Muffler modification caused sudden consumption issue

[Dartagnan66]

Hello guys, sorry for disturbing you whit some silly question, but I have got here some consumption issue.
Last week I've decided to little modify my original Scorpion muffler.
Nearly I cut it in half, the original 24" is 13" right now.
It's looks pretty cool, it's sounds pretty exciting... But... And there's a big BUT...
Since that cutting the consumption suddenly raised... And not just a little...
I am using the bike basically for commuting, a little in city, then few miles on motorway, then few miles in industrial area, in free time some fun riding. The speed on motorways 70-90 mph depends on traffic. The consumption was before the modification like 42-43 mpg (UK mpg) and after the cutting is 35-36 mpg.
Any ideas what's happened and how could I solve this?
Thank you for your help in advance!

[Dudeofrude]

Sounds silly but are you sure your not being a bit more heavy wristed because of the new noise? I for one have been guilty of it in the past. The bike sounds amazing so instead of shifting into 3rd at 5000rpm you keep it in second until 10000 because the noise is immense?
Just a theory

[Dartagnan66]

Tell you honestly, the funny thing is the real exciting noise coming by 4-6k, over that is similar like before only louder... So basically I don't rev too much higher than before....
Since I've posted here, I did some research, and I've read something about so called back-pressure in muffler, the airflow is more easy, shorter and faster, so the engine need more air on inside, but I didn't change for something high flow air filter.... But i am not sure, really would be great some advice from a real expert