Not really, but to keep it simple, anytime you add stuff that disrupts the airflow over and around the cross-sectional area -- the pointy end of your bike realtive to the wind -- you'll increase the net drag effects. So, adding a windscreen means you'll need to make more power/torque to move through the airmass at the same velocity before your addition. The drag effect gets worse the faster you go (since its a function of speed), so the superslab is where it'll be most noteable.

OK, that was longer than it should have been....

 

I would think that the windshield would slice thought the air much smoother and more efficiently then a body.

 

I can get 50 to 55 touring on back roads. High speed freeways or city driving will drop me down to 45 or so.l

 

Just did 1300 or so miles. Had a 149.1 mile run which consisted of 50 miles 4 lane country road with some lights running 55-75mph, 25 miles 2 lane rural running 35-55mph, and the last 75 miles on interstate running from 70-85mph. Put in just a little less than 3 gallons. Knocked my socks off as I'm usually around 42-44 mpg. I'm 180lbs and had a tour pak and a bag plus full saddlebags. I run a 6.5" KW windshield.

God I love this bike. It runs like a champ and as a bonus it gets 50 mpg on occasion.

 

been close to 50 , a couple tanks , not very often

 

I have a stock '04 E Glide Classic and loaded with my camping gear I've gotten as high as 51 MPG (on the interstate). I always use hi test but it depends on the gas (brand, ethanol, etc). It usually ranges around 45-47 which includes highway and in town.

 

That simple assumption contains a lot of variables that may, or may not, make it true. I suppose an example might be a thin, 5'5" person weighing 105 lbs slung low in the seat wearing an aerodynamic helmet offers significantly less surface area than say a person that is 6' 8", 300lb broad shoulder sitting upright and high in the seat. Obviously the two have different net drag values and efficiencies, and one may benefit with the addition of a windshield. There are a host of other factors involved here too, but the point about drag is that the faster you go the higher penalty paid.

Anyway, on a flat highway with no appreciable wind my RKC gets a pretty consistent 42 mpg running 75-80 mph. But if I back it down to around 60-65 mph my mpg shoots up to almost 48. So that extra 10-15 mph cost me 30 miles per tankful of gas, and some of the lonely desert stretches I cover could mean the difference between making the next fuel stop or not.

 

I have an 05 EGS, bone stock. I consistently get 48 – 50 MPG. Oddly enough I just finished installing fairing air deflectors, the between the forks air baffle, and the outside fork mounted air deflectors and my mileage went up to 56 MPG. Mind you I only have 1 tank so far, but I was surprised mileage went up at all, not down with the extra drag?? I should also add that I always travel secondary roads at about 50 to 60 miles per hour.

 

48.2 miles to the gallon on yesterdays ride....with a 15 to 25 mph winds
the whole ride, riding mostly main roads with top speeds of 70 while passing.
We did approx 440 miles from Bar Harbor, ME to North Conway, NH.....for a free bbq lunch at White Horse Press Annual Open House........
Riding 2-up my mileage drops slightly......
SE HD breather, Rush slip ons, and a set of Xied's...she purrs like a kitten

 

I finally got 50 mpg on a ride I did yesterday with the Run-For-The-Wall guys. Tailwind of about 15mph and never breaking 50 mph. Stayed in 6th gear. I usually get right around 40 mpg Other than that I can't get 50mpg unless I'm going downhill in Neutral. Also I've noticed that I like to hear the sound of my pipes so I tend to rev a little higher.