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Tyre size calculator

Here you can work out which tyre sizes fit your car without any hassle, and calculate the rim width for the tyres you choose. The calculator also works out the speedometer error, the change in ground clearance and the rest of the wheel and tyre parameters: the tool covers every parameter of the wheel at once.

Serhii Maslov Tool Updated:

Calculation

195/65 R15 → 205/55 R16
Standard tyre size

Reads as 195/65 R15

The size on the fuel filler flap or the driver’s door pillar.

You want to fit

Reads as 205/55 R16

Everything recalculates automatically.

Ground clearance will change by-1 mm The speedometer shows 100, your true speed is 99.55 km/h. Speedometer error -0.5 %.

Within 5%: the geometry is acceptable. Overall diameter -0.5 %, width +5.1 %.Over 5%: the tyre width Wt differs by +5.1 %. Fitting this tyre is not recommended: the load on the suspension, the clutch and the gearbox goes up, and the wheel may foul the bodywork or the suspension.

Tyre parameters
Parameter Standard New Difference
WtTyre width mm 195 205 +10+5.1 %
dtSidewall height mm 127 113 -14
ddRim diameter mm 381 406 +25
WdRim width inches 5.5-7 5.5-7 -
DOverall wheel diameter mm 635 632 -3-0.5 %
Decimal separator: point

True speed

Speedometer: instrument reading and true speed Inner numbers: the speedometer reading. Outer, orange: true speed for the new tyre size. 0 20 40 60 80 100 120 140 160 180 200 220 240 260 19.9 39.8 59.7 79.6 99.55 119.45 139.35 159.25 179.15 199.05 218.95 238.85 Speedometer reading
Speedometer reading 100 km/htrue speed 99.55 km/h

The speedometer counts wheel revolutions and assumes the car has the standard size fitted. Inner numbers: the instrument reading. Outer: true speed for the new size.

True speed, km/h (outer numbers)

  • 2019.9
  • 4039.8
  • 6059.7
  • 8079.6
  • 10099.55
  • 120119.45
  • 140139.35
  • 160159.25
  • 180179.15
  • 200199.05
  • 220218.95
  • 240238.85
Wheel diagram, side view: D overall wheel diameter. Values for the new size. D 632 side view Wheel diagram, cross-section: Wt tyre width, dt sidewall height, dd rim diameter, Wd rim width. Values for the new size. Wt 205 mm Wd 5.5-7″ dt 113 dd 406 cross-section, not to scale

In the diagram: the new size 205/55 R16.

Wheel and tyre parameters

The key parameters when choosing tyres and wheels are:

  • D - wheel diameter. Increase this significantly and the wheel may start rubbing against the bodywork or the running gear (wings, bumper, track rod ends and so on). It will also throw off the speedometer reading, in direct proportion to the difference in diameter from the car’s standard wheel.
  • Wt - tyre width. Tyre width is shown on the sidewall (the first number in the size). A wide tyre improves handling on a dry road and shortens braking distance; a narrow one is more comfortable in city conditions and on a slippery but firm road surface.
  • Wd - rim width. Rim width in inches is the first number in the wheel marking. It is very important for the rim width to match the tyre width (see the table), otherwise the tyre may fail while driving, or it may not be possible to fit it onto the wheel at all.
  • dt - sidewall height. In the tyre size the profile is given as a percentage of the width; in the table, in millimetres. Low-profile tyres increase the risk of damaging the wheels on a poor road surface (potholes, kerbs and so on), and they also increase the load on the suspension over potholes and bumps.
  • dd - rim diameter. This is given in the wheel marking in inches, and in the table in millimetres.

This tool quickly works out every change in wheel geometry if you decide to fit tyres of a different size. What sets it apart from other such tools is that all the wheel and tyre parameters (plus your true speed at various speedometer readings) appear on screen as soon as you choose the tyre size. There is nothing extra to click or type.

How to use the tyre size calculator

In the table, choose the tyre size shown on your car’s fuel filler flap (or on the driver’s door pillar), and the tyre size you would like to fit. All the tyre and wheel geometry figures, and the comparison between them, appear in the table straight away. Below the table, above the speedometer, you will see your true speed at the corresponding speedometer readings.

Assessing the calculator’s results. If the overall wheel diameter (D) exceeds the standard figure by more than 5%, or the maximum tyre width (Wt) exceeds it by more than 5%, fitting such tyres to the car becomes very doubtful, and it also increases the load on the suspension, the clutch and the gearbox, so it is strongly not recommended.

Question and answer

What is a tyre size calculator for?
If you want to fit tyres your car maker did not list, you need to make sure the overall wheel diameter stays the same. If it is significantly larger than the standard size, the wheel may foul the bodywork or the suspension.

See also

How do you choose wheels?What criteria should you follow first when choosing alloy wheels for a car? How do you choose tyre width for the season?In reality, all else being equal, tyre width only affects one parameter: the size of the tyre’s contact patch with the road surface. What does that mean? Wheel markingsWheel markings. What is stamped on a wheel? The main markings on a wheel usually look something like this: 6.5J x 15 5/112 H2 ET45 d 57.1.