"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?"
(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?"
(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 equipped with solid axles.
(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
