Tire Size Odometer Correction Calculator
Enter your factory and new tire sizes to see each tire's diameter and revs per mile, how far off your odometer reads, your actual miles and your true speed.
Tire Size Odometer Correction Calculator
Your tires
Your odometer reads in
Pick what your dash shows. Switching relabels the results. It doesn't convert the numbers you typed.
Tire sizes
The size your speedometer is set for. It's listed as the original tire size on the placard on the driver's door pillar. If a shop recalibrated the speedometer for another size, use that size.
Type it as it's printed on the sidewall, like 235/65R17, LT265/70R17 121/118S or 33x12.50R15LT.
Odometer and speedometer
Only the miles driven on these tires. For one trip, its distance. For everything since the tire change, today's reading minus the reading when the tires went on.
What the speedometer shows. Works for any speed.
—
| Factory | Now | |
|---|---|---|
| Diameter | — | — |
| Circumference | — | — |
| Revs per mile | — | — |
Revs per mile here are calculated from the size. Tire makers publish measured figures a few percent higher; see the FAQ.
Road44
Driving Hours Tracker & Logger
Log the corrected miles for each drive in Road44
Type the actual miles into a drive's Distance field. It overrides the odometer difference, is tagged manual and counts toward your Distance totals.
Free · No sign-up needed
Road44, the driving log app mentioned below, is our app. The figures on this page come from NIST, the federal tire placard standard and tire makers' published data, checked in October 2026. Results use nominal sizes, so a particular tire can measure a little differently. This isn't fitment advice: ask a tire shop before you change sizes.
How a tire size change throws off your odometer
Your odometer and speedometer don't measure the road. They count wheel turns and work out the miles from the size of the tires the car was built with. As calculator.net puts it, they're "calibrated based on stock tires", and SD Wheel notes that the odometer "calculates mileage based on the same tire rotation data as your speedometer". Fit a different size and each turn covers a different distance, but the count still converts at the factory rate.
Take the calculator's starting sizes. A 225/65R17 tire is 28.52 in tall and a 235/65R17 is 29.03 in, so the new tire is 1.79% taller. Each turn covers 1.0179 times as much road, and when the odometer shows 10,000 miles you've driven 10,179.5. The odometer reads 1.76% low: it shows 98.24 miles for every 100 you drive. When the speedometer shows 60 mph, you're doing 61.1 mph.
Smaller tires flip it. On 225/65R17 tires where the car came with 235/65R17, 10,000 miles on the odometer is 9,823.7 driven. The odometer reads 1.79% high, and 60 mph on the speedometer is really 58.9 mph.
On this page, "reads low" always means you drove farther than the odometer shows, and "reads high" means you drove less. The error is measured against the distance you actually drove, which is why it isn't quite the same number as the height difference.
Reading a tire size: metric, LT, ZR and flotation
A metric size like 225/65R17 holds three numbers: the width in millimeters (225), the aspect ratio, which is the sidewall height as a percentage of the width (65), and the wheel diameter in inches (17). The calculator works out the overall diameter from those three. A letter in front sets the tire type: Continental lists P for passenger cars, LT for light trucks, and no prefix for Euro-metric tires. A ZR can appear in the size of a tire rated above 149 mph, according to Goodyear, which cites the Tire and Rim Association yearbook. It doesn't change the size.
Flotation sizes, found on many light-truck and off-road tires, are written in inches. As tiresize.com explains, "The first number in a high flotation tire size is the overall diameter of the tire." So in 33x12.50R15, the tire is 33 in tall, 12.50 in wide and fits a 15 in wheel, and the calculator uses the 33 directly.
