I haven't seen it mentioned in this forum but SCT has come out with a firmware up grade for the SF3 and XCal3 that allows some adjustment of the speedometer based on tire size for 2005-2007 6.0L trucks. It's not available for 03/04 trucks yet but they're working on it. Nothing will be available for us Xcal2 users as they aren't doing any more to those units.

 

Awesome! Thanks for posting this up. Reps to you.

 

I REALLY hope they make this available for '04 trucks. Although I have corrected speedo displayed on my Insight CTS I would love for the factory speedo to be accurate, as I've ben told it will help improve shift timing. I've got an 8" lift and 37" tires and even with Matt's SRL tune my shifting seems to be a little "off".

 

Have you tried hand calculating the rotations per mile on the tires? I've found that many of the listed rotations aren't very accurate. I mark a corresponding mark on the tire and ground and move the truck one full rotation of the tire, make another mark on the ground, and measure between the two marks. Divide 63360 (inches/mile) by that measurement and you have tire rotations per mile. If your figure is 600, for example, then enter that into your SCT manually and see if it helps. I know on Jakes truck a lower number seems to help shifting more.

 

Yes, I think I have the rotations down pretty well. I actually got them from someone here (maybe you?). Can't recall the exact number but thought it was 570-ish?

At any rate, apparently none of that makes a huge difference in transmission shifting until you can get into the ABS control unit on the '03-'04 trucks to properly change the speedo. At least that's what I was told by Matt @ Gearhead.

On my truck, if I give half-throttle or higher at times it holds first gear for a LOOOOOONG time and shifts hard as hell into second. I thought about calling Matt about it, but figured it is due to the speedo issue. Hopefully SCT figures this out and gives us '04 owners the adjustability we need.

 

My truck does that weird shifting too. It's almost like every other day or 3rd day it goes through a relearn and occassionally the towhaul doesn't work until I reflash or disconnect batteries.