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

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.

RE: Need HELP with speedometer
 

thanks swartlkk I didnot know those faxs thay helped lots. I will have pic soon

RE: Need HELP with speedometer
 

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

Go to above link. No hard work needed.

RE: Need HELP with speedometer
 

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.

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

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.

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

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.

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 :D...so the 245/35/20 tire would be how many inches in diameter??....and stock is 28 in. diameter right