You can check out the gas prices for all gas stations on the GSM Bensín website. It was set up to help locals find the best deals on petrol and diesel. Notice that the navigation on the left indicates the different areas of the country: Southwest, South, East, North, Westfjords and West. A similar website is Gasvaktin (look at all the colours!).
Note that the price is represented in Icelandic króna (Icelandic crown), the currency in Iceland, and is representing price per litre.
These are the companies that sell fuel in Iceland:
Atlantsolía, Dælan, N1, Olís, Orkan, ÓB and Skeljungur.
Prices fluctuate quite a bit - see petrol prices graph from media outlet Kjarninn.
On the Car Owner's Association (FÍB) website, there's a handy calculator that allows you to see how much a specific journey costs in Iceland with your car. The variables are obviously the points where you start and end your journey and also the mileage of the car you're driving. If you know the registration no of the car, the website will fetch that information for you - handy!
C = Car milage, liters per 100 km
F = Fuel prices, Icelandic króna per litre
D = Distance you intend to drive
To calculate your cost of driving a specific vehicle a specific distance, use the following equation: C * F * D / 100
Example: Car milage is 12 l per 100 km. Fuel price is 230 ISK. Distance is 1.332 km (the ring road): 12 * 230 * 1332 / 100 = 36.763 ISK.
See our guide to driving in Iceland and get our 10 tips for your Iceland ring road journey.