@Jerry:
nicely stated but maybe a little off topic 
Speeddy's concern was not one or two possibly faulty gauges but a significantly different consumption with two more or less similar bikes. And to find out if that's so one should not believe in gauges (faulty or not) but calculate the consumption for an equal distance under equal conditions. Only after you did this you will know if you got really different consumptions or just faulty gauges - which is more likely I concede 
Hans, you're probably correct but when I read his original post it sounded more like he was just going by the gauge reading and not actually calculating fuel economy manually. I don't see any mention of miles per gallon, all I see is miles to empty. And if the gauge tells you your empty when you aren't even close to empty, that will definitely screw up the perception of total range. And of course when other folks jump in and talk about the lousy gauges and just using the odometer to gauge fuel useage it tends to set me off. I'm so very tired of the customers themselves making excuses for the p-poor quality and having to find workarounds for all the junk that doesn't work properly from the MoCo. I can't think of a single soul who would accept this kind of crap from their $15k Ford Fiesta, but for some reason people accept it all day long on their $30k-$40k Harley.
As for the actual consumption differences, you would think that identical powertrains in very similar motorcycles would in fact provide very similar fuel economy if ridden in the same manner. But there are a lot of variables that have to be taken into account, none of which are detailed in this thread. There is the weight difference, but it's not that significant. More significant are the aerodynamic differences between the two models. And any tuning differences can make a huge difference on a Harley. If folks only look at wide open throttle AFR's, and don't look at steady state cruise parameters, they can fool themselves into thinking they have the same tune when in fact they are different as night and day. Then there are the things like brake drag, tire rolling resistance, etc., etc., etc.
Speeddy, how about explaining exactly what you are talking about. Are you just talking about the number of miles until the gauge tells you your empty, or actual miles per gallon calculated manually?
Jerry