Tire size calculator
Type in your current tire and the one you are thinking of. You get the diameter in inches, sidewall and width, revolutions per mile, how far off the speedometer will be, and what bigger tires do to engine rpm and gearing. Metric (265/70R17) and inch (33x12.50R17) sizes both work.
Inputs
Result
Side by side
Speedometer reading vs actual speed
With the speedometer still set up for the current tire.
| Speedometer shows | Actual speed | Difference |
|---|
Engine rpm by axle ratio
Common ring-and-pinion ratios; which ones are made depends on the axle. “Effective” is what each ratio feels like compared with the current tire. The closest match to the old gearing is marked.
| Axle ratio | Rpm | vs now | Effective ratio |
|---|
How to read a tire size
Take LT265/70R17 121S. LT means light truck (P is passenger; no letter is the European metric size, which is built to the same dimensions as P-metric). 265 is the section width in millimetres. 70 is the aspect ratio: the sidewall is 70% as tall as the tire is wide. R is radial construction, and 17 is the wheel diameter in inches. 121S is the load index and speed rating, which do not change the size.
Inch, or flotation, sizes like 33x12.50R17 give it to you directly: 33 inches tall, 12.5 inches wide, for a 17 inch wheel.
The formulas
For 265/70R17: 265 × 0.70 = 185.5 mm of sidewall, or 7.30 in. Twice that plus the 17 inch wheel is 31.61 in tall, 99.3 in around, and 638 turns per mile. A 225/45R17 on a car works out at 25.0 in.
Tire size chart: metric to inches
Nominal sizes from the formula above. Inches.
| Size | Diameter | Width | Sidewall | Revs / mile |
|---|---|---|---|---|
| Cars | ||||
| 195/65R15 | 25.0 | 7.7 | 5.0 | 807 |
| 205/55R16 | 24.9 | 8.1 | 4.4 | 811 |
| 215/55R17 | 26.3 | 8.5 | 4.7 | 767 |
| 225/45R17 | 25.0 | 8.9 | 4.0 | 808 |
| 225/65R17 | 28.5 | 8.9 | 5.8 | 707 |
| 235/65R17 | 29.0 | 9.3 | 6.0 | 695 |
| 235/60R18 | 29.1 | 9.3 | 5.6 | 693 |
| 245/45R18 | 26.7 | 9.6 | 4.3 | 756 |
| 255/35R19 | 26.0 | 10.0 | 3.5 | 775 |
| Trucks and SUVs | ||||
| 245/75R16 | 30.5 | 9.6 | 7.2 | 662 |
| 265/75R16 | 31.6 | 10.4 | 7.8 | 637 |
| 265/70R17 | 31.6 | 10.4 | 7.3 | 638 |
| 275/65R18 | 32.1 | 10.8 | 7.0 | 629 |
| 275/60R20 | 33.0 | 10.8 | 6.5 | 611 |
| 275/55R20 | 31.9 | 10.8 | 6.0 | 632 |
| 285/75R16 | 32.8 | 11.2 | 8.4 | 614 |
| 285/70R17 | 32.7 | 11.2 | 7.9 | 617 |
| 275/70R18 | 33.2 | 10.8 | 7.6 | 608 |
| 295/70R17 | 33.3 | 11.6 | 8.1 | 606 |
| 315/70R17 | 34.4 | 12.4 | 8.7 | 587 |
| 315/75R16 | 34.6 | 12.4 | 9.3 | 583 |
| Flotation (inch) sizes | ||||
| 31x10.50R15 | 31.0 | 10.5 | 8.0 | 651 |
| 33x12.50R15 | 33.0 | 12.5 | 9.0 | 611 |
| 33x12.50R17 | 33.0 | 12.5 | 8.0 | 611 |
| 33x12.50R20 | 33.0 | 12.5 | 6.5 | 611 |
| 35x12.50R17 | 35.0 | 12.5 | 9.0 | 576 |
| 35x12.50R20 | 35.0 | 12.5 | 7.5 | 576 |
| 37x12.50R17 | 37.0 | 12.5 | 10.0 | 545 |
Metric sizes that come out close to 33 inches: 285/70R17 (32.7 in), 285/75R16 (32.8 in), 275/60R20 (33.0 in), 275/70R18 (33.2 in), 295/70R17 (33.3 in). Close to 35 inches: 315/70R17 (34.4 in), 315/75R16 (34.6 in). 37 inch tires are almost always sold in flotation sizes.
Speedometer and odometer error
The speedometer counts how fast the wheels turn and assumes the tire it was set up for. A taller tire covers more ground per turn, so you go faster than the speedometer shows, and the odometer under-counts by the same percentage. Going from 31.6 in to 33 in tires is 4.4% taller, so an indicated 60 mph is really 62.6 mph. Smaller tires do the opposite.
Most newer vehicles can be recalibrated with a programmer or by a dealer; older ones use a speedometer drive gear in the transmission.
