I've heard mixed things. Some say performance motorcycle exhausts reduce fuel consumption, others say it increases. What's your experience?

gasoline engines are sir pumps. Every inch of the path of the air affects the power and mileage. Exhausts need to match the intake/valves/air speed to get the best scavenging of exhaust gases out of the cylinder. You need a way to measure this to know what you got by changing the exhaust. Engine tuners use flow benches. You, the end user, can tell by power deliver across the rpm range and gas mileage!

I can often smell the gasoline fumes long before I catch up to and see a group of HD's ahead of me. I seriously doubt their loud pipes are increasing fuel economy. 

maybe they’re mixing some Klotz in the fuel? A nose like that reminds me of my wife lol