If you have 35" tires or bigger I highly recommending you set your RPM's on your tuner in the options screen to match your specific tire size. Your changes will overide what your vendor had put in there.
I have had custom tunes from two different vendors now that set the RPM much less than the actual RPM for my tire size. I told them I have 35'' tires when having my custom tunes built and they we way off.
Once I made the change, I immedieatly noticed better shifting and MUCH quicker downshifting. That dreaded Lag is gone...you know..the one where you slightly give it more throttle and nothing happens...this is seperate from the know turbo lag.
Now while going 40+ MPH and when pressing the pedal it downshifts right away and takes off..! Much improved! If you have that lag also, and have large tires identify what your RPM's is for the tire and get it as close as you can on your tuner. Incriments of 5's is as close as you can get unless you call have your vendor do it. It's much faster just to make the change yourself.
I have Toyo Open Country M/T Tires and mine are 35x12.50x20 which is 592 RPM per Toyo's website. I selected 590 RPM on my tuner and it made a noticable difference.
Here are some other Toyo sizes:
35X12.50R17LT = 596 RPM
35X12.50R18LT = 593 RPM
35X12.50R20LT = 592 RPM
37X14.50R15LT = 566 RPM
37X13.50R17LT = 560 RPM
37X13.50R18LT = 560 RPM
37X13.50R20LT = 560 RPM
37X13.50R22LT = 560 RPM
37X13.50R24LT = 561 RPM
All 38's on 16/17/19/20 inch rims are 550 RPM
If you have a different brand you will most likely have to look up your tire makers website to identify actualy RPM for your brand.
Hope this helps, I just wanted to share what worked for me.
2005 F350 Crew Cab LB FX4 Lariat
Loaded-Leather, Sunroof, 06 Chrome Package
5'' Tuff Country Lift
Toyo MT's 35's with Race Line Renegade 20's
ARP Studs, EGR Cooler Delete, Coolant Filter
MBRP 4'' Exhaust Turbo back, No Cat
SCT X3 Tuned by TSD & IDP
Stock Airbox, NON-MAF Truck
GT3788VA POWERMAX Turbo
DiProcol Optix EGT & Boost Guages , Scangauge II