Woolworths petrol - effective $0.10 to $0.14 per litre discount
Need to bear with me here for a moment as it's a combination of deals to get the overall discount.
If you get a wishcard from Infiniterewards (see other 'freebie' post - and I promise this is the last time I'll do this!), you get that at a 5% discount (if you can't pick it up, then you also need to allow for postage etc). Most Woolworths Caltex petrol stations do accept Wish cards, so....
Everyone knows that if you buy over a certain amount ($30) at a Woolworths/Safeways you get a discount fuel voucher. Well, with petrol costing around the $1.30 per litre mark now (in Melb), that 5% discount is equivalent to over six cents per litre discount. Combine that with your existing $0.04 discount voucher then you save some $0.10 per litre. If you buy $5 worth of something at the station, then you get an extra $0.04 discount. All up you would be saving a minimum of $0.10 per litre. My car has an 80 litre tank, but on average I probably go to the station when I have between 20 and 10 litres left.... so I could be saving around $6 to $7 per tank. More if I buy something in the shop.
Drawbacks? Well, a few - for example the wishcard is like cash so you need to be careful with it. Lose it, and the money's gone. Also, like I said you may have to pay for postage for the card, plus you need to pay for these cards up front rather than as you need it. So definitely some nuisance factor, but possibly worth it if you do a lot of driving and/or you have to pay for a lot of petrol (kids etc).
Before you ask, unfortunately Coles Express don't accept Coles gift cards... they have their own card that is not available from Infiniterewards....