The 3% rule
A common guideline in tire and wheel fitment is to keep the new overall diameter within about 3% of the original. Inside that range speedometer error stays small and ABS, traction control and transmission shift points behave as designed. It is a rule of thumb, not a law; lifted trucks routinely go well past it and then recalibrate.
When changing wheel size (“plus sizing”, such as going from a 17 to an 18 inch wheel), drop the aspect ratio to keep the diameter close, and choose a tire whose load index is at least the original’s. The placard on the driver’s door jamb lists the original size and pressures.
Bigger tires and gear ratio
A taller tire acts like a numerically lower axle ratio. On 31.6 in tires and 3.73 gears, moving to 33s makes the gearing feel like 3.57; to get back the original engine rpm you would need 3.89, so 3.73 or 4.10. On 35s it takes 4.13 from 3.73, so 4.10 or 4.30; from 3.55 it takes 3.93, so 3.73 or 4.10.
The 336 is 63,360 inches in a mile divided by 60 minutes and by π. At 65 mph in a 1:1 gear, 3.73 gears and 31.6 in tires turn 2,578 rpm; on 35s the same gears turn 2,328.
Axle ratio needed from 31.6 in tires (265/70R17)
| Current ratio | To 33 in | To 35 in | To 37 in |
|---|---|---|---|
| 3.21 | 3.35 | 3.55 | 3.76 |
| 3.42 | 3.57 | 3.79 | 4.00 |
| 3.55 | 3.71 | 3.93 | 4.16 |
| 3.73 | 3.89 | 4.13 | 4.37 |
| 4.10 | 4.28 | 4.54 | 4.80 |
Engine rpm at 65 mph in a 1:1 gear
Multiply by your top gear ratio for overdrive: a 0.70 overdrive turns 70% of these figures.
| Axle ratio | 28 in | 30 in | 31 in | 33 in | 35 in | 37 in |
|---|---|---|---|---|---|---|
| 3.08 | 2,403 | 2,243 | 2,171 | 2,039 | 1,923 | 1,819 |
| 3.21 | 2,505 | 2,338 | 2,262 | 2,125 | 2,004 | 1,896 |
| 3.42 | 2,669 | 2,491 | 2,410 | 2,264 | 2,135 | 2,020 |
| 3.55 | 2,770 | 2,585 | 2,502 | 2,350 | 2,216 | 2,096 |
| 3.73 | 2,911 | 2,717 | 2,629 | 2,470 | 2,328 | 2,203 |
| 4.10 | 3,199 | 2,986 | 2,890 | 2,715 | 2,559 | 2,421 |
| 4.30 | 3,355 | 3,132 | 3,031 | 2,847 | 2,684 | 2,539 |
| 4.56 | 3,558 | 3,321 | 3,214 | 3,019 | 2,847 | 2,693 |
| 4.88 | 3,808 | 3,554 | 3,439 | 3,231 | 3,046 | 2,882 |
Questions
How do I calculate tire diameter?
Multiply the width in millimetres by the aspect ratio, divide by 100 and by 25.4 to get the sidewall in inches, double it and add the wheel diameter. For 265/70R17 that is 265 × 0.70 ÷ 25.4 = 7.30 in, times two, plus 17 = 31.6 in.
What size is a 33 inch tire in metric?
Common metric sizes near 33 inches are 285/70R17 (32.7 in), 285/75R16 (32.8 in), 275/60R20 (33.0 in), 275/70R18 (33.2 in), 295/70R17 (33.3 in). Exact measured height varies by brand, so check the maker's spec if clearance is tight.
What size is a 35 inch tire in metric?
Metric sizes close to 35 inches include 315/70R17 (34.4 in), 315/75R16 (34.6 in). Most 35s are sold in flotation sizes such as 35x12.50R17 or 35x12.50R20.
How much will bigger tires change my speedometer?
By the same percentage as the diameter. Going from 31.6 in to 33 in tires is 4.4% taller, so at an indicated 60 mph you are really doing 62.6 mph, and the odometer under-records every mile by the same amount.
What is the 3% rule for tire size?
A fitment guideline: keep the new tire's overall diameter within about 3% of the original so the speedometer, ABS, traction control and shift points stay close to how they were designed. Beyond that, recalibrating is recommended.
What gear ratio do I need for 35 inch tires?
Multiply your current ratio by the new diameter and divide by the old. From 31.6 in tires with 3.73 gears, 35s need about 4.13 to restore the original rpm, so 4.10 or 4.30 depending on what your axle is made in; from 3.55, about 3.93.
How many revolutions per mile does a tire make?
63,360 divided by the circumference in inches. A 31.6 in tire makes about 638 turns a mile; a 33 makes 611 and a 35 makes 576. Makers' published figures are measured under load and are usually slightly higher.
Does a wider tire change the speedometer?
Not by itself. Only overall diameter changes the speedometer. A wider tire with the same aspect ratio is taller, though, because the sidewall is a percentage of the width: 285/70R17 is taller than 265/70R17.