Type the size as it's printed on the sidewall. The calculator reads all of these, and ignores the load index, speed rating and anything else after the wheel size:
- P225/65R17 102H
- LT265/75R16 123/120R, with a dual load index
- 245/40ZR18 97Y and 255/35ZR19 (96Y)
- 205/55-16 and 225/45 R17 94W XL
- 33x12.50R15LT, 35X12.50R15 113Q and 33x12.5x15
It can't compare a size that doesn't say how tall the tire is. For a size with no aspect ratio, like 185R14, or one like 7.50R16, you'll need the tire maker's spec sheet. Under each field, the calculator shows what it read, such as "225 mm wide, 65% sidewall, 17 in wheel", so you can check it before you rely on the result.
Your factory size is on the tire placard. The federal tire standard, FMVSS 110, has it "permanently affixed to the driver's side B-pillar" on most cars, under a heading such as "original tire size" or "original size".
Calculated vs published revs per mile
The calculator works out revs per mile from the size: 63,360 inches in a mile divided by the tire's circumference, which is π times the diameter. For a 225/65R17, that's 63,360 ÷ (π × 28.52) = 707.
Tire makers publish a measured figure instead, taken on a loaded, rolling tire. SAE J966 is the "Test Procedure for Measuring Passenger Car Tire Revolutions Per Mile". A tire carrying the car's weight turns a few percent more times per mile than its unloaded size predicts, so Michelin lists its Defender2 225/65R17 at 730 revs per mile, not 707.
Most of that gap cancels out, because the correction uses only the ratio between your two tires. On Michelin's own Defender2 figures, the measured and calculated ratios nearly agree:
| Factory vs now | Michelin revs per mile | Measured ratio | Calculated ratio |
|---|---|---|---|
| 225/65R17 vs 235/65R17 | 730 vs 716 | 1.0196 | 1.0179 |
| 225/65R17 vs 255/65R18 | 730 vs 670 | 1.0896 | 1.0889 |
Those are just two Michelin pairs, though, not a rule for every tire. If you have published revs per mile for both of your tires, actual miles = odometer miles × factory revs ÷ new revs.
An odometer isn't exact to begin with. NIST Handbook 44 allows odometers used to charge for rent or hire an error of "4 % of the interval under test" and expects the tires to be kept "at the posted pressure". This calculator isolates the part of the error that comes from the size change.
Keep corrected miles in Road44
If you log drives from odometer readings, every one of them carries the same error. On tires that aren't the factory size, each logged distance is off by the same percentage the odometer is.
Road44, our driving log app, has a place for the corrected figure. Each drive has optional Start odometer, End odometer and Distance fields. A typed Distance wins over the odometer readings and is tagged "(manual)", so you can keep the readings as you wrote them and type the corrected miles beside them. With Distance as your tracking mode, the home screen totals and the all-time goal count each drive's distance, and both are free. Road44 doesn't read your odometer or track your location: every reading is typed in by hand. It's a personal record, not an official or certified logbook.
For a single trip, get the distance from two readings with the odometer trip mileage calculator, then correct it here. For fuel economy, put the corrected miles into the MPG calculator: odometer miles that read low make your MPG look lower than it is. The annual mileage calculator and the cost per mile calculator take miles too, so correct them first if your tire size changed.
Frequently Asked Questions
Common questions about tire size odometer correction calculator
Do bigger tires make the odometer read high or low?
Low. A taller tire covers more ground with each turn, but the odometer still counts turns as if the factory tires were on. calculator.net puts it this way: "a larger tire will result in a speedometer reading that is lower than the actual speed that the car is traveling; similarly, it will result in an odometer reading that is lower than the actual distance traveled." On 235/65R17 tires instead of the factory 225/65R17, an odometer showing 10,000 miles means 10,179.5 miles driven. Smaller tires do the opposite: on 225/65R17 tires where the car came with 235/65R17, 10,000 miles shown is 9,823.7 driven.
How do I work out actual miles after changing tire size?
