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About the tire size calculator
Before you buy new tires or wheels, check how the new size compares with the old one. A tire that is too big or too small in overall diameter makes your speedometer and odometer wrong, and can rub on the car or upset the ABS.
Type your tire now and the new tire, as printed on the sidewall — for example 205/55 R16. The calculator shows the difference in overall diameter, the speedometer error, the change in width, sidewall height and ride height, and a list of similar sizes with nearly the same diameter.
How to use it
- Type your tire now, like 205/55 R16
- Type the new tire, or press one of the common sizes
- Read the difference in overall diameter — under 3 % is usually fine
- Use the speedometer table to see what you really drive
- Look at the similar sizes list for other options
- Open More options to switch the speed between km/h and mph
How to read a tire size
205/55 R16 means:
- 205 — the width of the tire in millimetres
- 55 — the aspect ratio: the sidewall height is 55 % of the width
- R — radial construction
- 16 — the wheel (rim) diameter in inches
Overall diameter = rim × 25.4 + 2 × width × aspect ÷ 100 (in mm). For 205/55 R16: 406.4 + 2 × 112.75 = 631.9 mm.
Examples
| Old size | New size | New diameter | Difference | You really drive when the speedometer shows 100 |
|---|---|---|---|---|
| 205/55 R16 | 225/45 R17 | 634.3 mm | +0.4 % | 100.4 |
| 195/65 R15 | 205/55 R16 | 631.9 mm | −0.4 % | 99.6 |
| 215/60 R16 | 225/55 R17 | 679.3 mm | +2.2 % | 102.2 |
| 225/45 R17 | 235/40 R18 | 645.2 mm | +1.7 % | 101.7 |
| 205/60 R16 | 215/55 R17 | 668.3 mm | +2.4 % | 102.4 |
| 265/70 R16 | 285/70 R17 | 830.8 mm | +6.9 % | 106.9 |
The last example is too big — the speedometer would show 100 while you really drive almost 107.
Similar sizes for 205/55 R16
Sizes with nearly the same overall diameter (within 1.5 %), good for "plus sizing" to bigger wheels:
| Size | Diameter | Difference |
|---|---|---|
| 205/55 R16 | 632 mm | 0.0 % |
| 225/50 R16 | 631 mm | −0.1 % |
| 195/45 R18 | 633 mm | +0.1 % |
| 215/35 R19 | 633 mm | +0.2 % |
| 185/40 R19 | 631 mm | −0.2 % |
What to check besides diameter
- Load index and speed rating must be at least as high as the original
- Width must fit the rim and the wheel arch
- Lower sidewalls (bigger wheels with low profile) give sharper steering but a harder ride and a greater risk of rim damage
- Wheel offset and clearance — a tire shop can check
- Your car's owner manual lists the approved sizes
Frequently asked questions
How much difference in tire size is OK?
Most makers say within 3 % of the original overall diameter. Less is better. Check the speedometer error in the table.
Does a bigger tire change my speedometer?
Yes. A larger diameter turns fewer times per kilometre, so the speedometer shows less than your real speed. The table shows the real speed.
Can I fit wider tires on the same rim?
Often a little (10–20 mm), but check the rim's allowed width range and the clearance. A tire shop knows the rules.
What is a plus size?
Fitting a bigger rim (plus one or plus two) and a tire with a lower sidewall, so the overall diameter stays the same.
What is the sidewall height?
Width × aspect ratio. For 205/55 it is 112.75 mm. A taller sidewall gives a softer ride.
Is this advice from a tire shop?
No. It is a calculation guide. Always check with a professional and your owner's manual.
Good to know
- Your sizes are saved only in this browser
- Nothing you type is sent anywhere
Related tools
- Fuel Cost Calculator — what the car costs to run
- Fuel Economy Converter — mpg, L/100 km and km/L
- Auto Loan Calculator — finance the car
- Unit Converter — mm, inches, km/h and mph
- Percentage Calculator — the percent difference
