Is it possible to re-calibrate the speedo for larger tire sizes? I drive a 2004 2500 cummins 4x4. I replaced the stock rubber with 315/70R17 BFG All Terrains. The speedo is off about 10 percent according to my GPS.

 

I'm sure someone a little more knowledgable about this than me will chime in here soon, but since they haven't....I'm pretty sure you can take it to the dealer and using the computer they can input your current tire size which will adjust the speedo, odometer, etc. I've heard before that you might need some tire RPM info which you can get from wherever you bought the tires, not sure about that part but I've heard it before somewhere.

 

I took mine to the dealership and had the computer reprogrammed. My tires are the same, told them to set it for 601 revs per mile. It's spot on now. Cost was about $50. Some will do it quick for free.

 

From what I was told at the dealer, the StarScan tool only calibrates to some preset sizes and does not allow recalibration to a specififc overall diameter. The problem with this is they don't have anything bigger than a 285/70 (or something like that) or a 245/40 on a 19.5 (or something close). The biggest overall diameter is only like 32.5" HOWEVER, the Smarty does! It will let you calibrate to the nearest 1/10" so I was able to do my 36.9s and so my speedo is dead on.

 

I saw this a while back .


http://www.ricksontruck.com/speedometer.html
Rickson Truck Wheels and Accessories

 

I had a TruSpeed in my 99 Diesel and I had bigger tires and gears and I could never get the speedometer accurate with it. It took my wife driving it and my laying on the ground with their little adjusting tool and a friend driving beside us with his truck at 60 to try and get it close. Withing 5mph but still not dead on.

 

On an 04 it can be done. the biggest pre set tire size is 285 but they can manually put in the revs per mile for any tire. 595-601 revs per mile should do it. The newer trucks starting in 06 (I think) don't have this capability.

 

Thanks Guys. I really appreciate the info.
Happy Truckin'.

 

Telling them you need a recalibration will sometimes confuse them. Tell them you need your pinion factor reset for 601 revs per mile. It only takes them 10 minutes if they are slow and they should only charge you for a half hour labor. Some will do it for free....

 

I just took my 06 Mega in yesterday and they recalibrated for the 285's. 616.6 revs per mile. Worked fine.

 

Yep dealer did it for me, had to go to 4-5 diffrent dealers before I found one that wasnt going to charge a full hour of labor, I was getting quotes around 80-90 bucks to do this and I had to leave the truck with them, I just said no thanks turned around and walked out. Finally found a deal down the road from where I work did it in like 20 mins(once they found the scanner and the tech, took a little looking around to find that guy) and only charged me like 30 bones, set my 315s to 601 revs per mile.
JW

 

That's what I wanted to hear, I have 315's and want to get them calibrated also.
I will wait tell my next dealer visit and hopefully they will do it for free.
Mark

 

I had my 99' cal'd for my 225/70/19.5's by my local dealer and the tech (he is a great diesel guy) told me that depending on the rev of the software on my PCM that there may be only a few tire size selections to choose from but there was always the option of reflashing to enable selection of a specific rev per mile. In the end no reflash was needed

I plan on by-passing the dealer on my 07' this spring when I fit my 19.5's by buying a Smarty and doing it myself.

 

Yeah, schedule an oil change first and then mention the re-calibration, might grease 'em a little.

 

none of the dealers by me would do it so ihad to get the super lift one....works good, but it should for $200