Multiply the odometer miles by the new tire's diameter, then divide by the factory tire's diameter. For a metric size, the diameter in inches is the wheel size + 2 × width × aspect ratio ÷ 100 ÷ 25.4. So 225/65R17 is 17 + 2 × 225 × 0.65 ÷ 25.4 = 28.52 in, and 235/65R17 is 29.03 in. Then 10,000 × 29.0276 ÷ 28.5157 = 10,179.5 miles. Keep the unrounded diameters: the rounded 29.03 ÷ 28.52 gives 10,178.8 instead. A speedometer reading converts the same way: 60 × 1.01795 = 61.1 mph.
Why are the odometer error and the tire height difference two different numbers?
They're measured from different starting points. The height difference compares the new tire with the factory one, the way Omni does: "percentage change = 100 × absolute difference / first tire diameter". The odometer error compares what the odometer shows with the distance you actually drove. That's how NIST Handbook 44 states odometer tolerances: as a "% of the interval under test". For small changes the two are nearly equal, and for big ones they drift apart. A 255/65R18 is 8.89% taller than a 225/65R17, and the odometer reads 8.17% low: it shows 10,000 miles for 10,889.1 driven.
Where do I find my factory tire size?
On the tire placard, usually on the driver's door pillar. Federal standard FMVSS 110 (49 CFR 571.110, S4.3) requires the placard to be "permanently affixed to the driver's side B-pillar", with fallbacks for cars that don't have one. It lists the "Tire size designation, indicated by the headings 'size' or 'original tire size' or 'original size'" for "the tires installed at the time of the first purchase". If a shop has recalibrated your speedometer for a different size, enter that size as the factory size.
Why doesn't the revs per mile match the tire maker's spec sheet?
The calculator uses geometry: 63,360 inches in a mile divided by the tire's circumference. Tire makers measure a loaded, rolling tire instead (SAE J966 is the test procedure for passenger tires), and a loaded tire turns a little more often per mile. Michelin lists its Defender2 225/65R17 at 28.5 in and 730 revs per mile, where the formula gives 707 from the size's calculated 28.52 in (708 from Michelin's 28.5 in). The correction uses only the ratio between the two tires, and on Michelin's own figures that ratio barely moves: 730 ÷ 716 for the 235/65R17 is 1.0196, against 1.0179 calculated. If you have published revs per mile for both tires, actual miles = odometer miles × factory revs ÷ new revs.
How accurate is an odometer on the factory tires?
Not exact, even before a size change. NIST Handbook 44 section 5.53, the weights-and-measures code for odometers used to charge for rent or hire, allows "4 % of the interval under test" either way, and 5% under or 3% over for unloaded trucks. It also requires tires to be kept "at the posted pressure". This calculator measures only the change caused by the new size, which adds to whatever error the odometer already had.
Can I recalibrate my speedometer and odometer for new tires?
Often, yes. SD Wheel says "A handheld tuner or programmer can recalibrate your speedometer to account for your new tire size," and "Many dealerships and local repair shops can recalibrate your speedometer to match your new tire size using diagnostic software." It notes that the odometer "calculates mileage based on the same tire rotation data as your speedometer", so ask the shop whether the change covers both. After a recalibration, enter the size the gauges are now set for as the factory size. SD Wheel also notes: "Some vehicles use wheel speed data to determine when to shift gears."
What do P, LT, ZR and sizes like 33x12.50R15 mean, and can I type them?
You can type the size just as it's printed. Continental describes P as a "Passenger car tire designed for cars, minivans, and small SUVs", LT as a "Light truck tire for pickups, larger SUVs, and commercial vehicles with higher load capacity", and no prefix as a "Euro-metric tire, similar to P-metric but with slightly different load standards". Goodyear, citing the Tire and Rim Association yearbook, says that above 149 mph "a ZR may appear in the size designation". In a flotation size like 33x12.50R15, 33 is the overall diameter in inches, 12.50 the width in inches and 15 the wheel size (tiresize.com). The prefix, ZR, load index and speed rating don't change the height, so the calculator ignores them.