This is for the Chevy vans 2003 and up, maybe earlier, until 2010 6 speed models came out. The method for change would be the same for others, but the numbers would change.
Your Chevy dealer will likely not be able or willing to do this change as it is not in their prebuilt tire size flashable modifications availability.
This is for the fairly common move up to 265-75-16 tires on the Chevies mentioned above. It is what we have and size change we made quite a while ago. The size increase puts the speedo and odometer reading 3-4% low but doesn't have a major detrimental effect on the running and shifting.
The GM program is accessible with readily available tuners from HPtuner and BullyDog as examples. The programmers and software are not cheap at all, but reputable "tuner" shops for hotrods or trucks often have the tuners and bulk licenses to tune for hire. I would expect this change, if they are given the numbers to enter might be in the $100 range to have done in a fair shop. The whole thing only takes about 15 minutes to do.
Here is a screen print from my HPtuner as it shows up on the laptop screen after uploading our 07 van program. It is of the "speedo" section and includes the settings that would be changed.
It is best to change two of the settings. Gear Ratio and Tire Size, plus Sensor Calibration. One controls the trans shift points and the other controls the speedo calibration. They move together by using the same % change to both of them. You need to get both of them to read higher by increasing the settings when tire diameter is bigger.
The settings I will give are based on how I see our tire life to probably work out. New tires are bigger than worn tires and will cause changes of the two setpoints. I chose to use the Agilis tires we have with the 657 rpm per mile spec on Tire Rack for the new tire basepoint. The tires have 12/32" of tread so we would typically be replacing them by the time they are 6/32 worn (because of age). If I split the error from where it gives a 3/32 or about .100" reduction in diameter. That .100" wear would change the speedo/odo by about .6%.
We then did 100 mile odometer checks against the highway markers on our recently competed trip and found out how far off the odometer was. I changed it to a percent off and then included the .6% for future wear and came up with the new settings. I just did what should be the final change after the last check showed a .4% correction should put it where I want it to be.
Here are the settings I put in:
Trans revolutions per mile 2698 (enter this number and it might read a bit different one in place as it rounds to increments of the sensor)
VSS sensor pulses per mile 108,432 Be sure you use a comma and not decimal point as it looks like a decimal on the computer screen.
Save it on the laptop and then flash it into the program, is all the shop will need to do.
If you have a Scangauge or other monitor you can change the "speed" adjustment to zero base on how ours behaved and probably will need to recalibrate the fuel compensation % setting for a few fillups.
This should take care of the about 3.5% error in speed, odo, and fuel mileage you calculate from the odo which is about .5mpg at 15mpg.
I welcome any and all checks on my calculations as this is kind of a convoluted process.
Personally, I will be glad to have accurate speed, odo, and most of all mpg from the Scangauge on future trips so it is worth the effort.
Oversize tires speedo/odo calibration
Threaded Mode
