Small windshield, when to tuck?

My 650R has a pretty upright seating position. My knees have more of a bend to them than others I am sure, but from the waist up it is entirely comfortable to sit and not hunch over. At speed, say 70+ there is a lot more wind hitting my chest and helmet. I've been experimenting at at 70+ if I tuck in below the windscreen obviously I have less wind hitting me. I can tuck in and my eyes and the top of my helmet are slightly above the windscreen. I can also feel additional downforce on my back at that angle. Usually this isn't an issue because I am in 55-60 mph roads with little traffic. However, I've taken a new route to work that involves a stretch of wide open almost no other cars 4 lane highway. Maintaining 70+ is easy, and the motorbike has more to go (I'm not ready to go there yet though...) but you get what I mean.

My questions:

Is there a recommended speed to tuck? Or is it more of a comfort get out of the wind feeling?

Will I notice any MPG, performance, etc, type of benefits from tucking?

Also, keeping in mind I have a topcase on my motorbike, does tucking really help that much? Or is that wind I am deflecting now hitting the top case instead of my chest? (Aside from going in a wind tunnel I don't know how to tell if it is hitting the topcase or not)

I feel like my field of vision is reduced when in a tuck position, is that a pretty normal feeling?

You might read thru the coast down stuff JG wrote about a while back. There is also a calculator to do some coast down calculations here on the website somewhere. The data collected/generated will give you an idea of how much drag you are reducing by tucking.
Found the links
http://www.grassrootsmotorsports.com/media/files/DragCalculationsV2.xlsx
http://grassrootsmotorsports.com/articles/discover-your-drag-coast-down-testing/
I know the author, so if you have questions shoot them my way.
Steve Stafford

If what you are talking about is "buffeting" rather than just being in the wind I'd adjust the screen size/angle to put me in calmer air. It is extremely fatiguing for me to have my head bobbling all over the place due to wind. I get a sore neck fighting it. It's a funny balance - with no windscreen you have a steady stream of air so you don't get it - it's caused by the air detaching from the edge of the screen and your head position. Like riding 40' behind a big rig you get hammered with tumbling air but get right up close and there is a dead pocket. I'd tuck anytime that my head is shaking but then you get fatigue/discomfort from the slouch... so I start moving things around.

The poor man's wind tunnel is to tape little streamers of paper to the areas and use your gopro to video what happens while you ride.

As far as MPGs... just notice how much of a sail you are... so, yeah, getting yourself out of the airstream is going to help during steady state cruising. I think if you can adjust the screen so the air stream is smooth over you it will give you a similar benefit though.

you might try either a laminar lip or a taller touring screen

Zero Gravity Double Bubble windscreen time.

I have a zero gravity on the turbo bike and a puig bulge screen(its like a 1/4 bubble) on the '04. The important thing is to keep your sight line above the windscreen but still be tucked. Just getting your helmet below the wind flow will help as well. You should also notice the bike being more stable in traffic at interstate speeds.

It was a rude awakening for me coming from your area to TX. I never reached 70mph the entire time I lived in TN. Compare that to here where most roads outside of town centers have a 75mph limit, for miles on end. Yeah, first big ride was an eye opener, and a huge pain, literally.

I have my fairing adjusted to where the clean air over it breaks at the chin on my helmet. I get smooth airflow in the helmet so the venting works as designed, important thing here when the temp gets and stays high, forever. But the important thing is my shoulders and chest are in the pocket so I don't get fatigued. I've had no neck pain from having my head in the wind. Without that full airflow into my helmet I get way too hot way too quickly.

If it's really windy, and it frequently is, or I'm enjoying these arrow straight roads that go on for miles at 80+ speeds I tuck down, the height is perfect when I'm laying down on the tank and the wind flows over the top of my helmet.

I definitely think it's time to look into aftermarket windscreens for your bike. It really makes a world of difference in the quality of the ride.

In reply to Nick_Comstock:

The aero protection is already decent on his bike, just needs a bit taller screen to put the air onto only his head. We're talking less than an inch in height should make that difference.

In reply to octavious:

Check out this link and look at the comparison picture between the 3 zero gravity screens available. The link is for the Double Bubble which IMHO is the one you're going to want. The SR is too low and the Sport Touring is massively tall(like you'll FUBAR your bike's aero too tall)

http://www.revzilla.com/motorcycle/zero-gravity-double-bubble-windscreen-for-kawasaki-ninja-650r-2006-2008