Tire Size Calculator
Decode a tire size like 225/45R17 into diameter, sidewall and revs per mile, compare a second size side by side and see the speedometer error at each speed.
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About this toolWhat is the Tire Size Calculator?
Type the size printed on your tire, such as 225/45R17, and the Tire Size Calculator turns the code into real measurements: overall diameter, sidewall height, section width, circumference and revolutions per mile and per km. Add a second size and it lines the two up side by side, draws them to scale and tells you the percentage change in diameter.
That one percentage drives most of what changes when you swap sizes. A taller tire makes the speedometer read low and adds a little ground clearance; a shorter one makes it read high. The speedometer table shows your true speed at common indicated speeds, so you can see that a +3% change means doing about 62 mph when the dial says 60.
The calculator reads metric sizes with or without a P, LT or ST prefix, ZR speed-rated sizes and the flotation sizes used on trucks and off-roaders, like 33x12.50R15. Load index and speed rating characters after the rim size are ignored.
- Parses metric (P, LT, ST, ZR) and flotation sizes
- Diameter, sidewall, width, circumference and revs per mile and per km
- Side-by-side comparison with differences in mm, inches and percent
- Speedometer error table in mph or km/h
- Scale drawing of both tires
- Clear ±3% guideline check with an honest caveat
- Shareable link and copy-ready summary
How to use the Tire Size Calculator
- Enter your current tire size exactly as it appears on the sidewall, for example 225/45R17 91V.
- Optionally enter a new size to compare, or tap one of the common sizes below the boxes.
- Choose inches or millimetres for the measurements and mph or km/h for the speed table.
- Read the diameter difference and whether it falls within the ±3% guideline, then check the dimension table, the scale drawing and the speedometer table.
- Use Copy result or Share link to send the comparison to a tire shop or forum.
Worked examples
Plus-one wheel upgrade
Input
225/45R17 compared with 235/40R18
Output
Diameter 24.97 in vs 25.40 in, +1.72%; speedometer 60 mph = 61.0 mph
The larger wheel is offset by a lower sidewall (94 mm against 101 mm), so overall height grows by under 2% and stays inside the guideline. Revs per mile drop from 808 to 794.
A matched plus-one size
Input
205/55R16 compared with 225/45R17
Output
+0.38% diameter; 60 mph indicated = 60.2 mph
This pairing is a classic plus-one swap: wider tread and a larger wheel with almost no change to the speedometer.
Bigger tires on a pickup
Input
265/70R17 compared with 285/75R17
Output
31.61 in vs 33.83 in, +7.04%; 65 mph indicated = 69.6 mph
Well beyond the 3% guideline: the truck gains 28 mm of clearance under the axle but the speedometer reads about 4.6 mph low at highway speed and may need recalibrating.
Background and insight
A metric tire size has three numbers. The first is section width in millimetres. The second is the aspect ratio, the sidewall height as a percentage of that width. The letter is the construction (R for radial) and the last number is the wheel diameter in inches. So 225/45R17 is 225 mm wide with a sidewall of 225 x 0.45 = 101.25 mm on a 17 inch wheel. Overall diameter is the wheel plus two sidewalls: 17 x 25.4 + 2 x 101.25 = 634.3 mm, or 24.97 inches.
Flotation sizes state the overall dimensions in inches directly: 33x12.50R15 is 33 inches tall and 12.5 inches wide on a 15 inch wheel, which leaves a 9 inch sidewall. Revolutions per mile follow from the circumference: one mile (1,609,344 mm) divided by pi x diameter. For 225/45R17 that is about 808.
When changing sizes, the widely repeated rule of thumb in the tire trade is to stay within about 3% of the original overall diameter, so the speedometer, ABS, stability control and gearing stay close to how the car was calibrated. It is a guideline, not a law or a manufacturer approval. Load index, speed rating, rim width and clearance to suspension and bodywork matter just as much, and the sizes on your door-jamb placard are the ones the maker has tested.
Common use cases
Choosing new wheels
Find a tire size for a larger or smaller wheel that keeps overall diameter close to stock.
Lifted trucks and off-roaders
See how much taller a flotation or LT size is and how far the speedometer will be off.
Winter tire sets
Check that a narrower winter size on smaller wheels stays within the guideline.
Speedometer correction
Work out your true speed, or the correction factor a programmer or GPS check should show.
Decoding a sidewall
Understand what the numbers on a used tire or a listing actually mean in inches and millimetres.
Tips and limitations
- Results are design dimensions from the size code. Under load a tire squats a little, and tread depth varies by model, so published revs per mile can differ by 1 to 3%.
- The ±3% figure is a rule of thumb used by the tire trade. Your vehicle maker's approved sizes and a tire professional have the final word.
- A taller tire also raises effective gearing, which can dull acceleration and change where automatic gearboxes shift.
- Wider tires need the right rim width. Check the tire maker's approved rim range for the new size.
- Never fit a tire with a lower load index or speed rating than the original specification.
Frequently asked questions
What do the numbers on a tire mean?
In 225/45R17 91V, 225 is the section width in mm, 45 is the sidewall height as a percentage of the width, R means radial construction and 17 is the wheel diameter in inches. 91 is the load index (615 kg per tire) and V is the speed rating (149 mph).
How do you calculate tire diameter?
Multiply width by the aspect ratio to get the sidewall: 225 x 0.45 = 101.25 mm. Overall diameter = wheel diameter in mm + 2 sidewalls = 17 x 25.4 + 2 x 101.25 = 634.3 mm, which is 24.97 inches.
How much bigger can I go on tire size?
A common guideline is to stay within 3% of the original overall diameter so the speedometer and safety systems stay close to calibration. Beyond that, check clearance, the maker's approved sizes and whether the speedometer needs recalibrating.
Do bigger tires make my speedometer read slower?
Yes. A taller tire covers more ground per revolution, so you are going faster than the speedometer shows. With a tire 3% taller, an indicated 60 mph is really about 61.8 mph. A smaller tire has the opposite effect.
What is the difference between P and LT tire sizes?
P-metric tires are built for passenger cars and light SUVs. LT (light truck) tires have stronger construction and higher load ratings for towing and heavy loads, and often run at higher pressures. The size arithmetic is the same.
How do I read a size like 33x12.50R15?
Flotation sizes give the overall diameter (33 inches), the section width (12.5 inches) and the wheel size (15 inches) directly. The sidewall is (33 - 15) / 2 = 9 inches.
Can I put 245 tires instead of 225 on the same wheel?
Often yes, if the rim width is within the range the tire maker approves for the 245 size and there is clearance. Keep the overall diameter close by adjusting the aspect ratio, for example 245/40R17 instead of 225/45R17 (-1.0%).
Is there an in-depth guide to the Tire Size Calculator?
Yes. “Fuel Costs, MPG and Tire Sizes: The Driving Maths Explained” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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