A verdict, not just numbers
The 3% diameter rule is applied for you and colour coded, so you know at a glance whether a size is a straight swap, a borderline call, or a change that will upset ABS, traction control and the speedometer.
Tire Size Calculator
Compare two tire sizes side by side and see exactly what changes: overall diameter, section width, sidewall height, revolutions per mile, ground clearance and speedometer error. Enter metric sizes like 225/45R17 or inch sizes like 33x12.50R15, and get an instant fitment verdict against the 3% rule shops actually use.
Metric & flotation sizes
Speedometer error table
Visual size comparison
Runs in your browser
Current tire size
225/45R17
Diameter–
Sidewall–
Revs/mile–
New tire size
245/40R18
Diameter–
Sidewall–
Revs/mile–
Current tireNew tire
Safe fitment
+0.0%
Diameter change stays inside the 3% window most manufacturers and tire shops treat as acceptable.
The new tire is 0.00 in taller overall and lifts the body 0.00 in higher off the road.
Overall diameter––
Section width––
Sidewall height––
Circumference––
Revolutions / mile––
Ground clearance––
Speedometer error––
Odometer per 100 mi––
Speedometer reading vs true road speed
| Speedometer shows | You are actually doing | Difference |
|---|
A taller tire covers more ground per revolution, so the needle under-reads and you travel faster than it shows. A shorter tire does the opposite.
Every calculation runs locally in your browser. Nothing you type is uploaded, stored or tracked. Results use nominal published dimensions; real moulded sizes vary by a few millimetres between brands.
Decode the sidewall
How to read a tire size
Every number moulded into the sidewall means something specific. Once you can read it, the calculator above stops being a black box and the fitment verdict starts making sense.
- 225
Section width in millimetres. Measured at the widest point of the inflated tire, sidewall bulge included, not the tread contact patch. It is always a metric number even on cars sold in the United States.
- 45
Aspect ratio, as a percentage. Sidewall height equals 45% of 225 mm, so 101.25 mm. Lower numbers mean a shorter, stiffer sidewall and a sharper steering feel; higher numbers mean more cushion over potholes.
- R
Construction type. R is radial, which covers virtually every modern passenger tire. You may still see D for diagonal bias on trailers and older equipment.
- 17
Rim diameter in inches. This is the one number you cannot change without also changing the wheel itself.
- 94V
Load index and speed rating. 94 maps to a maximum load of 670 kg per tire and V allows sustained speeds up to 149 mph. Never fit a replacement rated below the figures on your door placard.
225/45R17 94V
Width in mm · aspect ratio · rim diameter in inches, followed by the load and speed code.
33x12.50R15
Flotation sizing used on trucks and off-road tires states overall diameter and width directly, both in inches. Switch the calculator to inch mode to work in this format.
Built differently
Why this tire size comparison beats the usual ones
Most tire size calculators hand you a table of numbers and leave you to work out whether the change is safe. This one answers the question you actually came with.
Scaled visual comparison
Both tires are drawn to scale on a shared ground line with the rim inside, so the difference in sidewall and overall height is something you can see rather than something you have to imagine from a decimal.
Real speedometer error
A full table converts indicated speed to true road speed at five real-world speeds, plus the odometer drift per 100 miles, which matters for lease mileage, warranty terms and service intervals.
Metric and inch in one tool
Switch between 245/40R18 and 33x12.50R15 without losing your entry. The calculator converts your current tire into the other format instead of blanking the fields, so plus sizing and lift planning happen in one place.
Numbers that affect driving
Revolutions per mile, circumference, effective ground clearance and section width are all shown with the percentage change, because those are the figures that decide gearing feel, fender rub and wheel well fit.
Fast, private, mobile first
No accounts, no uploads, no heavy frameworks. The whole calculator is a few kilobytes of vanilla JavaScript that works on a phone in a tire shop parking lot with one bar of signal.
Step by step
How to use the tire size calculator
Four steps, about thirty seconds, no sign up.
1
Enter your current size
Read it off the sidewall or the placard inside the driver door and type the three numbers into panel A. Tap a preset if your size is a common one.
2
Enter the size you are considering
Put the candidate size into panel B. If you are shopping truck or off-road tires, switch to inch mode and enter something like 33 x 12.50 on a 15 inch rim.
3
Check the verdict and the diagram
The badge tells you whether the change is inside the safe window. The scaled drawing shows how much taller or shorter the new tire actually sits.
4
Read the speedometer table
Confirm how far your speedometer will drift, then copy the full comparison to send to your installer or keep with your build notes.
Why it matters
What changing tire size actually affects
Diameter is not a cosmetic choice. It feeds almost every system on the car that measures wheel rotation.
