I have an auto with 35's and 4:10's and OD is useless until i hit around 65mph. I live in mostly flat area. Does anyone think the flashpaq would help this or just keep runnin with OD off.

 

I have 34s with 3.73s and don't have any problems on flat ground,
hills are a bit of any issue but thats expected. Also you might
want to consider the flashpaq if you drive into a head wind a lot,
it seems to slow me down quite a bit

 

2dr auto Rubi on 35's w/4.10's and Flashpaq here...I'm in southern Indiana with rolling hills. I can honestly say gears are pretty low on my to do list. Like others have said, if I were in the mountains I would want gears. In flatter areas it's not bad at all to run 35's on 4.10's.

One thing to consider...not all 35's are created equal. Sizes vary slightly, but more importantly, it seems like there is a pretty big range in weight between different 35's. Unsprung weight will rob power quickly so you may want to consider tire weight when shopping around.