Tire Size Comparison Calculator
Compare two tire sizes side by side: overall diameter, width, sidewall height, revolutions per mile, and how far your speedometer will be off. Made for anyone choosing between their current tires and a wider, taller or plus-size set. Free, and it runs in your browser.
1.8% taller overall: inside the common 3% guideline. Speedometer and odometer will be off by about the same percentage.
| Measure | Current 225/65R17 | New 235/65R17 | Difference |
|---|---|---|---|
| Overall diameter | 28.5 in (724 mm) | 29.0 in (737 mm) | +0.5 in (+1.8%) |
| Section width | 8.9 in (225 mm) | 9.3 in (235 mm) | +0.4 in (+10 mm) |
| Sidewall height | 5.8 in (146 mm) | 6.0 in (153 mm) | +0.3 in (+7 mm) |
| Circumference | 89.6 in | 91.2 in | +1.6 in (+1.8%) |
| Revolutions per mile | 707 | 695 | -12 (-1.8%) |
| Rim diameter | 17 in | 17 in | 0 in |
Ground clearance changes by about +0.3 in (+7 mm) at the axle, before load and tire flex.
| Speedometer shows (mph) | 30 | 45 | 60 | 70 | 80 |
|---|---|---|---|---|---|
| Actual speed (mph) | 30.5 | 45.8 | 61.1 | 71.3 | 81.4 |
| Speedometer shows (km/h) | 50 | 80 | 100 | 120 |
|---|---|---|---|---|
| Actual speed (km/h) | 50.9 | 81.4 | 101.8 | 122.2 |
Assumes the speedometer is accurate on the current size and reads from wheel rotation. Real speedometers can read a little high or low.
Estimates from the size codes only. This does not check whether the new size fits, rubs or is safe on your vehicle; ask a tire shop before you buy.
How to compare two tire sizes
- Find the size on your tire's sidewall. It looks like 225/65R17: width in millimetres, aspect ratio, construction letter and rim diameter in inches. If the code starts with P or LT, leave it out or keep it; the calculator reads both.
- Type it into Current tire size, then type the size you are considering into New tire size.
- Read the verdict line first. It tells you whether the overall diameter changed by more than about 3%.
- Check the speedometer table. It shows what you are really doing when the speedometer reads 60 mph or 100 km/h on the new tires.
- Look at the width and sidewall rows if you are worried about clearance. Then ask a tire shop whether the size suits your wheels and vehicle.
Need only one tire? Use the tire size converter to turn a metric size into inches, or learn how to read a tire size.
What the numbers mean
- Overall diameter
- The height of the tire from the ground to the top when mounted and inflated: rim diameter plus two sidewalls. It decides speedometer accuracy and, together with width, how the tire sits in the wheel well.
- Section width
- The first number in the size, in millimetres, measured across the widest part of the sidewalls. The calculator also shows it in inches.
- Sidewall height
- Section width multiplied by the aspect ratio. A 225/65R17 has a sidewall of 225 × 0.65 = 146 mm (5.8 in).
- Revolutions per mile
- How many times the tire turns to cover a mile: 63,360 inches divided by the circumference. A taller tire turns less, which is why the speedometer reads low.
- Circumference
- Diameter multiplied by pi. It is the distance the car moves in one turn of the wheel.
Speedometer error when you change tire size
Most speedometers and odometers count wheel turns and convert them to distance using the original tire size. If the new tire has a bigger circumference, each turn covers more road than the car thinks, so the speedometer reads lower than your real speed and the odometer under-counts miles. A smaller tire does the opposite.
The correction is a single ratio: actual speed equals displayed speed times new diameter divided by old diameter. Take the default pair above. The 225/65R17 is 28.5 in tall and the 235/65R17 is 29.0 in tall, a ratio of 1.018. When the speedometer shows 60 mph you are doing 61.1 mph, and 100 km/h on the dial is 101.8 km/h.
This assumes the speedometer was accurate on the original tires. Real speedometers are allowed to read a little off, and some vehicles can be recalibrated by a dealer or a tuner. Treat the table as a guide, not a measurement.
The 3% guideline and other things to check
Many tire shops and guides suggest staying within about 3% of the original overall diameter. Inside that range the speedometer error stays small and gearing, traction control and clearance usually stay close to stock. Outside it you should expect side effects, and some vehicles need a recalibration.
- Load index and speed rating. A new size must carry at least the load and speed your vehicle requires. The numbers are on the door-jamb placard.
- Rim width. Every tire size has an approved range of rim widths. A wider tire on a narrow rim changes how the tire behaves.
- Clearance. A wider or taller tire can touch the fender, suspension or liner when you turn or hit a bump. A calculator cannot see this.
- Brakes and offset. A bigger rim has to clear your brake calipers, and offset decides how far the tire sits out.
- Warranty and insurance. Some vehicle makers and insurers care about non-original sizes. Ask before you buy.
Popular tire sizes in inches
Truck and SUV owners often search for these sizes. Each page lists diameter, width, sidewall height and sizes that are close in diameter.
Popular tire size comparisons
Side-by-side pages for common upgrades, each with the speedometer difference worked out.
Tire size comparison FAQ
How do I compare two tire sizes?
Enter the size printed on your current tire and the size you are thinking of buying, such as 225/65R17 and 235/65R17. The calculator shows overall diameter, section width, sidewall height, circumference and revolutions per mile for both, and the difference between them.
What is the 3% rule for tire sizes?
A common rule of thumb says a replacement tire should have an overall diameter within about 3% of the original. A bigger gap changes the speedometer reading, odometer, gearing feel and possibly clearance. It is a guideline, not a guarantee that a size will fit, so check with a tire shop.
How much will my speedometer be off with different tires?
If the speedometer is accurate on the current size, the actual speed with the new size is the displayed speed multiplied by new diameter divided by old diameter. For 225/65R17 to 235/65R17 that is a factor of 1.018, so 60 mph on the speedometer is 61.1 mph in reality.
Does a wider tire change the diameter?
Not by itself. Diameter depends on the rim size plus two sidewalls, and the sidewall height is the width multiplied by the aspect ratio. A wider tire with the same aspect ratio has a taller sidewall, so it is also taller overall.
Will a different size rub or fit my car?
This calculator cannot tell. Rubbing depends on tire width, wheel offset and width, suspension, steering lock and the fender liners on your vehicle. Check the placard on your driver's door jamb and ask a tire shop before you buy.