One day I noticed that my speedometer is reading more than I am traveling. At 45mph it reads 55mph. At 27 it reads 35 etc. It off by about
20 or more percent and it seems consistent by a percentage of speed. I'm also running 32's instead of 31s which means that it should be reading wrong the other way so it must really be off. Does anyone have any ideas what could be wrong? I thought it was all computer controlled. I think the odometer is reading more miles also than I am travelling.

I also have a hypertech programmer. Could I just program in a tire size 20 percent smaller to trick it into reading correctly? I hate to do that, but I dont want to keep racking up miles without having driven them.

 

Yes whenever you change tire size you must recalibrate the speedometer and odometer. The easiest way to do this is from your programmer and this is how I did mine.

 

^+1 Widow so has hit it outa the park .again lol , Always recalibrate the computer to correct tire size ...your aftermarket tuner will do the trick nicely .......

 

Also if you get different gear ratios, more than likely it will affect the speedometer and odometer so a recalibration is also required.

 

I used my aftermarket programmer and I think something is wrong. When I try to update new settings it says it works, but the speedo still shows the same speeds. If I go from 26 to 32" tires in the programming it doesnt affect the speedo reading. Now that I think about it, it started acting up the last time I reprogrammed my durango to go back to stock tuning. I'm thinking a glitch must have happened. If the programmer doesnt affect anything is there a way to go back to the stock settings on the computer?

I originally had it for 31.75" tires and 93 octane.
I changed it back to stock but left the tire size the same, thats when the funny readings started occuring.
Now no matter what I program it still reads wrong by a lot.

 

First make sure your programmer is set back to the vehicle "Stock" and follow this procedure. Reset the PCM (instructions below in case you don't know how) and plug your programmer into your computer and go to the company website. Do a download of an update or if there's a way, restart the programmer from factory settings kind of like a reset.

It sounds to me like your programmer is out-of-date and thus trying to screw with your PCM on your vehicle and something is telling it No! This is one of the reasons I like to recommend SuperChips over some other brands because if you had a SuperChips, you just gotta ask them for a box and send it back and they'll reflash the whole thing free of charge.

 

Thanks. I tried resetting the pcm and it didnt work. It remembed the tire size thats in there programmed that must be wrong. I realized that the hypertech programmer isnt allowing me to program the tire size anymore. If I set it, the controller ignores the change. If I pick 2 options then it allows me to change the tire setting, but it doesnt do it because when I go back later to check it acts like it was never programmed. I have emailed tech support. Hopefully they can help me. The only thing that I can think of now is getting a new PCM or having the factory reflash it with the original programming.

 

Yeah factory can help you out. We do it at our dealership for like $120 so definitely better than having to buy a new PCM. You can also have us do a recalibrate while its in but that up to you.

 

I still have an old PCM that would overheat and I put it in the Durango to test out. I get the same false speedometer readings. Does anyone know if the programmer reprograms the PCM or is it something else? The odometer reading stays intact between PCMs so that info must be located somewhere else.
The more I play with my programmer the more I realize that it did reprogram something in the truck so an incorrect tire size and now it wont let me fix it. I guess I will wait for hypertech to see what they say, but I thought the old PCM would fix it for sure.

 

Yes odometer is stored in the gauge cluster and the false speedometer reading is due to the need for a recalibration!! Try using the programmer on the old PCM because it's not just going to recognize slightly larger tires.