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Need HELP with speedometer

  #1  
Old 03-14-2006, 06:46 PM
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Default Need HELP with speedometer

I just put 31x10.50-15 on my 93 s-10 blazer. it had 235-75-15 on it.
how far is my speedo going to be out ??? please help



93 s10 blazer 4.3 4x4
96grand AM 3.1
 
  #2  
Old 03-14-2006, 07:43 PM
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Default RE: Need HELP with speedometer

A 235/75R15 tire equates to about a 28.9" diameter tire with a rev/mile of ~697.8 revs/mile.

235 is the width in mm. 235mm = 9.25". 75 is the sidewall height as a percentage of tread width. 75% of 9.25" = 6.94" sidewall height. Now double that and add it to the rim size to get the overall diameter. 15" + 6.94" + 6.94" = 28.875" or ~28.9"

5280 ft/mile * 12 in/ft / (28.9 in * 3.14159) = 697.8 revs/mile

Now your 31x10.5-15 tire has a 31" diameter and 650.5 revs/mile.

5280 ft/mile * 12 in/ft / (31 in * 3.14159) = 650.5 revs/mile

The speedo will be reading slow by ~7.25%. So at a speedo reading of 60mph, you will be travelling ~64.4mph.
 
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Old 03-14-2006, 11:13 PM
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Default RE: Need HELP with speedometer

thanks swartlkk I didnot know those faxs thay helped lots. I will have pic soon
 
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Old 03-19-2006, 11:19 PM
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Default RE: Need HELP with speedometer

http://www.miata.net/garage/tirecalc.html

Go to above link. No hard work needed.
 
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Old 03-20-2006, 08:19 AM
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Default RE: Need HELP with speedometer

ORIGINAL: blazingsadle

http://www.miata.net/garage/tirecalc.html

Go to above link. No hard work needed.
I would hardly call that hard work, but the reason why I did not link to the Miata Garage is because it does not have a selection for 31x10.5 tires.
 
  #6  
Old 03-20-2006, 04:36 PM
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Default RE: Need HELP with speedometer

so what would be the speed differance between 235/70/15 and 245/35/20...and can i fit any bigger tire on that size rims with out rubbing....wouldnt mind a thicker sidewall
 
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Old 03-20-2006, 06:05 PM
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Default RE: Need HELP with speedometer

235/70R15 ==> 722 rev/mile
245/35R20 ==> 754 rev/mile

So at a speedo reading of 60mph, you will actually be travelling at 57.4mph. You will be going 4.3% slower than the speedo reading.

I believe that others have ran 30" overall diameter tires without problems. 235/70R15 is a 28" overall diameter. So you should be able to go up to a 255/50R20 tire depending on if that would fit on your 20" rims. That would give you a 30" tall tire, but would result in a speedo reading that is 7.5% slow or at a 60mph speedo reading, you would be travelling at 64.5mph.

If you wanted to stick closer to the 235/70R15 tire size (~28" tall), then you should go with a 255/40R20 which would be 719 rev/mile or 0.3% slow speedo or at 60mph speedo reading, you would be travelling 60.2mph. Again, this is dependant on the availability of the tires and if they would fit on your rims.
 
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Old 03-20-2006, 07:19 PM
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Default RE: Need HELP with speedometer

so do u think 255/45/20 would fit and how much does it cost to recalibrate your speedo at a tire and rims place
 
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Old 03-20-2006, 08:59 PM
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Default RE: Need HELP with speedometer

Not sure that tire/wheel places have the capability to recalibrate the speedo for different tire sizes.

I know that 255 width tires fit on my 17x7.5 rims on my Bonneville so I would think that if your 20's are atleast 7.5" wide, they should hold the tire just fine. Now the diameter, well... I am just going by the writeup that Hanr3 has created stating that 30" diameter tires fit with little to no rubbing. A safe bet would be to stick with 29" or smaller overall diameter.

I'm going to be checking out the tire size recalibration here in a little bit. I have a friend that has the full blown Snap-On scanner/reprogrammer and I'm going to see if we can get in and change the tire size / gear ratio with it. If we can, that should open the door for more options to get it done.
 
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Old 03-20-2006, 10:08 PM
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Default RE: Need HELP with speedometer

ok cuz thats what littledan911 has and he said they dont rub, he has 20s. I want the biggest sidewall without rubbing that i can get because i like the look of 20s but want the smoothest ride possible...im used to my michelins ...so the 245/35/20 tire would be how many inches in diameter??....and stock is 28 in. diameter right
 

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