waynebyrom

I bought and updated my cortex 3950 for my 2500 and I installed the updates. Then I used the setting that sets the tire size so that your speedo is calibrated correctly. And I went to a tire size calculator to get the size 35.35 inches tall for the tire. And I get on the road with my gps and its off by about 5 miles an hour. So I kept pulling over on the side of the road and resetting the tire size over and over going down in the cortex all the way to as if I had 34 inch tires and it was still about 1.5 miles off according to the gps. So I'm thinkin oh it might be that the original owner changed rear ends but it doesnt look like its been touched. Soooo, my question is:
Is the problem the speedo is off in the dash? I am at a loss and was wondering if anyone of you guys have come across this problem in the past? The computer was reflashed a few months ago and everything is updated on the cortex and it works fine as far as I know. So I have to actually go down to less than 34 inch tall settings on the cortex to get just about on the money. Freaky I think.

Mlucas

I can't say anything about the super chips, but when I checked my stock speedo calibration with a gps, it was 3.5km/hr off from the factory.

waynebyrom

Well thats good to know that it can be the factory setting. thanks

d.hodgee

I have a 06 with a super chip and 35's. I have recently found out that I'm about 5mph off as well.

Interesting!

So If I change my tire size to 34 it should be almost on?!?

waynebyrom

What I had to do was just keep going down in size with the superchips programmer and drive it and pace the truck with my gps until they were both the same then I left that size set. They say you cant go by what a tire size calculator shows you but to actually measure the tire size with a ruler.

waynebyrom

Just go .25 percent lower each time then drive again and see what the diff is with your gps

recone5

Same here with an 06, superchips, and 35's. I'm five mph off also. I could never figure it out why and everything is up to date. Maybe its time to get into the Superchips forum and ask around.

The 5 mph under is not a bad thing for me since i got one point left on the license before i need a retest. (All points not for speeding)