My question for those that have done the gears, what are your 1/4 mile trap RPM's with the automatics? Ideally you want to be at the top of 4th gear at the top end of your HP curve just as you cross the line. Any input?
LB
I have an 07 GT Coupe automatic with 4.10's. I go through the 1/4 mile traps at 102 mph, which is 5300 rpm's. HP dies off after 6500 rpm's, but peak torque is at 4200 rpm's, so the 4.10's fall right in that spread in 4th gear at the finish line.
More important to my decision was being able to go about 120 mph in 4th gear just in case I do a power adder with this car so that I don't have to shift into overdrive at the dragstrip. I also wanted to stay under 3000 rpm's at 80 mph in overdrive on the highway (4.10's = 2800 rpm's at 80 mph, FYI).
I had 4.30's in my 03 Mach 1, but it was a manual trans with a much steeper overdrive (.62 vs the .71 in my 07 automatic), so I could get away with more gear. Plus, that 4V motor pulls strong to 7000 rpm's whereas the 3V motor drops off after 6500 rpm's.
For those with 05-08 GT automatics who really want awesome launches, get 4.10's
and a high-rpm stall converter for your trans. The high-rpm stall converter allows my car to launch at 3800 rpm's - just like a stick shift car when you rev up and dump the clutch. I would recommend something in the 3000 to 3200 rpm range, though, as my 3800 rpm stall converter is a bit much for the faint of heart (but I love it!). The factory torque converter in the 05-08 GT automatic only flashes to about 2200 rpm's, so launches are lackluster when stock, although the 4.10 gears out back will help a lot.
As many have already advised, the 05-08 GT's are heavy cars and the automatics are 180 lbs more than the manuals, so gearing really helps make these cars get up and dance.