Plus sizing

Effects of tyre size changes on Speedometer & Odometer reading

Plus sizing

Plus sizing wheels and tyres has become the hottest trend among enthusiasts. Adding inches to the wheel diameter for wider, low aspect ratio tyres can increase your cars handling capabilities.

Plus sizing can be segregated into 3 categories i.e. Plus zero, plus one and plus two

The significance of each is as follows:

Plus zero (+0)

Increase the section width and lower the aspect ratio while retaining the wheel diameter. This replacement tyre increases the contact area and lowers the tread profile for improved traction and quicker response. An example for +0 sizing is 175 80 r 13 converted to 185 70 r 13.

Plus one (+1)

This sizing mounts tyres with an increased section width and lower aspect ratio on wheels of a wider diameter. A plus one fitment for a car with 13-inch original equipment wheels would use 14 inch wheels. As a general rule in plus one sizing

  • Increase section width by 10 – 20 mm
  • Decrease aspect ratio by 10 – 20 points
  • Increase rim diameter by 1 inch

An example for +1 sizing is 175 80 r 13 converted to 195 60 r 14.

Negative Offset

Increased wheel widths in plus sizing also contribute to greater tyre stability, improved cornering and maneuverability. A plus two fitment for a car with 13-inch original equipment wheels would use 15 inch rims. As a general rule in plus two sizing

  • Increase section width by 20 –30 mm
  • Decrease aspect ratio by 20 – 30 points
  • Increase rim diameter by 2 inches

An example for +2 sizing is 175 80 r 13 converted to 205 50 r 15.

tyre making ingedients

Effects of tyre size changes

When a change to a different size of tyre is made, the vehicle will travel more or less kilometers than shown on the odometer according to whether a larger or smaller tyre is fitted.

It is difficult to adjust an odometer to take care of this point, so a formula is given which will enable us to calculate how far the vehicle has actually traveled.

P = rolling radius of present tyre

F = rolling radius of future tyre

D = the distance registered by the odometer

The formula is as below

d x f / p

Example

A vehicle originally fitted with 175 80 r 13 converted to 195 60 r 14

P = rolling radius of 175 80 R 13 is 24.02 inches

F = rolling radius of 195 65 R 14 is 23.98 inches

D = odometer reading 100 kms

100 x 23.98 / 24.02 = 99.83

Therefore after fitting 195 65 R 14 tyres in place of 175 80 R 13 the vehicle will actually have run 99.83 kms each time the odometer registers 100 kms. A tolerance from – 1.5% to + 2% can be approved.

It should also be remembered that the speedometer would read slow with a larger tyre and fast with a smaller tyre.