The silly car of mine has yet again decided that it hasn't had enough money spent on it and decided to lose a part.

From what I gather, there is a yellow hose, which connects to the throttle body and the overrun cutoff valve - but the 90 degree rubber union has abandoned ship somewhere on the A74 (if someone finds it, please pm me to post it back to my address). You would never believe it, but the noise it makes is phenomenal when you're doing about 50 mph, a very loud hissing from under the bonnet! I'll need to pop down the dealership to get one ordered in, but the engine seems to be running fine in the mean time. Only thing I've really noticed is pants mpg, it used 3/4 of a tank for a 215 mile motorway journey @ an average of about 85 mph. I never really got a full gauge on how good the mpg is on the car because I've only done about 1000 miles in it since having it, and so its been difficult to judge, especially with the super lazy petrol gauge which takes about 5 mins to actually show a fully tank!
Will reconnecting this hose help my mpg woes? Also what might help brighten up the mpg front? I've got perfect inflated tyres, using Shell V-Power (99Ron I think), just had a service (cleaned up sparks - they looked fine to me, oil change and and belts).
Ben