So I wanted to get my speedometer recalibrated for my 35" tires, it seems to be 5-6 mph off when on the highway. So I called the first dealer in town and they said they could do it but it would be $130 for them to do it, I said Ill get back to them on when to make an time for it. Called another dealer about an hr away, they said they could do it an would only charge an hr of labor ~$65, I said ok ill bring it in later today. Iv been searching online and found that for a 35" tire they need to put in a code ranging from 600-605 revs per mile. when I talked to the mechanic after he was done (took less then 20min) he said that he put in 602 for the rev per mile. The nice thing is they only charged me $20 to do it, this has to be the cheapest thing Iv ever had done at a dealer, so I leave an notice right away that the speedo seems to be reading about the same as before. I get my handheld GPS out and sure enough its still reading off, I get on the highway at when the speedo shows 70mph my GPS shows 75mph at this speed my rpms are at 2k exactly what they were before they did anything. so my question is from what iv read they put in the right rev per mile, but when I calculate it i get nowhere near that number.

Tire diameter= D =35"= ~2.92'
circumfrince= 2*pi*r= pi*d= 9.163'
1 mile=5280'
rev per mile= 5280/9.163=576

so if they put in 602 then my speedo is thinking its a smaller tire right? but if you do that same calc for a 33" tire it comes out to 611 rev per mile, so where is everyone getting this ~600 number from? did I do something wrong? anyone had any luck recaling for a 35" tire? Im going to call them tomorrow and see what they have to say, luckly it only cost me $20 and not $130...this is the reason i refuse to go to the dealer unless i absolutly have to.

 

You are making several errors in your calculations. First, you are assuming a tire labeled 35" is actually 35" in diameter. They usually are not, that is just a nominal size. Secondly, you cannot calculate revs per mile from the diameter. You have to use the loaded radius of the tire in your calculations, which is much less than the unloaded radius measurement. The best thing to do is visit the manufacturer's website and look up the data for the tire you are using. The engineers have already performed the calcs for you.

As for the programming, if the tech entered 602 into the ABS module, then that's about as accurate as you can get with the factory set up due to limitations in the software.


Jay

 

find a fellow FTE user near you with AE they will be able to get ou dialed in