I ride 2 up all the time, I average 32 MPG on a two lane highway's running 60-67 around 2600 to 3000 RPM. If I'm running into a headwind my MPG will drop.
On interstate I average 25 to 26 MPG running 76 MPH around 3400 RPM and MPG will drop if running into a head wind.
I had a bad dyno tune at a dealership, I was only getting 18 MPG on interstate running 76 MPH around 3400 RPM. On two lane highway's I was getting 25 MPG running 55-67 MPH around 2600-2900 RPM.
While trying to figure out why my MPG was so terrible, visiting with a Harley Davidson service manager, he mentioned that my AFR was set very rich through the dyno run. If you have a dealer tune on a dyno, make sure the tuner knows what they are doing. The dealership that tuned my trike sold and changed ownership so the new owners couldn't do anything about the dyno tune.
We cussed and discussed my options, went over what voids the HD warranty. That is another story in itself, we decided to remove the Vance and Hines dresser duals which were installed by the dealer where I bought my Tri Glide. Went back to the factory exhaust, took out the TTS tune, flashed a HD factory tune.
After my factory HD warranty is up I will change the exhaust, install the TTS and dyno tune with a reputable tuner, hoping to gain a few MPG.
I had just rode back from the HOG rally in Ardmore, OK on interstate running 76 MPH around 3400 RPM. There was a good tail wind I averaged 36 MPG until I turned west and the wind changed.