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Discussion Starter #1
I have a speedometer question. I know when using bigger than stock tires it throws off the speedometer. I saw a show, maybe Stacey David's Gears, where he mentioned a company that, if you told them what size tires you were using, they would adjust a speedometer that would give the correct speed. Anyone aware of such a company? Or a formula to calculate the speed according to the tire size? If this is the wrong forum, let me know. Thanks.
 

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1994 GMC Jimmy SLE, 2010 Cadillac STS
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[url=https://www.tacomaworld.com/tirecalc?tires=245-60r15-235-75r15][/URL]
This is the website I use it has a good calculator, easy to use. I just used a gps speed box of my buddy’s to see how my 31’s threw my speedo off.

As a side note, it would be cool if we had our own calculator like this on our own forum... I have no idea how to do that but if someone does, it would be pretty cool.
 

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So with trucks 96+ getting a speedo calibrated isn't too hard but it can be a bit $$$. The OBD2 era computers have programmable memory in them for parameters such as speedometer and spark/fuel maps. Look in a phone book or google for a speedometer shop near you and give them a call. In my area the shop is called Tacoma Speedometer and they will do it for around $150. On earlier trucks it is not as easy. The ECM/PCM is looking for 1000 PPM @60 MPH and that is hard wired into it. There are gears in the trans that will make the sensor read what is needed, kinda like the spinning cable and magnet in the old school speedos. I think 1994 is when that changed but the computer didn't get flash memory until 96. Around 94(ish) GM went to a reluctor wheel sensor and gave up on the drive gear sensor. This then required the computer to be calibrated. Early computers would need to have the entire EEPROM flashed and reprogrammed to get a new cal for the speedo. This took specialized equipment that most shops have long since lost interest in maintaining.
 

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Discussion Starter #9
Thanks Rebel n Dirt. Mine is a 91. An I live wat down in South Arkansas and we don't have speedometer shops! :(
 

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94 4x4,01 Blazer
2001 Blazer 4dr 4wd LT
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buy a cheap gps, they have a speedo function on most. all done.
 

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On the 91 you probably have a DRAC module that adjusts for tire size etc. It can be modified pretty easily to adjust the speed output to your gauge cluster.


I soldered in a dip switch on my 91 to adjust, but like that site explains it is just as easy to move the jumper bars to the correct positions.
 
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