- Speedometer and odometer accuracy
A 3% taller tire means roughly 2 mph of under-reading at 70 mph and about 3 extra miles logged for every 100 you drive.
- ABS, stability control and TPMS
These systems compare wheel speed sensors against a stored rolling circumference. Large diameter changes can trigger warning lights or dull intervention thresholds.
- Effective gearing and fuel economy
A taller tire behaves like a taller final drive: slightly lazier acceleration, lower cruising revs, and a small shift in real world fuel consumption.
- Clearance and rubbing
Half of any diameter increase goes upward into the wheel arch. Extra section width eats into suspension components and inner liners at full lock.
- Ride comfort and steering feel
Plus sizing to a bigger rim with a shorter sidewall sharpens turn in but transmits far more of the road surface into the cabin.
- Load capacity and legality
The replacement must meet the load index and speed rating on your placard. Insurance and roadworthiness checks in many regions look at exactly this.
Questions
Tire size calculator FAQ
The questions people ask most before they commit to a new size.
How much can I change tire size without problems?
The working rule across the industry is 3%. Keep the new overall diameter within 3% of the original and the speedometer stays close enough to legal tolerance, ABS and stability control keep behaving normally, and clearance rarely becomes an issue.
Between 3% and 5% you are into territory where a speedometer recalibration is sensible and rubbing becomes possible at full steering lock or under suspension compression. Beyond 5% you should expect warning lights, noticeably wrong speed readings and, on lifted trucks, potential gearing changes to compensate.
What does 225/45R17 actually mean?
225 is the section width in millimetres, 45 is the aspect ratio meaning the sidewall is 45% of that width, R indicates radial construction, and 17 is the rim diameter in inches. Multiplying 225 by 0.45 gives a sidewall of 101.25 mm, or roughly 3.99 inches, so the overall diameter is 17 plus twice 3.99, about 24.98 inches.
Will bigger tires make my speedometer wrong?
Yes, in a predictable direction. A larger diameter tire travels further per revolution, so the vehicle covers more ground than the sensors report and the needle reads low. If your new tire is 3% taller, a displayed 70 mph is really about 72 mph, and your odometer records about 3% fewer miles than you have driven.
Smaller tires flip it: the speedometer over-reads and the odometer adds miles you never travelled. The table above gives the exact figures for your two sizes.
What is plus sizing, and does it change the diameter?
Plus sizing means moving to a larger wheel while keeping the overall tire diameter almost unchanged. Each inch added to the rim is compensated by a wider tread and a lower aspect ratio, so 205/55R16, 225/45R17 and 245/40R18 all land within a fraction of an inch of each other.
Done properly it improves steering response and fills the arch better without touching your gearing or speedometer. Done carelessly it changes diameter, which is exactly what this calculator is for.
How do I convert a metric tire size to inches?
Multiply the width by the aspect ratio and divide by 100 to get the sidewall in millimetres, divide that by 25.4 to get inches, double it and add the rim diameter. For 265/70R17 that is 265 x 0.70 = 185.5 mm, which is 7.30 in, doubled to 14.60 in, plus 17 gives an overall diameter of about 31.6 inches. Switching the calculator to inch mode does this conversion for you and keeps both sizes in view.
What size tire is equivalent to a 33 inch tire?
On a 17 inch wheel, 285/70R17 comes out at about 32.7 inches and is the usual metric stand in for a 33. On an 18 inch wheel, 285/65R18 measures roughly 32.6 inches. On a 20 inch wheel, 275/65R20 is close at about 34.1 inches, slightly over.
Because moulded sizes vary between manufacturers, always compare the specific tire model's published overall diameter rather than assuming the metric conversion is exact.
Do wider tires need a wider wheel?
Each tire size has an approved rim width range published by the manufacturer, usually spanning about two inches. A 245 section tire is typically approved for 7.5 to 9.5 inch wheels. Fitting a wide tire on a narrow rim balloons the sidewall and makes the car feel vague; fitting it on an over-wide rim stretches the tread and exposes the rim edge to kerb damage. Check the approved range before buying, since the diameter calculation alone does not confirm a rim is suitable.
Can I run different tire sizes front and rear?
Only if the vehicle was designed as a staggered fitment, which is common on rear wheel drive sports cars where the rear tires are wider but the diameters still match closely. Mixing different overall diameters front to rear on an all wheel drive or four wheel drive vehicle forces the centre differential or transfer case to absorb a permanent speed difference, and that is a genuine mechanical risk rather than a theoretical one.
Check the size before you buy the tires
A five minute check now avoids a rubbing fender, a warning light, or a speedometer that quietly lies to you for the next forty thousand miles. Enter both sizes, read the verdict, and take the comparison with you to the shop.
Compare two tire sizes now