I got my tires replaced the other day with 305/65 17 BFG AT KO's. Original size was 265/70 17 Michelins that I had the other thread on for cracking. I checked my speedometer with the GPS and when the speedo reads 60mph the GPS is reading 61.8mph. The dealer has stated that they will work with me since the tires were replaced under warranty and would hook up the scanner at no costand see if a tire size exists in there for what I have. These new tires are the same diameter and revs/mile as the factory 285 on the power wagon but that's a Hemi engine and different tranny. However,I don't know what all needs to be done by them and I want to make sure I'm covered. I've read other threads on speedo recal but I still didn't know what all has to be completed. If they hook up the scanner and recalibrate the speedo will this correct shift points, fuel mixture, timing, etc or will it just the correct the dash speedo reading? I didn't know if a reflash of any sorts was needed??? Any help or input is greatly appreciated.

 

all they will do is reset the ABS side of the computer and tell it the tires have a circumference of what ever the tire is. when they do that it will bring you VERY close to what your speedo should read matched next to your GPS.

 

Thanks Drew. Will shift points or anything else be affected by the tire size? If the dealer does this will it correct all needed items?

 

probably not. . . that is determined by the tranny programming

 

Does the tranny module need to be reflash? I'm not sure what is tied in with what. The dealer stated that the speed sensor is tied in with fuel mixture and injector timing so with the larger tires they said it may be off. Anybody go any ideas?

 

*SIGH*

another dealer that has absolutely no clue.

the speed is read off a ring gear in the rear axel. if you look on the top of the pumpkin you will see some wires coming out of the top, that is the actual speed sensor, and tied into a part of the ABS system that reads the signal comeing from it and then tells the computer what the speed is. then the computer sends a signal to the dash to display the speed.

the sensors that have to do with engine management are the crank, cam, MAP, IAT, and TPS. that speed sensor in the rear axel is a part of the ABS system since it shows how fast the rear wheels are turning.

 

Thanks Drew. I really appreciate it. They had me concerned. That helps a bunch.

 

dealers are good at doing just that.

 

I wish dealers that had no clue to what the hell they were talking about could be shut down completely. I gives the good ones a crappy name...

 

Got the speedo calibrated today at the dealership. They didn't charge me anything since the tires were replaced under warranty. The scanner had options for various tire sizes. It didn't appear to have an option for revs/mile. I knew the 285 70 17 BFG AT is the same diameter and revs per mile as my 305 65 17 so I guided the tech as to what tire size to select. The next option was rearend gear. There were 4 options and default was 3.21 but we selected 3.73. The next option was tone ring size and there were 4 options there also. Default was 9.25 but we selected the 11.50. The speedo is dead on the money now. I checked it with mile markers and also my gps and if it's off it's not enough that my eyes can see. The dealer was good to work with but I had to look over their shoulder and tell them what to enter. Thanks for the input on this. I'm good to go.