Nothing to do with Gas Mileage, but was just down in your area a couple week ago, nice area, great ride between there and the East Coast....Hocking Hills....was very cool....as for the topic....Stock 2001 Heritage Springer gets lows 40's pretty much coast to coast.

 

I don't have a 103 so this probably isn't a good reply but when 2 of us went to Sturgis in 2011, neither of us experienced such a dramatic drop in mileage as you did. My bike had an 88 and my friend's bike was a 883 Lo and with both bikes loaded with 3 weeks worth of gear and such, our mileage only dropped just a little from normal. We mostly cruised around 80 mph. Our trip was from Oregon, though Idaho, Montana, Wyoming and the Dakotas so we experienced several different elevation levels and gas stations but we always tried to stop at gas stations that we were familiar with and bought premium gas. So not really sure what caused your mileage to be so bad but ours, on the other hand, was quite acceptable considering our speed and gear we were carrying.

 

We just got back from sturgis last night and the first couple highway tanks me and my buddy both noticed a substantial drop in mileage compared to the trip there. We both ran the numbers and both ended up with almost exactly a 12% drop in mileage. I'm a stage 1 88" softail he rides an 03 triumph speed master. We chalked it up to very dirty air filters as we had rode on a lot of gravel paved dirt roads while there. We'll as we got closer to home (dayton Ohio) the problem seemed to fix its self. We figured maybe shitty gas?, altitude?changes?, voodoo? I'm open to any other theories.

 

Ethanol and higher speeds is the answer

 

We have ethanol in our fuel here in dayton so I don't think that's the problem