I have just bought a BMW Polo, and as I have never had a fifth Gear before, I am not sure what speed I should be in before i use it. I know this must sound pathetic to all you hardened drivers out there, but i am not to proud to ask for help.
As with any gear,if you want to drive conservatively,then change up as soon as your engine will accept the higher gear without labouring. Won't take you long to get used to it.
A bit late but here's a tip from my Mitsubishi handbook - use 5th gear as overdrive. That means 40+ on an open road. As for 5th in town traffic; don't think about it. You want a lower gear to accelerate and help with possible braking.