Vehicle · Tire sizes
Tire size comparison and speedometer error.
Enter the tire you have and the tire you are thinking of. You get overall diameter, width, sidewall height, revolutions per mile, how much taller the tire is, and the true speed when your speedometer reads 60 mph. It runs in your browser; nothing you type is sent anywhere.
Inputs
Result
Enter two tire sizes and press Compare.
How the comparison works
A tire size is a short code that fixes its geometry. This tool reads the code, works out the overall diameter, and compares the two diameters. Nothing else is assumed.
Metric sizes, such as 265/70R17
- 265 is the nominal section width in millimetres.
- 70 is the aspect ratio: sidewall height as a percentage of the section width.
- R17 is radial construction on a 17 inch rim.
Sidewall height = width x aspect / 100. Overall diameter = rim x 25.4 + 2 x sidewall. For 265/70R17 that is 265 x 0.70 = 185.5 mm of sidewall, and 17 x 25.4 + 2 x 185.5 = 802.8 mm (31.6 in).
Flat-float sizes, such as 33x12.50R15
The first number is the overall diameter in inches, the second is the section width in inches and the last is the rim diameter in inches. Sidewall height = (diameter - rim) / 2, so 33x12.50R15 has a 9 in sidewall and an 838.2 mm diameter.
What is calculated
- Circumference = pi x diameter. 49 CFR 571.109 S5.1(f) measures tire outer diameter the same way, as the maximum circumference divided by pi.
- Revolutions per mile = 63,360 in / circumference in inches. Per kilometre it is 1,000,000 mm / circumference in mm.
- True speed = displayed speed x new diameter / current diameter. The speedometer counts wheel turns and assumes the current tire, so a larger tire covers more road per turn.
- Odometer under-reads (or over-reads) by the same percentage as the speed.
- Lift = half of the diameter difference, the amount the axle moves up. It is also the extra clearance the larger tire needs in the wheel well.
- Gearing: engine rpm at the same road speed changes by current diameter / new diameter. That has the same effect as multiplying the axle ratio by that factor.
Limits of this tool
- Values come from the printed size, not a measured tire. Actual diameter and width vary by maker, rim width, wear, load and inflation, and a loaded tire is a little shorter than its free diameter.
- The error is only the tire part. A speedometer can also have its own factory error, which this tool cannot know.
- It does not tell you whether a size fits your vehicle, rim or load rating. Check the vehicle placard and the tire maker's specifications.
- Changing tire size can affect ABS, stability control and the speed and distance the vehicle's computer reports. Ask a dealer or tire shop whether the vehicle can be recalibrated.
Sources
- 49 CFR 571.109, FMVSS No. 109, new pneumatic tires (eCFR, text read October 2, 2026): defines section width as the linear distance between the exteriors of the sidewalls of an inflated tire, refers to aspect ratio and size designation, and in S5.1(f) defines tire outer diameter as maximum circumference divided by pi.
- 49 CFR 571.101, FMVSS No. 101, controls and displays (eCFR, text read October 2, 2026): covers the identification of vehicle controls and displays. The text read contains no speedometer accuracy requirement, so this page does not state a US legal limit for speedometer error.
- The size-code reading (width / aspect ratio / rim) follows the tire designation convention standardized in ISO 4000-1 and by ETRTO. Those standards are paid documents and were not read for this page; the formulas are the arithmetic the designation implies, shown above so you can check them by hand. The flat-float format is the common industry convention for inch-sized tires.
- NHTSA, Tires: general guidance to follow the vehicle placard for tire size and pressure.
Cite this page: TNAADO Tools, “Tire Size Comparison and Speedometer Error Calculator,” tools.tnaado.ca/calculators/general/tire-size-comparison-speedometer-calculator/, checked October 2, 2026.
Tire size and speedometer FAQ
How do I compare two tire sizes?
Enter the size your speedometer was set up for (usually the original size on the placard) as the current tire and the new size as the replacement. The calculator turns each designation into an overall diameter, then compares diameters. Metric sizes such as 265/70R17 give diameter as rim x 25.4 + 2 x (width x aspect / 100) in millimetres. Flat-float sizes such as 33x12.50R15 state the overall diameter in inches directly.
How much will my speedometer be off with bigger tires?
The speedometer counts wheel revolutions and assumes the original circumference, so true speed is the displayed speed multiplied by new diameter divided by old diameter. Going from 265/70R17 to 285/75R16 is a 3.9 percent larger diameter, so a displayed 60 mph is really about 62.3 mph. The odometer under-reads by the same percentage.
Is the speedometer error the same at every speed?
On the geometry used here, yes: it is a fixed percentage of speed. A tire 3.9 percent larger makes every true speed 3.9 percent higher than shown. Real speedometers also have their own factory error, and tire diameter changes slightly with wear, load, inflation pressure and speed, so treat the result as an estimate.
How much lift does a taller tire give?
The axle rises by half of the difference in overall diameter, because the tire grows in radius. A 31 mm (1.2 in) taller tire raises the body about 15.5 mm (0.6 in) at the wheel, before any suspension lift. It also needs about that much extra clearance in the wheel well, plus room for any extra width.
Does a bigger tire change my gearing?
Yes. A larger tire makes the engine turn slower at the same road speed, which acts like a numerically lower axle ratio, by the factor old diameter divided by new diameter. A 3.9 percent larger tire is like multiplying your axle ratio by about 0.963 (for example 3.73 acts like 3.59). Expect weaker acceleration and a change in fuel use; a shop can recalibrate the speedometer and set the new size in the vehicle.
Is the width in the calculator the real width of the tire?
It is the nominal section width from the size designation. The actual width depends on the rim the tire is mounted on and on the maker, and can differ by several millimetres. Check the tire maker's published specifications for the exact width, overall diameter and recommended rim width before buying.
Is anything I type sent or stored?
No. The sums run in your browser and nothing is transmitted or saved.
What this calculates, and what it does not
This tool compares the geometry implied by two size codes and estimates speedometer and odometer error from the difference in diameter. It does not confirm that a size fits your vehicle, carries your load or is approved by the manufacturer. Use the vehicle placard and the tire maker's specifications for fitment, and have a qualified shop check clearance and calibration.