Today, same trip, 'older' '08 bike, 12,500kms. Basically the same set-up, with one 'big' difference:
this bike has a large rear do-nut, Kenda Challenger 140/90-15. Todays result?  55MPG!!!!!

How to explain the diff? beats me.  Both are '08s, newer one with 5500kms.
Insurance goes off the older one on the 27th, I'll try another road with both bikes before then, and try to confirm these results.
cheers!

Is the weather noticeably different? Cooler dense air versus hotter loose and humid air?

Are you riding the 43mpg bike after a huge lunch, and the 55mpg bike first thing in the morning before you pack on the extra pounds?  

Have you checked the error in your speedos?
They are far from ''certified'' and can vary up or down with cable friction or maybe even the weather.
Mine was 7% optimistic stock, after changing the pulleys and rear tire size it was 22% under against a gps!
You can get free spedo apps for your phone. I taped one to a piece of cardboard and then to the handlebar and clamps to try. Ended up making a solid mount and the stock speedo is now in the attic.

zipidachimp wrote on 09/22/16 at 11:51:24:


I like to run higher than normal tire pressure. I think my back tire runs around 40 psi and the front is set around 35 (I think). I'm using Pirelli Route 66 tires and I think the max cold pressure is 45. Again, don't quote me on that. I'm not near the bike right now. Higher tire pressure makes the bike a little more predictable in turns and it handles better. It also improves gas mileage quite a bit. There's less rolling resistance with higher pressures. Around 32 psi my bike feels squirmy. You might try airing up a little bit.

Always check the max cold pressure of your tires and don't exceed that number!  

Doesn't look like speedo error is the cause.

If you are really curious...you might try switching carbs!