Calculations

Here's some of the mathematical formulas used to answer commonly asked questions concerning fitments of light truck tires.
"How do I figure out my gear ratios?"
Click here to download a ratio calculator spreadsheet created by Jeff Gauvin.

The way it works is this: you enter your gear ratios (it's already loaded with stock) and your tire circumference (you need to figure this out), and it tells you your overall gear ratios ("Final"), RPM@70mph in each gear, and mph/1000rpm in each gear.
"How much extra ground clearance under the axles will be provided?" or "How much will it raise the vehicles 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 ?"
Maybe 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.
(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
"How can I calculate how big a tire actually is?"
Tire width(first number)* aspect ratio(second number as a percentage. Ex. 75 = .75) = ??.?? Take that number, multiply by 2, and then divide by 25.232 and add rim width (16")

Example...
285 / 75 / 16
(285 x .75) x 2 / 25.232 + 16= tire height
How to tell how tall your tires really are (since what the maker says and what it is can differ), and at the pressure you run them at:

Mark the bottom of your tire and the ground at the center of where it meets the ground. Roll forwards (or backwards) exactly one revolution. Measure the distance on the ground between marks and divide by "pi"/3.141ish. The answer will be the actual/real/instant height of the tire. Repeat for front/rear tires to see what the difference is. (The difference is made up for by the center diff)

"How do I figure out my gear ratios?"

"How much extra ground clearance under the axles will be provided?" or "How much will it raise the vehicles 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 ?"

Maybe 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. (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

"How can I calculate how big a tire actually is?"

Tire width(first number)* aspect ratio(second number as a percentage. Ex. 75 = .75) = ??.?? Take that number, multiply by 2, and then divide by 25.232 and add rim width (16") Example... 285 / 75 / 16 (285 x .75) x 2 / 25.232 + 16= tire height

How to tell how tall your tires really are (since what the maker says and what it is can differ), and at the pressure you run them at:

(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.

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

(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.

Maybe 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.

Change the differential gears.
(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

Tire width(first number)* aspect ratio(second number as a percentage. Ex. 75 = .75) = ??.?? Take that number, multiply by 2, and then divide by 25.232 and add rim width (16")

Example...
285 / 75 / 16
(285 x .75) x 2 / 25.232 + 16= tire height