Back Packs and aerodynamics

[badlydamaged]

Today I made a trip to Burger King on my bike to bring back lunch for everyone. Once I was out onto the open road and doing more than 40mph I noticed the bike seemed to have more power than I have previously noticed, easily screaming up to 70+ mph (on the clock) and cruising happily though all the route at 60mph. The only reason I can think it would do this is because I was wearing a back pack, which could have lowered my drag (as I sit very upright on my unfaired bike).
Has anyone else noticed this on their own bikes (particually unfaired lower powered ones)? I certanly noticed a big performance increase, up to 10mph over what I would have expected running into headwinds. The bike also seemed to rev higher in the higher gears (high speed) before running out of puff when running on level ground than before. The back pack I was wearing was the average sized school type back pack, adjusted so it rested snug against my back (as a pose to resting on the seat).

Interested to here your expriances / thoughts on this.

Cheers, Jonathan
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My bike: 1988 Kawasaki KH100 Status: Polluting Devon with two stroke smog and noise pollution!
"Its not your engine size that matters!, It's what you do with it that counts"
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[hustler]

Same as a speed hump in leathers really I assume.
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As I said to that guy with the crutches and full camouflage gear, you can hide but you can't run.