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Looking to change tire size? Read me!

10K views 15 replies 8 participants last post by  mistert 
#1 ·
Found some very useful info.

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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#2 ·
"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
Are you sure that these are accurate? I'm running 33's and my speedometer reads 5mph under my actual speed at all times.
 
#5 ·
so it reads zero til you go over 5 mph? it should vary with the speed that you calculate at a slower speed your guage if only off maybe 1 mph and maybe 3 mph or more on the highway. well i would think the correct stock speedo gear has to be installed to calculate the change. from stock tires and speedo to new tire size
 
#11 ·
not sure where you read that but ok, thank you. yes larger tires = stock speedo reading slower than actual vehicle speed. its a thread for the people who want to figure out what they might have to change or what changes when upgrading tire size. if you can't figure out the formulas or don't believe they are correct don't use them.
 
#13 ·
dont get your panties in a bunch. I was adding important info that was missing. I have changed my gears before to correct power and speedo accuracy, and am fully familiar. Using the forumulas above will also fix the speedo for the most part. Mine was 99% accurate at 60mph after my swap.
 
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