The 2015+ cars apparently get the speed info from the wheel sensors, so no tune is required.I totally agree with MCM95403. Changing your 3.06's for 3.90's will be a night and day difference from a seat of the pants perspective. You may still need a tune for the gears and your mpg will suffer, but I'd do it before anything else personally. I changed from 3.55's (newly rebuilt 8.75 in a 73 Charger with a 340) to 3.23's and it is amazing what a difference even that small change made. But be warned that 3.06 to 3.90 may be a pretty big change. Someone else has already done this so it may be worth reading the thread and contacting them.
i've had this 3.90 gear set for months now. i will refill it with synthetic gear oil and install it in my challenger SE/E. now, the problem is you can't reprogram for different gear with the predator if you have a 4-speed. so i don't know how anyone else can do this swap.www.challengertalk.com