Read article about dynos that says dyno testing, etc. should be done with the car in as close to 1:1 ratio as possible - would guess this is 4th or 5th gear on a 2011 - 2012 GT. Makes sense I guess...

Just wondering how this is done on an automatic?

 

Pretty simple, the shift points are just changed around for dyno purposes so it stays in the appropriate gear at a low speed.. Almost like locking it in 3rd..

 

or, can't you just manually shift it to 3rd or whatever gear is required and leave it there?

 

Most will not stay in 3rd when you go WOT from a low speed.. The point that is changed is the 3-2 downshift..

 

Ah, gotcha. I forgot the 2011+ have that sport auto instead of 1-2-3-D (+ OD for 5) like the 05-2010

 

Depends on the car. Some can be altered in tuning, some have a stable manual mode, and some just need to be throttled carefully to avoid a downshift. I've usually done my pulls in 3rd to avoid a long, drawn out pull. I've compared pulls with different gears and the hp/tq doesn't vary enough to worry me. It's more about having the right amount of resistance...a higher hp car will want a higher gear or you rip through the pull too fast to get a decent reading (and also can get spin). Dyno manufacturers recommend 1:1 to maintain consistency from one shop to another.