I read on another forum that someone was getting an 2-3mpg increaces in fuel effenency with a top box on their bike so thought I'd try it on my Ten.

Well I'm pleased to say that I seem to be getting similar gains when I have the top box fitted compared to when it's not fitted for the same cruising speed. That works out at about a 5% gain, or reduction in fuel bill on the motorway. It also gives me somewhere to put my helmet if I go to the shops and an option to taking my Kreiga R35 rucksack with me (which I use on the motorway to hold my laptop)

At that rate the top box should literally pay for itself in 2 years (50k miles) for me.

Interestingly thats about the same gains as a 14 to 15 tooth change on the front sprocket (which would take about a year/25k miles to cover costs after you include speedo healer in the cost) but without the decreace in acceleration.

The free option of course is to just ride 5mph slower

Interesting result, I would have expected it to do the opposite, given the shape of the thing but I guess that's why I don't work for British Aerospace!

The reason I think this happens as it does is the upright seating position creates a lot of drag behind the rider (look at my avatar). The top box fills in an area of this drag where a vacume is created so makes the bike as a whole more aerodynamic.

The box by itself is not aerodynamic but that is not relevant in this situation.

Give McLaren a call and tell em you've found a diffuser for them to rival Brawn and Red Bull!

glad i read that I had been putting off fitting my box for that very reason now time to put it too the test, although my box is not the s****y Yam one but a cheaper bigger one thats been in my shed for a few years so will hope it also improves my mpg i didnt realise sw anky needed to be starred out ! ?