I down shifted earlier at about 85 mph from 5th to 4th as I entered a decreasing radius off ramp....(sweet!!) As I got into the curve I down shifted to 3rd at about 65 mph (again ..sweet!).........

My question is....what's the max speed you have successfully downshifted ( to 4th from 5th)....out on the interstate...when meeting a challenge....& your intention is max acceleration...

I know mags say max speeds in gears are 31, 53, 80, 111 , 138 & 151......but practically speaking.....does it make any sense to do down shift much past 85....(5-4)?

 

Your best bet is to look at a dyno chart of your car and see where all the power is made.

Here's a quick '101' scenario...

Let's say peak power is at 5000rpm.
Let's say the power curve is symmetrical about the 5000rpm line.
Therfore, you car is making the same power at 4500rpm as it is at 5500rpm (which is less than at 5000rpm).
Let's say when you change up a gear the rpm drops 1000 every gear.
So if you're in 5th doing 4500rpm and you drop to 4th and maintain your speed you're now doing 5500rpm, (and vice versa).
So, to keep in the max power band you should bechanging gears at either 4500 or 5500 depending on whether shifting up of down.