You didn't answer any of my questions...

Is speedo reading high or low vs GPS?

What is your current tire circumference in BCM?

What is your current tire size on the truck?

Also, is 3.73 your stock gear ratio or did you change to it from another gear?

 

Sorry, my. bad. Family wedding has me occupied. Here's what I know.:

Speedo now reading 60mph, but GPS reading 63-64mph

Currrent tire circumference is 2465mm

Current tire size is 275/55-20 - with 5k miles on them, so near new

Everything points to 3.73 gears with no reason that they were changed.

Thanks.

 

It could be a couple MPH off just in the cluster. If memory serves me correct they calibrated them all a couple MPH off. Only police cars had exact certified speedometers.
I dont think GPS is exact either.

 

I have worked with a couple others here that were able to get within 1 mph with non stock tire sizes. I see no reason a stock tire can not be adjusted within that range.

My current 2012 has 275/65/20's and my speedo currently reads about 1 km/h higher (less than 1 mph) than the GPS. I could play around and get that number tighter but I don't need it THAT accurate.

Nevada Bob - put in the 2500 I mentioned in the other thread for your tire circumference in the non AS BUILT file. This should get closer and may swing the other way (speedo higher than GPS). Then do the PCM relearn, clear the codes and go for a test drive.

 

Hi KRUISR, I used your calculator spreadsheet and wanted to run the numbers by you so I can feel good about inputting. Does the output look good for the inputs?
Thanks for putting the work in, I would have never been able to figure out on my own!

 

If the stock line 726-45-01 is as shown above then, yes the new line of 014B 09C6 xxxx should work and get you close. Some fine tuning can be done after that if you want to be closer.

Post up your speedo and GPS speed readings when you are done. Don't forget to do the PCM Relearn, then clear codes and then if they stay clear (no codes) go for a test drive and check accuracy.

 

Everything seems good! Speedo reads 1mph faster than GPS which I'm good with. Thanks again for making that spreadsheet.

 

Excellent. Glad it has helped. If you want to get closer/more accurate try changing the 09C6 to 09B5.

 

KRUISR - I have update by non-As Built file with 2500 for my wheel circumference, did a PCM relearn, and deleted codes. Drove twice today and I'm 2 mph off at 60. That is to say that the speedo reads 60-mph and and GPS reads 62mph. I can live with that. Of course, I'd love to be closer. What do you say? 2550? 2600?

 

If the information I have is correct the limit to how high you can increase it would be to 2532. You can try 2550, but it may not work. There might be no change or it will revert to your factory value in the PCM. Either way you will not wreck anything.

Looking where we started - tire circumference 2463 with speedo at 60, GPS at 63-64
last change - tire circumference 2500 (increase of 37mm) with speedo at 60, GPS at 62
If you change tire circumference to 2532 (increasing another 32mm) with speedo at 60 you should get a GPS reading of 60.5-61. I would try 2550 and if it doesn't work then put in 2532 and you should be really close.

Let us know.