Does changing the tire size throw off computer?
Team Owner
Joined: May 2005
Posts: 23,682
Likes: 9
From: Flagstaff Az.
Computor
You cannot change the ratio of front to rear diameter without getting the computor involved. I think that as long as you stay with the rear tire diameter staying 1" larger than the front, the width makes no difference. I'm sure those much more knowlegable than me will post. 
Team Owner
Joined: Jan 2002
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From: All that glitters is Gold - Hockey Is CANADA'S game
Cruise-In VI Veteran
Cruise-In VII Veteran
St. Jude Donor '05-'06
You can buy a hand held programmer such as Diablo Sport Predator or Hypertech which will allow you to change the tire sizes etc... 
Team Owner
Joined: May 2005
Posts: 23,682
Likes: 9
From: Flagstaff Az.
tires
When you go to buy your tires have the salesman look up the sizes in width you want. By choosing 30, 35, or 40 ratio tires you can keep the 1" diameter difference you need to keep the computor happy. Even a used programmer is going to be $200 or more.
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Race Director
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From: Newberry FL
Cruise-In VIII Veteran
St. Jude Donor '07
Originally Posted by Zilla
You can buy a hand held programmer such as Diablo Sport Predator or Hypertech which will allow you to change the tire sizes etc...
Micah
Race Director
Joined: Nov 1999
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From: Greer SC
Cruise-In II Veteran
Originally Posted by jules9
So there always has to be a 1 inch differance between th front and back.With the front being one inch lower?
I run
Front: 285/35 ZR18 25.8" diameter
Rear: 305/35 ZR18 26.3" diameter
Active Handling works fine...no problems at all.
Pro
Joined: Jul 2002
Posts: 686
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From: Ex suburban, now Chicago... IL
Originally Posted by jules9
What do you think an inch would be in the ratios?
Tire comparisons
Yellow C3/C5
Joined: Sep 2002
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St. Jude Donor '05-'06
This tire size calculator is very helpfull in comparing the difference in sizes.
Miata Tire Size Calculator
Miata Tire Size Calculator
Drifting
Joined: Mar 2005
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From: St. Louis MO
St. Jude Donor '06-'07
Safety Car
Joined: Apr 2004
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From: Louisville KY
Originally Posted by dougbfresh
I "think" as long as the tire diameter is the same, there should be no computer issues. Wider tires mean more friction, should loose a little gas mileage but hopefully get more traction.
From reading on it Yes it will effect the computer. MPH, MPG, odomator, Active, Handling, Traction Control, just to name a few.
Race Director
Joined: Nov 1999
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From: Greer SC
Cruise-In II Veteran
Originally Posted by Jim2003
I am no Geometry teacher, but I believe it is the circumference of the tire, in other words the rolling distance it takes to make one complete rotation, not the diameter or radius that makes a difference.
From reading on it Yes it will effect the computer. MPH, MPG, odomator, Active, Handling, Traction Control, just to name a few.
From reading on it Yes it will effect the computer. MPH, MPG, odomator, Active, Handling, Traction Control, just to name a few.
Race Director
Joined: Nov 1999
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From: Greer SC
Cruise-In II Veteran
Originally Posted by jules9
so i'm guessing 3/4 to an inch there will be no issues with the computer.
Use one of the tables in the posts above to compare rear tire size to stock to determine impact on the speedo.