Tire Calculator

PURPOSE:

Buying a new set of wheels for your car isn't as easy as some may think. You must take into consideration the wheel's offset, width, and diameter in addition to the tires width, aspect ratio, and outer & inner diameter. If you get any of these sizes wrong, your car may not perform as you would like it to, or it may not give you the look you want. Your best bet in buying new wheels is to keep the tires outer diameter and width the same as your factory tires, and keep your wheel's offset and width the same as your factory wheels. You certainly can deviate from from the stock size, but you may want to talk to others who already have your planned wheel/tire size to see if they have encountered any problems such as rubbing, poor handling, or the wheel just doesn't look good on the car.

FORMULAS:

Tire Height = Wheel Diameter + (2 x (Section Width / 25.4) X (Aspect Ratio / 100))

Rollout = Tire Height X PI

Wheel Rpm = (Vehicle Speed / 60) X (5280 / (Rollout / 12))

Engine Rpm = Wheel Rpm x Rear Gear Ratio x Transmission Gear Ratio

EXAMPLE TIRE SIZE:

Sample Wheel Size: 215/40-17

215

Section Width
Total width of the tire at the sidewalls in millimeters. This is measured from the outside of the sides of the tires when mounted and not the tread or rim width.

40

Aspect Ratio
Percentage of the section width that is the sidewall height. For this example the 255mm section width multiplied by an aspect ratio of 50 gives a sidewall height of 127.5mm

17

Wheel Diameter
Wheel diameter at the tire mounting surface in inches.

INSTRUCTIONS:

Find the dimensions of your OEM wheels and enter them in the "Current" column, then enter the dimensions of the wheels & tires you would like to buy in the "Planned" column. The difference between the two will be displayed immediately below under the "Results" heading, giving you a direct comparison as to how your new wheels and tires will affect you and your car!

INITIAL INFORMATION

Current

Planned

Section width (millimeters):

Tire aspect ratio:

Wheel diameter (inches):

Wheel width (inches):

Wheel offset (mm):

RESULTS

Current

Planned

Tire height (inches):

Ride height difference (inches):
- N/A -

Tire width difference (inches):
- N/A -

Tire rollout (inches):

Wheel/Tire Aspect Ratio (%):

Wheel diameter difference (inches):
- N/A -

Wheel width difference (inches):
- N/A -

Wheel offset difference (mm):
- N/A -

Wheel Inboard increase (inches):
- N/A -

Wheel Outboard increase (inches):
- N/A -

Tire & Wheel Inboard increase (inches):
- N/A -

Tire & Wheel Outboard increase (inches):
- N/A -

Effect on tire track (inches):
- N/A -

Wheel rpms at mph:

Actual speed with new tire/wheel combination:

Speedometer Error % with new tire/wheel combination:

Engine rpms with rear gear and transmission gear ratio:

Effective gear ratio with new tire/wheel combination:

Tire Calculator PURPOSE:

FORMULAS:

Tire Calculator

PURPOSE: