and sidewall damage. Most stock LT tires on half-ton trucks are Load Range C or D. Three-quarter-ton and one-ton trucks typically use Load Range E. Load Range E tires are significantly heavier and ride stiffer than Load Range C tires — the added unsprung weight can affect handling and the stiffer sidewall transmits more road impacts into the cabin. For trucks not used for heavy towing or hauling, Load Range D often provides the best balance of durability and ride comfort.
Tire width compatibility with a specific wheel is governed by the rim width range published by the tire manufacturer. Every tire has a specified rim width range (for example, a 265mm wide tire is typically specified for 7.5 to 9.5 inch wide rims, with 8 to 9 inch as the optimal range). Mounting a tire on a rim narrower than the minimum causes the tire to bulge outward, distorting the contact patch and handling. Mounting on a rim wider than the maximum stretches the tire, can cause bead seating issues, and creates an unsafe sidewall profile. Your vehicle’s original rim width is stamped on the wheel or listed in your owner’s manual. When selecting aftermarket tires, look up the specific tire’s rim width specification on the manufacturer’s website (Michelin, Bridgestone, Goodyear, etc.) to confirm compatibility with your current or planned rim width.
UTQG stands for Uniform Tire Quality Grading, a US government-mandated rating system administered by the Department of Transportation (DOT) under which tire manufacturers test and grade their tires in three categories: Treadwear (a comparative index number, where 200 lasts approximately twice as long as 100 under standard test conditions), Traction (AA, A, B, or C, measuring wet stopping ability), and Temperature (A, B, or C, measuring resistance to heat buildup at sustained highway speeds). A UTQG treadwear rating of 600 means the tire is expected to last six times as long as a reference tire in the test — translating roughly to 60,000 to 80,000 miles for most drivers. Performance tires often have UTQG treadwear of 200 to 300 (shorter life, higher grip), while touring and all-season tires run 500 to 800 (longer life, moderate grip). The UTQG is printed on every tire’s sidewall in the US.
Tire size affects braking distance through two competing factors: larger contact patch and rolling mass. A wider tire (larger section width) creates a larger contact patch between rubber and road, which in theory provides more friction force available for braking. However, heavier tires (which larger sizes typically are) have more rotational inertia, requiring more force to decelerate. In practice, a modest tire upsizing of similar construction and compound makes minimal difference in braking distance. The tread compound and design make far more difference: a summer ultra-high-performance tire in the same size stops significantly shorter than an all-terrain truck tire. For winter driving specifically, a narrower tire actually stops shorter on snow because it cuts through surface slush to reach firmer snow below. The US EPA’s fuel economy database at fueleconomy.gov is a useful reference for verifying OEM tire specifications by make, model, and year.
The correct inflation pressure for your specific vehicle is on the driver’s door jamb sticker, not on the tire itself. The maximum pressure printed on the tire sidewall is the tire’s structural limit, not the recommended operating pressure — inflating to the tire’s maximum pressure typically overinflates the tire for road use, causing excessive wear in the center of the tread, a harsher ride, and reduced wet-weather grip from a smaller contact patch. For aftermarket tires in a larger size, use the door jamb placard as a starting point but recognize that LT tires replacing P-metric tires require adjustment: LT tires in Load Range E should be inflated to 10 PSI above the placard recommendation when carrying heavy loads, per tire industry guidelines. After any tire size change, confirm your TPMS low-pressure threshold is still set appropriately for the new minimum safe inflation pressure.
Tire size calculations use standard industry formulas: sidewall height = width (mm) x (aspect ratio / 100); overall diameter = (2 x sidewall height in inches) + rim diameter (in); circumference = pi x diameter; revolutions per mile = 63,360 / circumference. Inch-to-metric conversions use width rounding to the nearest standard 10mm increment and aspect ratio rounding to the nearest standard value. Speedometer error is calculated as (new diameter / stock diameter – 1) x 100%. Actual vehicle fitment, speedometer accuracy, and legal speed compliance are the responsibility of the vehicle owner.
Tire fitment information is provided for general reference. Always verify compatibility with your specific vehicle’s fender clearance, wheel offset, load rating requirements, and TPMS compatibility before purchasing tires. Consult your vehicle owner’s manual and a qualified tire professional before making sizing changes that affect safety-critical systems. Editorial transparency: No tire brand, retailer, or wheel manufacturer paid to influence the content or calculations on this page.