Assuming you are driving from Madrid it should take
about 4/5 hours (depending on traffic and Stops) to get to Oviedo and 1 hour more to get to Ribadesella.
Gas price is around 0.915 € for unleaded gas and 3 cents less for diesel, plenty of gas stations on the way (except in the highway stretch from Benavente to Leon 60 Km, where you would have to take a little detour to find one)so don't worry about this.
Road is excellent, similar to an Interstate in the US, four lines, two each way (3 sometimes), there are two strechtes where you have to pay toll, Villalba-Adanero 8.40 € and then Leon (Onzonilla)-Pola de Lena 9,25 €.
Hope you find this info useful!
Have a good trip!