I needed 4.10 gears to get there with the heavy RR.
I actually tried 3.91's and it did not change the et at all I was out of motor/tuning and it was stock pistons rings and bearings from a 85K mile 79 wagon
In my heavy car 3.55 to 4.10 was like 4-5 tenths. I suspect 4.10 to 4.88 might be another 2-3 tenths.
