# Tire Tech

## Light Truck Fitment Formula

(Lea en español)

Below are some of the mathematical formulas used to answer commonly asked questions concerning fitments of light truck tires.

### How much extra ground clearance under the axles will be provided? or How much will it raise the vehicle's center of gravity?

(New Tire Diameter-Original Tire Diameter) / 2 = Approx. change in ground clearance and center of gravity

Example:
(32.5" - 27") / 2 = 2.75" change (increase) in ground clearance and center of gravity

### How will it affect the acceleration performance of the truck?

The answer is that it has the same affect as changing the axle ratio of the truck.

(Original Tire Diameter / New Tire Diameter) X Original Axle Ratio = Effective Axle Ratio

Example:
(27" / 32.5") X 3.50 Original Axle Ratio = 2.91 Effective Axle Ratio

### How will it affect the performance of the truck on the highway?

(Original Tire Diameter / New Tire Diameter) X RPM@MPH = New RPM @ actual MPH

This example is for 55 MPH

Example:
(27" / 32.5") x 2400 RPM = 1985 RPM @ 55 mph with new tires.

### How far off will the speedometer be?

May be speeding ticket time!

(New Tire Diameter / Original Tire Diameter) X Indicated Speed = Actual Speed with new tires

Example:
(32.5" / 27") X 55MPH = 66.2 MPH Actual Speed with new tires

### What can I do to regain the performance?

Change the differential gears if the truck is equipped with solid axles at a cost of about \$400 - \$500 per axle.

(New Tire Diameter / Original Tire Diameter) X Original Axle Ratio = Required Axle Ratio

Example:
(32.5" / 27") X 3.50 Original Axle Ratio = 4.20 Required Axle Ratio

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