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Old 10-15-2009, 10:26 PM   #1
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Looking to change tire size? Read me!

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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Old 10-15-2009, 10:48 PM   #2
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Originally Posted by 95riogrande View Post
"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.

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Old 10-15-2009, 10:51 PM   #3
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what kind of TRUCK is this refering to? JEEP TRUCK JEEP TRUCK JEEP TRICK TRUCK
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IF YOU CAN'T EAT IT OR PLAY WITH IT,
JUST PEE ON IT AND WALK AWAY.
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Old 10-15-2009, 10:53 PM   #4
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Originally Posted by NAVYCHIEF View Post
what kind of TRUCK is this refering to? JEEP TRUCK JEEP TRUCK JEEP TRICK TRUCK
Touche
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Old 10-15-2009, 11:05 PM   #5
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Are you sure that these are accurate? I'm running 33's and my speedometer reads 5mph under my actual speed at all times.
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
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Old 10-15-2009, 11:13 PM   #6
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Honestly I've never looked at it while going that slow.

your post is acting weird. If looking at it on the forum it reads:

Quote:
Originally Posted by 95riogrande View Post
so it reads zero til you go 10 mph?
However when i went to quote you it read:

Quote:
Originally Posted by 95riogrande View Post
so it reads zero til you go 10 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
But anyway, I know for a fact that when my speedometer says 20, 25, 30, 35 my actual speed (according to the police "Your Speed" signs) is 25, 30, 35 ,40
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Old 10-15-2009, 11:14 PM   #7
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And now it reads even more... i'm not quite sure whats going on
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Old 10-15-2009, 11:15 PM   #8
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didnt realize it re-post or shows each edit when I make changes lol
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Old 10-15-2009, 11:24 PM   #9
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but yeah it would be off about 5mph at 25mph compared to a stock 27 or 27.5 inch tire to a 32.5 inch tire
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Old 10-16-2009, 12:35 AM   #10
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a larger tire makes your speedo run slower, not faster
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Old 10-16-2009, 01:24 AM   #11
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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.
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Old 10-16-2009, 06:04 AM   #12
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Originally Posted by NAVYCHIEF View Post
what kind of TRUCK is this refering to? JEEP TRUCK JEEP TRUCK JEEP TRICK TRUCK
Quote:
Originally Posted by 95riogrande View Post
didnt realize it re-post or shows each edit when I make changes lol
If not calling it a jeep, I'd much rather the term truck was used instead of car!

It is after all a small truck!
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Old 10-16-2009, 08:49 AM   #13
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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.
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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Old 10-16-2009, 05:50 PM   #14
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running 33 with stock spedo gear in my yj at 5 mph its dead accurate at 10 its like 2 above and so on till like about 55 then 55 is 64 and thats my jeep running as fast as she can! haha dont ask me why but thats what i have found..
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Old 11-10-2009, 05:17 PM   #15
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Quote:
"How far off will the speedometer be?"

May be speeding ticket time!
Have any of you guys tried that GPS program that alerts you to speed traps. looks interesting. I was thinking of trying out phantom alert. anyone with personal experience in here?

phantom alert review
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Old 09-17-2010, 08:19 AM   #16
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According to my GPS, when I ran at 55 mph on the speedo, I was traveling 57-58 mph. That was with 32x11.50's. My stock tires were 30x9.50. When I
changed to 31x10.50's my GPS reads 56 when the speedo reads 55.

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