Hey Mike,
We have a 2010 RT AC and love it. I use 5th gear as the overdrive because I like to keep the RPMs up and by doing that, I get better MPG. I had to get use to this, because my HD was a low RPM machine. The best I ever got with the RT was 186 miles and it started missing as I pulled in to the gas station. I put 5.9 gallons in it and got the last 1/2 gallon in by slowly topping it off (which you don't want to do because the gas goes in to the overflow, you get a gas smell for days). That gave me 31.5 MPGs and I average 30MPGs.
Now, to get in to the 40 MPG range, you probably will need to go with a 2 wheeler. I had a HD Classic that got 40 MPGs before I put a cam in it, and it got 32 to 35 after the cam. The buell I had got 55 MPG...
If you have to get over 40 MPGs, you may want to look at a VW bug TDI, or a ford festiva...:joke:
I ride with several 2 wheelers, and several trikes. The trikes are a tri-glide, a HD classic that has been triked, and a goldwing that has been triked. When we hit 100 miles on the clock, we have to stop for gas becuase at 110, all three of them are on the side of the road. Some times the tri-glide can be pushed to 130, but it is close. The 2 wheelers are a screamin eagle street glide and a ultra limited. Those to have been modified, but can go to 175 before the light comes on.
I like to get off and stretch my legs and drain the bladder around 100 miles, so I will fill it up also.