Changing Speedo for Bigger tires & transmission shifting
 

I've put on a set of 35's & would like to know if I should recalibrate my speedo.
I have a gps & know that I am driving approx 7%faster than the speedo says...but my biggest question is does the transmission shift based on engine RPM's or road speed.

Now that I am rolling faster-the tranny doesn't go into 4th gear till about 90kmph. If I recalibrate the speedo-will the tranny shift earlier? (mileage kinda sucks now) Driving around town is frustrating cause I like driving in 4th lockup & not getting speeding tickets...

Yes, a lot of the transmission control is predicated around wheel speed.

Fueling also uses wheel speed and other parameters to determine where to set it.

I am pretty sure your 06 like mine will only support, computer pick list, tire size upto a 285, a standard Dodge tire size. But the folks at SMARTY give us all the choices. I just wish I had the bucks to spend on one so I could enjoy the HP and get my speedo/computer and 315s on the same page.

I thought that the ecm could handle just about any tire size...some others in here say that there dealer reprogrammed their trucks for 35's...

I have a 2005 and after calling various dealers in my local area and getting replies from it can not be done to it will be $90. I found dealer that said he would do it for free. It took him all of 1 minute. Once he hooks up his DRB computer it is under the anti-lock function under pinion factor. If the tire size you have is not listed as an option there should be a selection to enter revolutions per mile, most tire companies provide this number.

Above 285s you have to enter the rpms of the tire

from what I gather from this site: up to 2005 can be programmed for any size tire by inputing the RPM (revolutions per mile), 2006 and up only let you select a tire size up to 285/?/17 unless you use an aftermarket programmer such as Smarty.